S T UDI E S ·
LABORATORY
E
Xc
I T E .D
R E
A
0 F
C T I 0 N 8
A thesis presented for the degree of Doctor
of Philosophy in Chemistry in the.University of
CanterblU'Y, New Zealand ..
by
Tf .. S. Wauchop
·t:
1969
PREFACE
.
i
I would like to thanlc Professor L•F. Phillips
·and Dr M.J. McEwan f'ql"' their advice and interest
during the course of' this work, and the Uni vel"'sit y
Gr-ants Committee for the award of' a Post Graduate
Scholarship ..
T.
s.
iVauchop
March 1969
Page
Contents
List o:f Figures
Introduction
Chapter I
Fevi e\'1: Excited. a ,toms, thei,r product.i_qn
and pro..J2§rt ies
The excited states o:f
1
~tomic
0
1
The :first excited state o:f atomic
hydrogen
21
Chapter II (i) Quenching o:f resonant radiation in
an atomic vapour
25
.
(ii) Imprisonment o:f resonant radiation
Chapter III
Experimental
27
31
1
Part I
To study 0( D)
31
Part II
To studY H(2l?)
35
Pressure, li'low control and measurement
Lyman
i
37
Lamps
41
.Spectr•ometers
44
Experimental conditions and procedure
48
Of
Chapter IV
1
E._q§ults and discussion of o( D)
studi~
50
Reaction with N 0
2
Reaction with co
2
1
Possible methods of studying o( D)
50
Chapter V
2
Chemiluminescent reactions of H( P)
Chapter VI
Trapping pf
Lyman-~
51
52
radiation in an
72
Calculation of trapping time for
the case of Doppler broadening
73
Calculation oft rapping time for
Chapter VII
the case of pressure broadening
75
. Compal"ison with experiment
77
Quenchinp; of H( 2P) fluorescenc.e b_l
Q2.z...l{2 , NO, !{ 2.0.t. CO, co 2 and so 2
Results
79
79
Discussion of errors
87
General Discussion
90
Aeroryomlt appl ic,a t ions
9l.
Appendix I
94
References
105
LIST OF ILLUSTRATIONS
Figure
Followi:ng
Page
1
Energy levels of atomic oxygen
1
2
Energy levels of carbon dioxide
1
3
Energy ley,els of atomic hydrogen
21
~.
Trapping of 2537~ radiation in Hg vapour
29
5
Apparatus for production of
6
Vacuum line for o( D) Study
31
7
Reaction vessel and lamp for 0( 1D) study
31
1
o3
31
8 and 9
Properties of the Hg lamp
2
The r"low system for H( P) study
34
36
12
First reaction vessel for H(~) study'
cond reaction vessel for H( 2P) study
36
1.3
2
Third reaction vessel for H( P) s~udy
36
10
11
35
14 Reaction vessel for studying quenching of
2
H( P) fluorescence
15
Bridge circuit for the isothermal
calori~1
38
meter
16
'Pt'-detector
for the isothermal calorimeter
17
The pumping and filling
Lyman-« lamps
sy~tem
for the
38
18.
Block diagram of circuits used with the
McPherson spectrometer
19.
45
Sensitivity CUPVe for the McPherson spectrometer and the photomultiplier
H(~P)
59
II
"I
59
II
II
59
II
59
20.
OH(~
2?t) emission from
02 plus
21.
NH( 3?t
32:)
II
22.
II
II
11
II
II
23.
II
II
It
II
11
2L~.
25.
N
2
NO
~<))
59
II
59
II
II
59
II
II
59
-
II
27.
SH(~
29.
II
NO"
2
Unmodulated emission from N 0 plus
2
II
II
II
II CO
CH(~
~)
26.
28.
II
II
21C)
tl
II
II
It
II
CO
il
so
L17
2
2
Light in an atomic H gas
2
Quenching·of H( P) fluorescence
76
79·
INTRODUCTION
Recent interest in electronically excited
~toms and molecules has stemmed from a !desire to know
more about some species found in the upper atmosphere.
This thesis reviews studies of two excited atoms
I
Attempts were made to study the reactions of
O(~D) with N20 and co 2 •
The difficulties encountered
in this study are discussed.
A method of' distinguishing luminescence arising
from primary reactions of a short lived excited
species from other light emitted in the reaction
system vras used in the study of H(~).
In this
method the ground state hydrogen atoms were excited by
a Lyman-or lamp.
The lamp wns operated from the a.c.
mains so thnt its output at 1216]{ was modulated at
100 Hz. Consequently the population of the short-lived
2
H( P) was modulated at 100 Hz and any luminescence
arisin!': i'rom allowed transit ions or electronically
excited species produced by its reactions was therefore
also modulated in phase with the lamp.
This luminescence
Q;)OI<J be distinguished from scattered light from the
lamp by its dependence.on the simultaneous presence
of both ground-state atomic hydrogen and the molecular
reactant.
Using this method in the following reaction
mecl~ani sms we detected: OH( 2 L:) from H( 2P) plus o2 ,
plus N , NO and
o, CH(~) from
2
2
H(~) plus CO and co 2 , and 8H( 2 L:) from 80 2 plus H(~).
To determine whether there was a relation between
NH( 3?C) from
H(~)
N
i
!
the. chemiluminescent reactions and the overall reactions of H(~), the Starn-Volmer equation was applied
to quenchings, by each of the reactants, of 1216i
fluorescence in an atomic H gaso
Difficulties
encountered owing to trapping of 1216.2. radiation in
the atomic hydrogen
~apour
are discussed.
From the
quenching measurements rate constants and crosssections for the quenching of' Lyman-.x fluorescence by
~o
2 , N2 , NO, N2o, co, co 2 , 80 2 , H2 and Ar were ob-
tained.
C H
AP T E R
I
Review of :B;xcited Atoms, their Production
and Properties
The excited states of' atomic oxygen:
The .excited states
of' oxygen have been the subject of' intense study in
recent years.
One of' the main reasons for the interest
shown in these excited states is the occurrence of' the
red (6300A and 6374X) and green (5577AJ lines of' atomic
oxygen as strong features in the aurora and airglow.
1
Only the lowest energy levels of atomic oxygen ( D and
1
s) are involved in these transitions, as shovvn in
:figure 1( 1 ).
Because of the long lifetime of the parent level
i
1
( D), the green lines are emitted with appreciable intensity only at very high.altitudes (usually at about
120 km and 250 km).
This section of the review is an
account of the reactions which cause deactivation of
1
o( D) at lower altitudes, together with some other observed deactivation of o( 1 D) and o( 1s).
Production: The main methods of laboratory production
1
1
o:r o( D) and o( s) are the photolysis of oxygencontaining gases.
Common gases used are ozone, oxygen,
T :
0 ·74 sec
4·17 eV
1s
'
.,.
..
>. :,577 A
110 sec.
1 ·96 eV
A. 63 00 "'A
0
Fig.· 1.
i\2 972 A
1D
..
A. 6374 A
"
3p
E.
( Kcal /mote )
P.
200
100
. ,Reaction coordinate
Fig. 2
Energy levels of carbon dioxide
Energy levels
of atomic
oxygen.
2
nitrous oxide, nitrogen dioxide and carbon dioxide.
(1)
Ozone photolysis has long been known as a
source or atomic oxygen in the laboratory and in the
upper atmosphere. Heidt( 2 ) showed that quantum yields
up to 6.7 for o
3
decomposition were possible in dry
ozone.
McGrath and Norrish3 '4 in a study or flash
photolysis of ozone obtained quantum yields or 7 for dry
ozone and several hundred in the presence of moisture.
The allowable processes or dry ozone photolysis
vary with wavelength as shown below.
(The wavelengths quoted correspond to the lower
energy limits or the reactions).
o ( 1A) ~ o2 ( 3zg-)
3
o(3P)
11800~.'
( 1)
1
( Ag)
+ 0(3P)
6110Jt
(2)
~ 0 2 ( 1z g)
+ 0 ( 3p)
~
0
~
0
~
0
~
0
2
2
2
2
+
(3)
(~g-) + o( 1D)
411oX
(4)
1
( Ag)
+ o( 1 D)
31102
(5)
1
( 2 g +) + 0 ( 1D)
·. 266oJl
(6)
Of these, only (1), (5) and (6) conserve spin, i.eo
correspond to allowed transitions in the
o
3
molecule.
3
Thus the process (1) is expected to predominate at wave-·
0
lengths greater than 3100A and processes (5) and (6) at
shorter wavelengths.
Experimentally the 0( 1 D) quantum
yield decreases rapidly above 3100A and the quantum yield
for ozone decomposition falls to 2.
6
De More and Raper 5 ' photolysed
o3
in liquid N2
Ill
hey found a quantum yield of unity
and liquid argon.
t:J
for 0( i D) production below 3000A.
In exchange experiments involving 0 2 they showed that o( 1 D) and o(3P)
are produced above 31001 with a total quantum yield of 1.
McGrath and Norrish3 '4 proposed the following·
deta
ed mechanisms to explain the high total quantum
yields of' ozone destruction.
Dr~
q3
Initiation
o3 ( 1A) +
hv
~ o2 ( 1 ~g) + o( 1D) - 91 k cal/mole
1
1
() ( D) + o ( A)
3
~ o2
*< 3z:g-)
(7)
+ o ( 3z·.g-) + 138 k cal/mole
2
(8)
where ~ indicates a Vibrationally excited species.
Propagation
02
*
(3~.c.rg-) + 0 3 (1A)
(9)
where the energy of 0 :;: in x•eact:i.on (9) must be greater
2
4
than' 69 k cal/mole so that only o2'T with v greater than
17 will pPopagate the chain. Unless there is sui':ficient
energy available in (9) to produce o( 1D) the "energy
chain 11 is unable to propagate.
Termination
0 + 0 + m
~
02 + m
( 10)
( 11 )[ '
(12)
Initiation
o3
1
( A)
0 (1D)
~ o2 ( 1ag)
+ H20
+
o( 1n) -
91 k cal/mole ·
~ 20H + 34 k cal/mole
( 1.3)
(The OH spectrum was observed in absorption during this
reaction.)
Propagation
OH +
o3
H0 2 +
o3
~
~
H0 2 +
o2
+ 25 k cal/mole
OH + 20 2 + 27 k cal/mole
( 14)
( 15)
Ter•mina tion
( 16)
5
. 20H + m
-to
H o + in + 46 k cal/mole
2 2
( 17)
In their original mechanism McGrath and Norrish
postulated
20H
_., H + 0 + 12 k cal/mole
2
2
( 18)
instead of'reaction (16). Work by Kauf'man and Kelso 7 and
Phillips and Schif'f' 8 has shovvnit is reaction (16) that
occurs at low pressures.
Two other termination reactions that .have been
postulated aPe:
(approximately thermo neutral)
( 19)
(20)
AJ.though other work has f'ollowed the basic
mechanisms have not been disproved.
Snelling, Baiamonte
and Bair 9 conf'irmed the g_uantum yields f'or dry ozone and
obtained k ~ 2.3 x 10 9 litre mole- 1 sec- 1 •
(2) Photolysis of N o.. Direct photochemical
2
'
decomposition of' N20 in the ultraviolet was first investigated by McDonald 10 at 1849~ and later by Duncan and
Noyes
11
in the range 2150~- 2350R.
6
Zelikoff and Aschenbrand 12 suggested
1
N 0 ( 21: ) + h v
2
-:;.. N 2 ( 1 21 ) + 0 ( 3P ) , 0 ( 1 D) , 0 ( 1S )
( 21 )
1
N 0 ( 21) + hv
2
~
(22)
NO + N ( 4s)
as two possible processes.
They were, however, unable
to estimate the relative importance of the two processes.
The existence of a nitrogen atom primar-y process
as in (2·2.) has been questioned by Kistiakowsky and
1
Volpi 3 who point out that because of the speed of the
r-eaction
N +NO
=
-+
N
2
+ 0
(23)
1.8 x 10 1 0 litre mole -1 sec -1
1 4)
it is unlikely that any NO VIOUld be found in the products
of N 0 photolysis if nitrogen atoms were produced.
2
However, N could be lost by usual Wall reactions at
moderately low pressure, since the product k
23
(N)(NO) is
expected to be quite small under steady state photolysis
conditions ..
Doering and Mahan
15 16
'
Q
showed that at 1236A,
1470i and 1830i, the two primary processes both occurred.
1
When they used a mixture of N 5NO and NN 15 o the reaction
produced N230 , which could be explained by the following
mechanism:
7
N15NO + hv
N20 + hv
Cf' + N20
...
...
...
N15 + NO
(24)
N 29 + O*
2
(25)
N 29
+ 92
2
(26)
0* + N NO
...
N15o + N14o
(27)
N15 + N15 0
...
N 30 + o( 3 P)
2
(28)
15
It is difficult to account for the production of
N 30 without assuming the occurrence of both processes
2
(21) and (22).
Yamazki and Cvetanovic 17 point out that production of 0 ( 1 s) has only o.8eV excess energy at 1849~,
which may not be sufficient for a vertical li'rank-Condon
transition. They therefore assume that the production
1
of 0 ( D) by reaction (20) is the main primary process
of N20 photolysis.
18 1
Young and Black ' 9 detected the 5577~ emission
1
line of 0 ( 8) during photolysis of N20 at 1470~,
1
showing that at shorter wavelengths 0 ( 8) is produced
1
as well as o( n). They did not detect o( 18) in photolysis at 1849~·
(3)
Carbon dioxide photolysis in the wavelength
region 1200-1700X.
Work on the photolysis of co
2
in
8
this region has failed to result in a genera•lly accepted
mechanism. Groth 20 concluded that oxygen atoms were
formed in the primary process with a quantum yield near
unity.
Tucker and Ridea1
in
co 2
21
found a quantum yield of CO
photolysis close to 2, and
conclud~d
.that atomic
oxygen was not involved in the photolysis..
They
proposed the following mechanism:
(29)
(30)
Mahan 22 examined t~is mechanism and that of Groth
in detail at 1~70j and 1236j and found the excited
co 2
mechanism to be inconsistent with his observations.
He
showed from spectroscopic data that an excited oxygen
atom was produced in the 1 n state. Further work by
Warneck 23 agreed with that of Mahan, although a recent
I
attempt by Young and Black 1 9' 2 ~ to detect the charac1
teristic radiation of o( D) failed.
The reactions Which
occur subsequent to the formation of o( 1 D) are still controversial.
(See the next section).
photolysis at 228d~. was used by Preston
2
26
and Cvetanovic 25 ,
to produce o( 1 n). They state. that
(LJ.)
N0
the energetics of the light absorbing process prohibit
9
1
the possibility of 0( 8) formation.
28
In two notes by WaPneck 27 and Cvetanovic
the
natuPe of O* produced by
discussed.
co 2
co 2 ,
N 0 and N0 2 photolysis is
2
1
Warneck showed that the o( D) produced by
photolysis was not quenched by
co 2 ,
but underwent
isotopic exchange.
He therefore doubted whether O*
produced by N 0 photolysis was in the 1n state, as
2
.
17 28
assumed by Cvetanovic ' '
since the O* produced by
and N 0 photolysis were rapidly deactivated by co •
2
2
2
The excited oxygen atom produced from N 0 has since
2
18 19
1
been shown to be o( s) , ; therefore the mechanism
N0
suggested by Preston and Cvetanovic for N0
2
photolysis
is open to doubt.
(5) Photolysis of H2Q. Experiments by Stuhl
and Welge 29 performed at 1236X and 1470~ showed that
H29 dissociates into H2 and 0 as well as OH(x ~~) and
2
H( 8).. Formation of o(3P) is spin forbidden when the
H2 is in the stable singlet state; therefore 0 is formed
1
1
in the 8 or 1D state. The threshold for 8 formation
0
0
is 1300A whicbli in the case of 1236A photolysis, leaves
little extra energy for the transition to the 18 state,
..
and it therefore is assumed that o( 1D) is formed at
both wavelengths.
10
at 1470~ has been used by
2
• 2Lj
1
3
Young and Black · to produce 0( D) along with 0( P)
(6)
o2
Photolysis of 0
+ hv
-*'
o ( 1 D)
+
o (3p)
(31)
This is not a good source of o( 1D) because of the
quenching reaction
(31a)
) 30
The reaction with N 0. Katakis and·Tauhe
2
have shovm that photolysing a mixture of N2 0 and o at
(1)
3
2537~ produces acidic oxides of nitrogen.
Detailed
2
chemical analysis showed that N was: .also pX"oduced..
Al-
though nq quantitative data was obtained because of
qbsorption of light by nitrogen oxides, they showed that
75~b
of the
o3
had reacted vvith N20, and of this
produced acidic oxides, the balance forming N •
2
suggests
i
had
This
(32)
predominates over
(33)
11
Yamazaki and Cvetanovic 17,3i used N 0 photolysis
2
25
at 1849~ and Preston and Cvetan6vic
N0 photolysis at
2
0
2280A to observe the same reactions, but were unable to
come to any quantitative conclusions about the relative
1
l"ates of the two processes. That o( s) reacts in a
1
:
similar way to 0( D) is confirmed by Yang and Servedio
32
•
Reaction with co • Using o photolysis as a
2
3
1
30
source of o( ,D) Katalds and Taube
showed that rapid
(2)
isotopic exchange took place with
co 2 •
(34)
The reaction was assumed to occur through an
intermediate excited
They
co3
molecule.
soshowed that at high pressures a three
body reaction with C0
2
to give a stab
ized CO] molecule
is possible&
(35)
They proposed that the
co3
radical was removed by
the reaction
(36)
B~"lch and Brechenbridge 3 3 demonstrated the
1
oqcurrence of isotopic exchange between 0( D) and
.
co 2
12
using photolysis at 1470l\.
Warneck 23 ,34 noted that the gaseous products of
co 2 photolysis were deficient in o2 , and attributed this
to co formation. He also observed a slow increase in
3
the ratio [o ]/[co ] over a period of one hour after
2
2
photol3mis.
This loss of oxygen was shovm to be due to
a wall reaction b,.r Slanger 35 t · who f'ound it could be
·reduced b;v suitable treatement of the walls.
Slanger nor
[oJ /[ coJ
~a"lch
Neither
noted the increase in the ratio
l"eported by Warneck.
The reaction mechanism suggested by Slanger is
(37)
(38)
co3
+ walls
--.
C0
2
+ solid :products
(39)
(36)
with an approximate value~of k
molecule
-1
sec
-1
36
=4
~ 10- 1 3 cm-3
•
Clutten, Moll and Thompson36 carried out photolysis
of a
co 2
matrix at 77°K with 1470~ radiationand observed
a new infx•a-red absorption band.'which they attributed to
co •
3
Further irradiation at wavelengths between the
13
1
(f'or o( D) f'ormation) and
2
0
3650A caused the bands to disappear. Clerc; and
-13em 3molecules -1
= 2 x 10
Rei:Cf'steck 37 have deduced 1c
38
-1
sec •
dissociation limit of' co
7..8
Taylor~
has produced evidence that the oxygen
1
loss occurs through reaction of' o( n) with the walls of'
the glass reaction vessel, and has denied the formation
of stable
co3.
Yamazaki and Cvetanovic 17 showed that the
energetic fragment produced in the photolysis of' N 0 at
2
0
18Li.9A was deactivated by co • In a parallel experiment
2
18
Young and Black
have studied the quenching of' the
0
5577A line of'
1
s) by co • They obtained a rate of'
2
1
1
3
2.5 x 10- 4cm molecules- sec- 1 for the reaction.
0(
(40)
or
An interesting sidelight to this work arises from
recent studies of' the atmospheres of' Mars and Venus.
'fhe
co 2 atmospheres on these planets, as revealed both by
rocket probes and by infrared absorption spectroscopy,
should have long ago been dissociated to CO and 0
sunlight.
co
2
by
The reaction
3
+ co
_.,. 2co
2
(41)
14
gives a possible explanation for the absence of detectable CO and the persistence of 00
(3)
Reactions with CO.
o( 1D)
that of
co 2
with
2
on these planets.
An analogous reaction to
was proposed by Katakis and Taube3°
DeMore and Ha:per 39 reported that the major loss
1
of o( D) in the presence of CO was by deactivation.
Although spin-foi•bidden, electronic deactivation is
stated to .be
ficient because of the
possibilit~
of a
transfer to a triplet surface of the o( 3P) - CO interactions ..
trhe detailed mechanism is as follows
o +
3
hv
~ o2 + o( 1 D)
1
0( D) + CO
co 2 *
~
(4)
(41)
(L~2)
co 2 *". + m
CO
2
~H,'c
~
-+
co
co 2 * +
+
m
(43)
o( 3P)
(44)
03
( 11 )
'Z
o(.JP)
+ 02
~
Figure 2 illustrates the relevant approximate
potential energy curves for
co 2 •
15
In work by Verdurmon40 N o photolysis was used as
2
q source of excited oxyeen, and he proposed a similar
O*.
mechanism for deactivation of
recent papel''s on co
Bal'at
L~ 1
2
photolysis Clerc:: and
LL2
' • px•oposed
(45)
1
as the main renction between GO and 0( D), and measured
10
-·11
(l.J.)
photolysed
3 molecule -1 sec -1 •
em
The reaction ·with
o3
N ..
2
De .lMmre
0
(at 2537A) in liquid N ..
2
and Ra.per5
They found that
N 0 was formed during the reaction..
The principal fate
2
1
of o( D) was deactivation to the ground state, which
was 75 times faster than the formation of N 2 o.
0( 1D) + N2 (+ m)
~
1
0( D) + N2
+ o(>p)
N
2
o (+ m)
'7.
~
N
2
(46)
(L~7)
In contrast to the efficient quenching of o( 1 s)
reported by Young and Black they measured
1
k < 1o-17 cm 3 molecule- sec- 1 for
or
(48)
16
However, Snelling and Bair43 report
sec
=
(5)
Reaction with
o3 •
-1
•
The reaction
(8)
ns originally postulated by McGrath and N6rrish 3 , has
been generally accer>ted, although30 ' 4 4 a competitive
exchange reaction
(49)
is lmovm to occur at a slov1eP rate ..
(6)
Reaction with
o2 •
McGrath and Nox>rish3
suggested ozone fox>mation as the main process occux>cing
with moleculaP oxygen.
( 11 )
Recently mensur>ements by Young and Black 24
requix>ed the rate fop the reaction betw~en o( Dl) and o
2
11
1
3
to be greater than 10cm molecule sec- to explain
1
difficulties in detecting radiation from 0( 1 D)•
Young and Black have also shown that
(50)
is extremely efficient.
The
o2 ( 1 2.:)
was detected by ob-
sel"Vation of radiation from the process
(r.. "' 7618~)
(51)
Katakis and Taube 30 suggested that the·exchange
reactj.on
(52)
is about ~O as rapid as the exchange reaction with 03'
so reactions ( 11) and (50) would pr•edominate..
and Sullivan 45 have measured k
52
-1
-1
molecule
sec .
=4
Warneck
x 10- 15 cm3
React ions with H ,- CHl.j.' H 0, NH and HCl.
2
2
3
L~6
No1·rish and Wayne
photolysed ozone mixed vlith several
( 7)
hydrogen-containing substances.
A general react ion
mechanism was proposed:
(53)
(where R
= H, cn3
OH
NH
2
or
Cl.)
f'ollowed by
( 15)
18
It does, however, seem likely that other reactions
would occur·in this system, notably the two very fast
reactions
OH + 0
~
0
2
+ H
(54)
(55)
were reported by Kaufman and Kelso7, and Kurzius and
1
11
Boudant 47 found k
= (2.8 :to .. 6) x 1ocm 3 sec- •
54
The cases where R = H and CH have been s tudiied
3
by De More and RaperL~B, Li.9 using o photolysis as an
3
1
o( D) source in liquid nitrogen. They showed that H
2
and CH
react at approximately the same ratet and about
4
4 times slower than the 0( 1 D) '!-.0 reaction .. The
3
reactions were shovm to probably involve an intermediate
exci te.d H20* and CH 0H* formed by an insertion reaction.
3
1
0( D) + H 2
(H
~
20*)
~
H
+ OH
(55)
+m
·~
1
0( D) + CHL~
H20, + m
(56)
'''
~
(CH OH"•)
3
+m
~
CH 0H
3
+ m
~
OH
+
CH~
.)
(57)
(58)
As H 0*: ·has fewer degrees of freedom than cH oH'\C
2
3
19
it is expected to dissociate at a greater rate.
(8)
1
Reaction of o( D) with
photolysis of an
o3 -
0~
0~ 2 •
Flash
transient
2 mixture produced
[
absorption· spectra of ON and NCO..
0
Morrow 'and McGrath 5
postulated the occurrence of the reaction
(59)
and explained the production of vibrationally excited NO,
which vtas also present, by
(60)
( 61 )
They observed. a new absorption spectrum which was
attributed to the radical NCN, produced by:
ON + NCO
_..
NCN + do
(62)
Another possible reaction with cyanogen is
o( 1 D) + c 2N2 _., NO + C2N
(63)
would be about 20 k cal/mole exothermic and would also
probably produce vibrationally excited NO.
However, no
absorption features could be attributed to the
c 2N
radical, and reaction (1::/3) was therefore thought to be
unimportant.
Table I.,
1
1
Reaction of' 0( D) and o( s)
1
o 8
1
o n
ctant
Rate
Rate
Process
(cm3sec -1,)
Ref',.
0
.-:
02
0 +0!:: ( I D)
2
-+
o(1n) +O~'o
4x10- 15
45
1
;;:;:10-11
24
1
0 2 +0( D) ..... 0(
co 2
) +02( z)
1
1
CO +0"'< ( D) _, coo::~+o( D)
2
"'
1
C0 2+0( D)
->
;)
H2
1
H2 +0( D) ...., OH+H
N20
0( D)+N 20
1
-+
1
0( D)+N Q _,
2
''J
1'l2
2x1o- 13
CO~
Ref.'.
1
0( 8)+0( 3?)
-+
1
20( D)
1. 8x1 o-13
18
1
co 2 +o( s)
o( 3 P)+Co 2
2 • .JX
- /0-14
I
18
5·9xio--13
18
<10 -17
18
-+
37
1
0( S)+N 2 ) _, N2+02
21~0
0 ( I 8) +N 2) ..... 2NO
.,
)
0-.)
03+0( D) ..... 20
co
1
CO+O( D) ..... co
He
He+O( D)
I
2
2
He+O(
Ar
quenching
Xe
quenching
2.2x1o- 11
49
j.jxio- 10
7xio- 12
-1
-J.
(cm3sec- 1 )
N2 +0
2
1
N2+0( D) ...., N2 +0(
1
Process
)
/. ">
:J.y
45
10-11
40
1.5xio- 15
45
_.,
quenching
<10-17
18
quenching
< 10-17
18
quenching
< 10-17
18
~~
8.3xi0 ·5.8x1o- 11
g_uenching
,.._,.
~s
21
(9)
Reactions with Rare gases.
1
0( D) has been
found tore strongly quenched by Xe43 and weakly by He
and Ar43 ,45..
The first excited level of atomic lwdrogen
Also notable in the recent upsurge of interest in
excited atoms is the 2P level of hydrogen.. The energy
levels of atomic h.yd1·ogen are shovm in figure 3·
The
absorption of solar Lyman-a (1215.7X) by ground state
lzydrogen atoms in the upper atmosphere was demonstrated
1
by Purcell and 'rousey5 .. T11is absorption of Lyman-a by
atmospheric hydrogen indicates the production of H( 2P)
~n
the atmosphere, and suggests that its reactions may
be significant in the airglow, especially the dayglow.
Laboratory studies relating to H(~) and its
reactions are reported in a few recent papers by Tanaka
., 2
and McNesb.y 5 and Koyano and Tanaka53, The main reason
for so little worl( having been done in this field is the
difficulty,encountered in optically exciting atomic hydrogen. Vanier 54 states that self-reversal of the
Lyman-a line 1'rom a typical discharge lamp causes it to
have as much as ten times the normal Doppler width.
This
makes it difficult to excite hydrogen at low pressure ..
~--~==~~~~=l~l~l~~~n;= 4
---:-+-t--1---:~ n
=3
Ptlschen
1
5·
n
=2
8 almer
>
n ::: 1
15
Fig .. 3
Lyman series
J£nergy levels of' atomic hyd.Pogen.
22
He recommended broadening of the absorption line of atomic
H with an electric field applied to the absorption cell..
Tanaka and McNesby 52 overcame the problem by developing
a lamp which emitted a sufficiently narrow line to excite·
a reasonable percentage of the atomic hydrogen present,
and by using a relatively high pressure in the reaction
cell.
The excitation process is
H( 2s) + hv
~ H( 2P)
(64)
( 121 s. -r~)
The radiative lifetime of H( 2P) is 1.2 >< 10- 8 sees while
the time between collisions of H( 2P) and a reactant gas
is several orders of magnitude longer than this.
Tanaka
et al assumed imprisonment of Lyman d. radiation within
the reaction cell in order to explain the observed
efficienpy of reactions of H( 2p) •
2
As any reaction of H( P) must take plape in a
Lyman-a, beam the reactant products must not absorb too
strongly at wavelengths emitted by the lamp.
Also, in
2
order for the J)I'Oducts of the reaction with H( P) to be
detectable any reactant gas must react slowly with the
more abundant ground state hydrogen atoms.
23
(1)
Reaction with N • Tanaka suggested two
2
:processes to explain the production of NH observed.
3
2
(a) Energy transfer: H( P) atoms have 10.02eV energy
above ground state and are therefore capable of rupturing the N-N bond (dissociation energy of N 9.76eV)
2
2
1
H( P) + N ( zg+)
2
~ H( 2s) + N2*
(65)
2N( 4s)
(b) Atom transfer mechanism:
(66)
The 37!:
-
3z emission band
of NH was detected in the reaction of H( 2P) with N ,
2
which suggests the most Pl'Obable reaction to be
(67)
Tanaka and McNcshy thought it likely that ammonia
is formed either by reactions of NH with H or H, or that
2
interaction;of N with His involved. The interaction
betweenN and H is discussed by Mannella55 and Philli:ps 56 ;
.it is thought to be very slOvY unless a third body is
involved.
Further work by I\ayano and Tanaka 53 gave addi tional evidence for the reaction of H( 2P) with N • Using
2
a deuterium lamp they showed that
H + N + D Lyman a.
2
~
no products
(68)
24
comfirming the importance of H( 2P) in the reaction,;
(2) rl'he reaction with 0 : . Tanaka 57. has proposed
2
2
that H( P) is capable of participation in a chemionization
reaction with oxygen
(69)
25
C HA P T E R
I I
Quenching of Resonant Radiation in an Atomic Vapour
The equation of Stern and Volmer 58 correctly
describes many examples of fluorescence quenching in the
gas phase or in solution.
The processes 'considered in
deriving the Stern-Volmer equation are:
Rate·
Absorption
A + hv
Fluol"'escence
A*
A* + Q
Quenching
~
All.'
robs
~
A + hv
11f [A>:c]
~
Products
k
Q
[Al,'(J [·Q]
.
Here A and A* represent ground-state and excited
fluorescent molecules Pespectively, I b
is the rate of
s
absorption of light quanta, and Q represents a quencher
0
molecule.
When a stationary state has been achieved
the concentration of excited species is constant
i.e.
=
0
=
I
obs
- lc [A*] - kQ [At.'<] [Q]
11'
The fluorescence emitted in the stationary state
is pl"'opoPtional to the concentration of the excited
species [A>:<] so that if F is the intensity of the
0
fluorescence emitted in the absence of the quencher and
26
F the f'luorescence in the presence of' the qu,encher then
Fo
F
1 + KQ [ Q ]
=
( i)
k
where KQ (= ~)
is the experimental quenching constant.
The mean lif'etime of' the excited species is given by
=
1
(ii)
11_;.
in the absence of a quencher ..
F
So
_Q
F
=
The measurements of Norrish and Smith 59 on quenching of' resonant radiation of sodium vapour by various
~dded
gases illustrate the applicability of' equation
(i)
to the quenching of' f'luorescence in the gas phase.
The ef'f'ective quenching cross-section is a convenient quantitative measure of the quenching ef'ficiency
of a gas.
It is def'ined as f'ollows.
collisions sec
-1
em
The nUDlber of
-3atom -1 is, according to kinetic
theory, given by the expression:
=
=
(iii)
27
where n is the number of molecules of quenching gas per
c.c., m the atomic weight of the vavour, and m the
1
2
molecular weight of' the quenching gas; a is the maximum
distance between centres :Cor which collision occurs.
The value of a
2
which must be used in this relation to
give the number o:f collisions per excited atom per c.c.
required to account :for the observed quenching, assuming
every collision to be ei'fective, is called the effective
cl~oss-sect
ion.
FOl.' the SteJ•n-Volmer equation ( i) to hold in gases
two conditions must be fulfilled.
(1) The vapour px•essure of the absorbing atoms
must be so low only primary resonance radiation is
emitted.
( 2) ~rhe effect of Lorentz broadening on the absorption line is negligible.
Iffiprisonment of Radiation:
A resonant quantum emitted by an individual atom
can be captured by another atom before it reaches the
walls of the enclosure.
This excites the new atom to the
same excited state as the original atom.
'
Such a transfer
of energy may occur many times before the photon reaches
28
the walls of the vessel, and radiation is said to be
imprisoned.
such secondary radiation is emitted by
the atomic vapour the effect of radiation imprisonment
on the lif'etime of quanta in the varJour 'must ·be taken
into account vthen a:pplying the Stern-Volmer equation.
garly theoretical treatments of imprisonment
60
.
61 .
62
were developed by Compton
and M1lne • Kenty
was
first to take into account the shape of an atomic line
in his treatment of the problem.
Holstein 63, 64 derived an expression for the
average number of' times a resonant photon is recaptured
in an enclosure containing atomic vapour • . His
derivation is discussed in A.Qpendix I, as is the later
work of Walsh 65 •
Holsteints work gave the following expressions:
( 1) For a Doppler broadened line and where the
gas forms an· infinite slab of' thiclmess L
g
=
(iv)
where g is the reciprocal of the number of emissions and
absorptions of an individual unit of excitation before
it escapes from the enclosure.
(2) For Doppler broadening in an infinite cylinder
of
s of radius R
29
g
=
(v)
k R
0
(3) For a pressure broadened line in an infinite
slab
g
=
1
1 • 1 50 ( ?C kpL) -~
(L~) l~'or
a
pressu1~e
(vi)
broadened line in an infinite
cylinder of gas
g
=
(vii)
The use of Holstein's theory and later Walsh's
involves a number of complications.
The first is related
to the shape of the enclosure which is nevel" the same as ·
the idealized systems described above.
Second, there can be complications caused by
byperfine structure of the spectral line..
of experiment and theory made to date
Comparisons
inv~lve
measure-
ments of the 2537~ line of mercury which has five hyperfine components.
This problem is discussed briefly in
Appendix I.
A plot of experimental and theoretical values of
.
0
im1·n•isonment lifetimes of the 2537A line is sho\m in
figu1,e Lt..
These were plotted by Walsh using experimen-
tal data from Alpert et a1 66 •
The dotted line (fig. 4)
/
/
/
-
calculated
experimental
10
Density
li'ig.
(atoms/ c. c.)
4 T:eapping of 2537 :A radiation in He; vapour.
gives the calculated curve
resonant line~
transition
o~
a Doppler broadened
2
2
The theory is applied to the P - s
~or
atomic hydrogen in Chapters V and VI.
31
C HA P T E R
I I I
Experimental
Part 1.
To study
o( 1 n)
1
To obserYe the reactions of 0(
i
D), o
3
.
was photo-
lysed in a static system :in the presence of the other
reactant species.
The vacuum line and reaction cell
which were used ar•e shown in figures 5, 6 and 7.
Ozone was prepared in the section of' the vacuum
line shown in figure 5..
over KOH ~nd P
o
Commercial tank 0
was dried
2
, before passing through the needle .
2 5
valve ll!_ The gas flowed through a dry ice trap
followed by the ozoniser, o67 • The latter• consisted
of an inner tube filled with aqueous copper sulphate as
one electrode and a second electrode of copper foil
wrapped around the outside.
The oxygen passed between
these electrodes between which was applied a 6kV a.c.
potential from a 12 kV neon lighting transformer with a
variac in the primary circuit.
The ozone/oxygen mix-
ture was collected in a liquid nitrogen tx•ap, T.
This
trap was surrounded by a length of six inch diameter
N
H, T. ac
to re-actior
Vesse-l
'---.....1..---;...
T
Fig. 5
N
2
o
and
from
Appm~atus
co
for> Pl"oduction of'
o •
3
2
dryer
N
'II
to
to pump
pressure~---i
to reaction
gouge
vessel
s 14
Socket
to sample
bulb
Fiz. 6
Vacuum line for 0 ( 1 D) study.
to rotary
pump
1
Atomic
1 i-ltcr
aird
---!--microwave
cavity
Cold
finger
Fig.
7 Reaction vessel
and lamp for 0 ( 1 D) study.
3'2
steel pipe as a precaution against explosion; no explosions wePe experienced during this wor•lc.
Unconverted
oxygen was pumped off by a Welch model ·1402 two stage
rotaPy pump.
The solid
0~:
.:;
remaining was then evaporated
into a 10 litre stor•age bulb B.
Initially a column
containing soda lime was placed between the ozoniser Q
and the trap ! to remove any oxides of nitrogen.
This
technique was unsuccessful as it removed ozone as well
as oxides ..
Medical grade nitrous oxide was dried over KOH
and P2o and stored at atmospheric pressure in a five
5
litre bulb (B in figure 6).
1
Solid co was evaporated into bulb B which had
2
2
previously been evacuated.. The co was then trapped in
2
a liquid air tr-ap and any residue of air pumped of:t\, Care
was taken to ensure that water- vapour did not enter the
bulb by leaving some solid co
in the trap. Later sam2
ples of co 2 were obtained from an Industrial Gases
cylindel" and dried in a similar manner to N 0.
2
The reaction vessel R shown in figure 7 was
filled with
o3
to a pressure of about 1 torr and between
1 and 20 torr of the reactant gas was added.
The
pressure in the reaction vessel was measured at first
by using a mercury manometer; and later- with a Texas
3.3
Instruments quartz spiral gauge..
All mercury in the
system was isolated from the reaction vessel by liquid
air or dry ice traps to prevent photosensitisation of
the r-eaction.
'rhe mixtures were photolysed f'or a pePiod
of' between 1 and 3 hours.
The Lamp:
Dif'f'iculty in obtaining a high-inteno
sity source of' 2537A light made it necessary to develop
a
low pressur•e electrodeless mer-cury lamp.
This lamp,
which is shovn in f'igure 7, consists of a cold finger
~~
which holds a small amount of mercury, a quartz seo.tion
r·
1'
"
to hold the microwave discha;r:ge, and a quartz window
.
The dischar•ge was excited by a Micr-otrnn 200 watt
microwave generator in conjunction with an E.M.S •. 213L
resonant cavity
~·
Initially a drop of' triply-distilled mercury was
plac9d in the cold finger, then the lamp was evacuated
and degassed at 10- 6 torr.
The lamp was then f'illed
with welding grade argon to a pressure of 200 microns.
(This pressure was chosen after studying the characteristics of the discharge at various argon pressures).
Once the lamp was seen to be oper·a ting properly it was
'sealed of'f.
The lamp spectrum was observed using the Hilger
medium c1uart z spectr•ometer vvi th E · 720 scanning unit and
a 1P28 photomultiplier.
The relative intensities of' the
three main lines in the spectrum at
0
L~363A,
0
3130A and
0
2536A were obscr•ved at varying temperatures of the cold
fl.nger (0°c - 50°c).
'fhe ratios
r2537.A!r 31302_
and
i/I
i decreased with increasing temperature
2537
4363
while the overall intensity of the 2537Rline increased
I
with increasing temperature.
:figures 8 and 9·
These results are shown in
The best results were obtained (having
high intensity and lower relative intensit
s of' the
other two lines) when the cold :finger temperature was
19 - 20°C.
'l'he lamp was therefore opera ted with the
cold finger at room temperature.
A 2660~ Baird Atomic interference filter F
eff'ectively- isolated
the 2537~ line, although some
3010R radiation did pass the filter (2.% of the unfiltered intensity).
'rhe filte'l" also reduced the inte1).sity of'
of' the 25372 line to about 30% of' the unfiltered value.
I
The lamp filter· combination was used fop photolysing o •
3
PPoducts of yhotolysis were analysed using the
two mass spectrometers.
~he
used by Phillips and Schiff
68
:first was similar to that
in Montreal.
It was a
""
first-order, dir•ection focusing 'instrument having a 90°
six-inch radius magnetic analyser.
described in detail elsewhere 6 9.
This instrument is
1 ·9
t/j
0
1 •7
.....
0
0
0::
0
(2537
A )
(3010
A. )
@
,. -
1·5
<2537A)
C43soA:)
1. 3
0
2 0
1 0
3 0
4 0
Temperature
0
10
2 0
3 0
( °C
4 0
Temperature
li'i~s.
8
&
9
Properties of the Hg lamp.
(°C)
35
The second instrument was similar to the first
except that it had a 6 inch radius Rola 4005 gauss permanent magnet.
The mass
ctrum was observed by
scanning the accelerating voltage.
The mass range of
the instrument was 12 to 90 atomic mass units and it
could not satisfactorily be used for a lower mass range
because it was impossible to accelerate low mass ions
to a high enough velocity to pa'ss through the analyser ..
In
later.
eXJ?ePiments the reaction vessel
was attached to the mass spectrometer samgling leak
which permitted continuous mass spectrometric analysis
during photolysis..
Samgling leaks v1ere pr>epared by
sparking a hole 20 microns in diameter in the end of a
pyrex thimble, the walls of which were blown to a
thickness between 20 and 50 microns.
Results and dis-
cussion of this work are given in Chapter IV.
Part 2.
To study H( 2P)
The f'low system used for studying react ions of
2
H( P) is shown in f'igure 10.
Gases entered the system
at atmospheric pressure, the fl?ws being controlled by
the needle valves li and monitored with flow meters E.·
The argon and hydrogen flows were ]nixed upstream of the
.Q9v
c9v
F
F
F
I
reactant
Ar
0
0
;--
"""'
D
L
.v
ga ugc
l
I
~
I
I
....___
2
'l'he f'low system for H (. P) study.,
to
pump
J
Fig.10
to pres sure
.36
microwave dischar>ge
:g.
This discharge was placed so
that noneof its light would enter• the spectrometer .§..
The discharged llydrogen was then mixed with the reactant
vvhich entered tln•ough jet !l,., and flowed between the win-
l·
dovrs of the Lyman-a, lamp L and the ion chamber
The
spectrometer § was positioned to observe the gases as
they passed through the Lyman-a. beam.
Stopcocks in the
system wePe lubricated with row-Coi-ning high vacuum
silicone grease.
Reaction vessels used in conjunction with this
flow 1 ine are shown in fieures 11, 12, 1.3 and
j
4.
The vessel shown in figuPe 11 was used to take
preliminary observations of luminescence in the near
ultraviolet and visible Vli·th the Hilger medium quartz
spectrometer, model E498, and later with the JarrellAsh model 8411
i
metre Ebert spectrometer.
FoP uce with the McPherson vacuum spectrometer a
second reaction vessel, shown in figure 12, was made.
With this vessel it was convenient to monitor the output of the lamp using the ion chamber
I·
The Peaction
took place along the length of the reaction vessel and
was obse:r•ved by the spectrometeP about half v'lay along.
r.l'he intensity of the chemiluminescence v1as vepy low
under these conditions, and it was necessari to develop
a third reaction vessel shown in figure 1,3.
Ar
•
H
2
react ant
ED 150 rotary__..
l
quartz window
Pig. 11
Pirst reaction vessel for study of H ( 2P) •
reactant
w
TO pump
Pig. 12 Second reaction vessel
~or
2
stu4Y o£ H ( P).
H +H 2 and Ar
,I
XBLG66•3(lll
Pia. 13 Third reaction vessel for study of H ( 2P).
L
to pressure
W
Gl'
.l.'l/J.
14
Renction ve~sel: stuQying quenching
of H ( P) · fluoresence.
gauge
pump
~
37
This conical shaped vessel prevented reflections
of light from the lamp off its wall from entering the
spectrometer, and allowed the spectrometer to view a
laroge volume of roeaction.
The vessel was used to ob-
tain the chemiluminescent spectroa discussed in Chapter
v.
Figure 1Lj. shows a reaction vessel :designed f'or
· the study of the quenching of Lyman-of f'luorescence.
The optical path length
of~
the reaction vessel was
determined larogely by the need to minimize the trapping
_0
of 12·16A light on the way to the spectrometer..
As it
was necessary to keep the pressure relatively high to
give sufficient intensity of fluorescence, it was irnpossible to eliminate the effects of light trapping ..
(This is discussed in detail in later chapteros)
4
Press.ure, Flow Control and Measurement
Flow rates were measuroed by the throee meter•s E
in figure 10.
capillary type.
These were of the simple interchangeable
The manometPic fluid used was mercuroy
with a Di - N - Butyl phthalate covering to exclude
mercury vapour from the system.'
'l,he flow-meters were
calibrated by measuring the time taken
fol~
a soap
38
bubble to t1•aver•se a 100 ml burette.
Meters wer•e
calibrated. for their respective gases to within :t:.2tC:.
The total pressure in the reaction vessels and
the partial pressures of gases were monitored with
either a tilting hlcLeod gauge or the Texas Instruments
QUar>t z spiral gauge.
To carry out the correction for light trapping
· it vtas necessary to lmow the partial pressure of atomic
hydr>ogen.
This was determined using the isothermal
calorimeter probe 70 • The circuit used with the iso-
thermal calorimeter is shown in figure 15.
Two
precision arms of' the bridge, R and R2 were standard
1
1 ohm and 1000 ohm resistances respectively (these were
kindly lent by the Physics Department).
The variable
resistance box R was a 0-10,000 ohm manganin precision
resistance box.
The detector D was mounted on a standard B14
gPound j?int as shown in figure 16.
The f'ilament .E
was of 0.002 inch diarnetei' platinum, and was about 5cm
·in length.
leads.
It was spot welded to the nickel Tungsten
The nickel section of the leads were shielded
from contact with the reaction gases by soft glass
skirts S, which reached down to the glass seals arou:hd
the leads.
The detector" f'ormed the fourth arm of the
D
Fig. 1 5
R
?>ridge
circuit for the
isothermal
o -1o,o oo.n.
Fig. 1 6
calorimeter.
Pt detector fol"
·.,
for the isothePmal calorimeter•
814
g I ass j oint
39
wheatstone bridge.
The bridge balance indicator Q was a Tinsley type
SS1-5269A
mj.1~ror
galvanometer.
'rhe cux•rent in the probe
at balance point was measured using a Solartron Lm902.2
digital voltmeter (input impedance 1m g) connected in
parallel VIi th R.
The flow rate of atomic hydrogen was calculated
as follows.
r.rhen H atoms are present the power
dissipated in the coil is
(i)
where IH is the cur1,ent f'lovdng.
RF is the resistance
o:t' the coil, and w is the heat liberated by recombining
hydrogen a toms ( 1 OLj.• 2 k cal/rnole).
The power
dissipated with H atoms absent is
p
=
0
I
0
2~i1
(ii)
Now if the temperature in the coil is constant
the value of' RF is constant, and if the current is
adjusted to keep the bridge in balance the power
dissipated in the coil is equal in both cases.
Therefoi•e w = (I o 2 - I H 2 ) RF
Now
=
and
::::
(iii)
40
± 0 .. 005 ohms and E0 , EH are measured
where R ::: 1. 000
8
across R •
8
So
(u
RF
:::;
(T!l
R 2 . 0
2
2
- EH )
8
FoP the recombination of' hydrogen atoms
sed
v:he:r•e f'
LiH
:::;
H
:::;
H atoms flO VI rate in !.1. moles sec
1 OL1 .. 2 k cals mole
'fhis gives fH
-1
= 4.586
-·!
-1
and
.
(E
2
0
- E
2
H
) !J. moles sec
-1
The efficiency of' the isothermal calorimeter vss
checked by measuring the dissociation produced in an
Argon mixture with 0.75% hydrogen by a
charge.
crowave dis-
100~~ d ir;socia tion was assumed in this mixture ..
Knowing the flow rate of' moleculaP hydrogen it is possible to calculate the percentage dissociation, and f'rom the
partial pressux>e of' molecular> hydr>ogen in the reaction vessel
one can then obtain the pax>tial pr>essure of' atomic hydr>ogen ..
'fhe measurements were taken with different lengths of'
filament in the probe.
The same values of p
H
were obtained
on both occasions.
1.
Hydr-ogen from a commercial cylinder was
purH'ied over hot copper and then dried through liquid
41
air t:r:'aps ..
2.
Welding grade argon was also purified over hot
copper and then dried through dry ice/acetone traps.
7.
.J•
Reactants: (a) Nitrogen of Matheson pre-
purified grade, Oxygen of Matheson ultra high purity,
and industrial erade carbon monoxide and carbon dioxide
were dried at liquid air or dry ice temperatures.
(b) Dental grhde N 0 was dried
2
through a dl"Y ice/acetone tra11.
(c) Matheson NO was purified by
trap to trap distillation and stored in a 10 litre glass
sto1~age
bulb at atmospheric pressure.
(d) British Drug Houses liquid so 2
was evapopated and used without further purif'ication.
A lamp similar to that described by Tanalm 52 and
co-workers was used initially; it is s·hown in :Cigure 13
as~I:_.
The water-cooled lamp has electrodes of pure
aluminium and a. lithium fluoride window
Y!..
which was
sealed to the lamp by means of Apiezen W wax.
The lamp
was filled to a pressure of 3r4 torr with pure neon,
and hydPogen was added until its partial pressure vvas
42
between 10 and 20)4
The power source for the lamp was a
3.5 kilovolt transformer (1.5 kva).
The transformer
was run, through a variac, :from the 50 Herz mains supply,
so the output of the lamp was modulated at 100 herz.
The electrodes deteriorated with use and it was necessary to replace them every two months.
cleaned with hydrofluoric acid, then
The lamp was
~insed
with dis-
tilled water and ether before it was degassed and filled.
A second, more intense, source of Lyman4X
shovm in figure 14.
is
This lamp which was developed to
obtain quenching measurements, wus powered by a Raytheon
P.G.M. 100 micpowave generator.
25~~
A mixture of He with
hydrogen was made up and the lamp was filled to a
total pressure of 200 microns.
Modulation of the out-
put at 100 Hz was readily obtained by switching out the
filteP in the microwave unit.
Pilling procedure:
A similar filling procedure
was used foP each lamp. The lamp was pumped out and
degassed at 10- 6 torr by the tPapped two stage mePcury
diffusion pump f. in figure 17.
Hydrogen from a cylinder
entered bulb B, through a porous disc in a i'unnel .E,, in-.
verted in a dish of mercury, and was stored at atmospheric pPessure.
The hydrogen was then purified by
passing it through the palladium thimble
!
into a second
T
T
To ro tar
v
pump
p
h------'---0
G
The pumping
and filling system for
the Lyman-a. lamps.
M
L
43
storag,e
B2 ..
A third bulb B , containing spectroscopically pure
b~lb
3
neon (supplied by the British Oxygen company) was sealed
to the system.
seal to B
3
A magnetic capsule was used to break the
and allow the neon into the vacuum line.
Helium :for tlle electrodeless lamp was taken :from
the Matheson high purity tank and stored in a bulb in a
similar manner to the hydrogen.
A liquid air tr?:p v1as
used to condense out any water vapour in the helium.
To :t'ill the lamp, the required pressure of' hydrogen
was let through the volume y and measured using the
calibrated McLeod gauge. M. (an intervening liquid air trap
kept mercur•y vapour :from entering the lamp)..
Then the
required pressure ·Of' Neon or Helium was added and the
greaseless stopcock
closed ..
The Lyman-~detector 71 :
The ion current produced by
0
photoionization of' carbon disulphide by 1216A light was
used to monitor both the lamp output and the absorption
by atomic hydrogen.
The electrodes weT•e sealed in the
glass envelope I shovm in :figure 13..
A.R. gr>ade cs
2
was degassed and purified by trap-to-trap distillation
bef'ore the chamber was :filled with v agour to a pressure
of' 10 torr.
This pressure was obtained using a slush
44
bath of Chloi•obenzene at -Lj.2. 5°0.
of
cs 2 is 10 torr at -L~L~. 7°C).
window
YY.
(The vapour pressure
The li thiurn fluoride
was sealed to the chamber with Apiezon W wax.
The spectral response of this chamber is des1
cribed by Carver and Mitchell7 •
Windows:
Ultraviolet q_uali t;}' Harshaw lithium fluoride.
windows were used for• the lamps and ion chambers.
For
the reaction vessel shown in figure 11 a q_uartz windovv
was used for the observation position; in all other cells
a LiF window was used.
The transmission cut-off for LiF is givpp by the
manufacturer as 1 040JL
The Lyman-!)( line f'req_uency falls
in the fundamental bands of Lithium fluoride crystals and
exposure to 1 216lt r•adia t ion has been shown to cause rapid
2
for1nat ion of colour centr•es 7 ' 73 • These centres can be
removed by annealing the windows for• twenty-four hours
at 500°C.
This procedure proved necessary after a win-
dov/ had been used for .30 minutes in the lamp..
Annealing
restored the transmission, but repeated use affected the
surface, as reported by Gilman and Johnstone7.3.
1..
The Hilger medium q_uartz spectrograph.
4B
Emission
~rom
the reaction was recorded photo-
gr•aphically with Ilfor-d ID?3 and long Pange spectrun;
plates, or• by an R. C.A. IP28 photomul tiplier• mounted in
.a HJ.lger E720 scanning unit.
The signal from the photo-
mul tiglier was tal<:en to the in};ut of a 1OOH.z tuned
amglifier and phase sensitive detector.
The reference
signal for the phase sensitive detector was.derived from
the 50 Hz mains.
2.
The Jarroell Ash half-metroe Ebert spectrometer
was fitted with a 1180 lines per m.m. grating blazed at
5000)\.
The detector here was also an R.C.A. IP28 photo-
multiplier, used in conjunction with the same amplifying
system as that described for the Hilger spectrograph.
3.
The McPherson spectrometer fitted with a
·1200 lines per m. m. grating blazed at 5000~ was used ~or
monitoring visible and near ultraviolet radiation.
block diagram
o~
A
the electronics associated with the
McPherson is shown in fit:,rure 18.
The output of the
E.r~i.I.
951LJ.SA photomultiplier situated at the exit slit of the
spectrometer was taken to a 100Hz tuned amplifier and
phase sensitive detector.
· emission was
measur~d
The ·intensity of unrnodula ted
by switching a
into the circuit at the input of the
field-ef~ect
amplifier~
chopper
A
50 c/
s mains
/1.....-).--_ -_-_-_---J~---r--)-~~
1
~-35v
}JF
rll-----,
supply
],_
power
·supply
I
r:::/
a~~~~~er
l:
r--1/t-------o"
1
-·:_-::·.·:: E
I I
'I
I
I
.- --, -
:)514
I--<) c.'-_·_·-·---·
- - - - l- - - - I Phase sens1 t. r.-1 ----co ( i )
A
:~·
I
,
-
+
detector
'-----r-----1
I
I ' - - 1- - -1
' I
~
Variable
circuit
delay.
'.
.
¥
-
-,
I
_----------------~chopper
· ·-- · - - · - - - - - - - ·r c i r cui t
L- -
-r -
lamp
I
I
I
- 'J
T
B
( i )
to chort'recorder
Scope
A
test
points
VVLM
nJLJI
c f\.f\_;
a
Fig .. 1 8
.,
Block diagrom of circuits used \'lith the
McPherson spectrometer.
variable delay circuit on the input to the chopper and
phose-SGnsitive detector made it possible to adjust the
chopping signal to bring it precisely into phase with
the lamp.
Oscilloscope test points (a), (b) and (c) were
used to check the phase and waveform of the signal at
three points in the circuit.
The amplifier bandwidth and
gain were adjustable and the voltage supplied to the
photomultiplier could be varied between 1000 and 2200
volts.
'rhe output of the phase-sensitive detector was
taken to a Varian G14 chart recorder through an RC
smoothing network of time constant 0.7 sec.
The variation of the spectrometer and detector
sensitivity with wavelength was determined from the response to a tungsten strip filament lamp of known colour
temperature, as described by Gaydon74.
The emission of
the lamp was calculated with the aid of Wein's Law·
which may be expressed as
,
2EA. A G A.
-5
°2
exp (- A.T ) d A.
0
for values of A.T < 0.5 em K. · IA. is the intensity of
emission in ergs
c~
-1
-1
ster , EA. is the emissivity of
tungsten at wavelength t.. 7 5 and A is the 31-.ea of the in-
47
2
2
em sec
candescent surface in em •
and
c2
is 1 .. 438 cm°K.
centimetres.
-1
'rhe wavelength 1\ is expressed in
'rhe sensitivity cur·ve for the McPherson
spectrometer and the photomultiplieP is shown in Figure 19.
Fon the quenching experiments in the vacuum
ultra-violet the J1lcPherson spectrometer was f'i t ted v1i th ,
.!
_,
0
a 2400 lines ,m.m. grating blazed at 1500A.
A sodium
salicylate coated perspex disc was placed in front of'
the photomultiplier.
The coating of sodium salicylate was obtained
by spraying a saturated solution of sodium salicylate
dissolved in methyl alcohol onto the perspex disc.
A
heat gun was used to facilitate r.apid evaporation of' the
alcohol, soas to give a fine cr>ystalline layer of thick2
ness 1 mg cm- , as recommended by Seya and Masuda 76 •
'ro avoid the agemg effect , reported by Samson 77 the disc
was recoated after two to three months of use.
'
Since
the McPherson spectrometer was evacuated with a Vacian
purnp, so that oil contamination was minimized, this may
well have been unnecessary.
The spectrometer vtas evacuated for vacuum
ultra-violet woi'k and maintained at about 10- 5 torr with
a Varian model 911500 8 litre sec- 1 vacion pump.
I
)
!
i
'
I
I
l
j
'
I
I
(..
rj)
l
l
'())
l
0
I
3
...J
I
I'
!
I
i
l
; 1
I
2
3000
4000
50 0 0
oOOO
Wave-length
Pie. 1 9
Sensitivity curve for tho
and tho
~cPhorson
~hotomultiplier.
spectrometer
=
A
48
Experimental conditions and procedure.
The chemilumineseent spectra discussed in Chapter
V were obtained using about 2 torr of' Argon mixed vtit h
1.0 torr of' H ; ca. 0.10 torr of' the reactant gas
2
entered the react ion vessel through !I·
This mixtur•e
was irradiated by the lamp and the spectrometer scanned
:f'rom 2500Ji to 65001( with the slits wide open and the
photomultiplier on maximum gain.
To distinguish betv1een
primary and secondary luminescence scans were made
with and without the
eld e:f'fect chopper activated. As
2
the lamp was modulated at 100 Hz, it produced H( P) in
pulses.
If the pPoducts of the reaction o:t' H(~) so
formed wore in short-lived states they would luminesce
at 100 Hz and be detected by the phase-sensitive detector.
Runs wer•e repeated with the microwave discharge
off' and with zero flow of the reactant ~as.
Features
dependent on the presence of' hydrogen at.oms and on the
presence of the reactant gas were observed a second
time in more detail.
Quenching measurements were taken with the
following approximate partial pPessures.
l'\r
Par= 1.5 torr;
= O. 3 torr; PH = 0.1 torr and PQ = 0 - o. 5 torr.
2
With the photomultipl r voltage high and the slits of
49
the spectrometer• open to 2 mm the 1216)( line was scanned
with PQ :::: 0, and then with a known value o:L PQ.
ratio of scattered Lyman-« at zero P
Q
The
to that at :finite
PQ was recorded and the process repeated about four
times for each reactant.
Stel~n-Volmer
obtained by least squares analysis.
given in Chapter VI.
plots we:r.,e
These results are
After each day's measurements the
window in the lamp was replaced and the process reJ)eated
fop a new reactant when the lamp had been degassed and
refilled.
50
1.
Reaction with N20.
Mass spectra of N 20, o , N 20 t o mixture, and
3
3
irradiated N o + o mixture are shovm in Table II.
2
3
Peak intensities are given relative to the mass 30 peak
Pressures involved in each case were 3 to 4
of N 0.
2
torr for o
3
Table II..
Mass
and about 20 torr for N o.
2
Mass spectra for the reaction of o( 1 n) + N 0.
2
Ion
+hv
Gx•ound
No.
14
N+
16
o+
18
H o+(impurity)
2
28
N+
<. 01
1 .. 0
2
2.17
0.03
1.6
0.59
0.78
0.1
0.65
o.L~7
1.22
2.94
0.62
0.37
53·3
NO+
1
0 +
2
2. 4Lj.
N o+,
2
Lj.B
Back-
co
2
+
1
4.76
1
34·3
1
26.8
4. 7
11.3
3·34
NO +
2
0 .. 12
0.02
0 +
0.06
0.01
3
20~4
1
51
The concentrations of N 0 and o are both
2
3
reduced by photolysis. The concentration of N0 in2
creased showing production of NO during reaction. This
Discussion:
means the mass 30 peak would also increase, therefore
there is probably an increase in masses 28 and 32
showing that N and 0 are products of photolysis.
2
2
Hence reactions 32 and 3.3 (:; compete as reported by
Preston and Cvetanovic 25 •
The problem of absor•ption of 25.37Jl light by the
acidic oxides of ni tr•ogen could be overcome by using a
small concentration of
o ,
so reducing the amount of
3
oxide product 8nd with it the Jlroblem of absorption.
However, problems with the mass spectrometer in 1966
halted atte;npts to. measure the ratio k /k
(page 10),
32 33
2
and the project on H( P) discussed in later chapters
1
proved so interesting that 0( D) vvork was discontinued •
• The reaction ·with co • An attempt to observe co by
2
3
mass spectrometric methods failed. It was hoped to
notice a change in mass 60
co 2
and
o3
CO +
3
while a mixture of
was phqtolysed with 2537R radiation.
This
was carried out while.the cell was connected to the mass
spectrometer through the leak described in Chapter III.
I believe that co
was not detected in this ma1mer
3
because it disappeared by a wall reaction before it was
52
able to be ionized in the ion chamber of the mass spectr·ometer.
mass
If' the walls of the r>eaction vessel and the
spect1~ometer
wer>e tr>eated in the manner used by
Slangcr 35 it might be lJOssible to detect co
3
by this
method ..
1
The method used here to study 0( D) proved unfruitful but it is possible that the kinetics of reactions involving this f:Wecies could be studied using a mass
spectrometer and a flow system similar to that used by
Phillips and Schif'f 78 to f'ollow the concentrations of
reactants with time.
A photolysis cell upstream of' the
x•eactant jet would overcome the problem of absorption o:f
light by pPoducts, and
o( 1 n),
having a lifetime of 110 sec,
would 1 i ve long enough to f'low dovm to· a region where it
would react between the jet and the mass spectrometer
sampling leaks.
This
metho~
could be used to obtain the reaction
rates of o( D) vvith such reagents as co , H , N ~, Vihich
2
2
have already been studied, and with new reagents such as
1
2
53
C HAP T E R__y
Chemiluminescence from Reactions of H( 2P)
During the experiments outlined in Chapter III
the majority of emission bands observed were identified
as long wavelength emissions from the lamp (Table III).
Ideally to prevent the r-esults being complicated by
photolysis of the reactant species the reactant used
must have low abs~rption at 1216~.
From Table IV,
which gives the extinctton coefficients, e, at 1216~
f'or the reactants used in this study it can be seen
that significant absorption does occur for all molecules
other than oxygen and nitrogen.
However, the observ-
ad.ons of only very weal<: emission from the products of
direct photolysis ~ a9pears to rule out the reaction of
1
such species with ground state atomic or molecular
b,_ydrogen as a sour•ce of det eatable radiation. React ions
2
of H( P) with the .Pr•oducts Qf previous reactions of
photolysis could account for some of the modulated
emission but these processes are believed to be slo'N
compared with reactions involving the much more abundant
reactant molecules.
Rate constants for the reactions
54
between ground state atomic hydrogen and the reactant
molecules in this investigation are shovm in Table IV.
These are seen to be
atively slow by comparing the
half lives of the reactions with the resident time of
gases in the Lyman- a. beam ( 0. 29 sec).
Emission spectra were recorded both with and
without the field ef'f'ect chopper activated.
The follow-.
ing bands (Tob\l.e V) wer•e observed only when the lamp was
on, the microwave discharge was on (See Chapter III) and
the reactant species was present.
Emission bands identified as long wavelength features of
the lamp ..
Feature f...
~
43LJ.O
LJ583
Orir;inating
species
H
System
H
"(
Intensity
12
9
4532
Lj.861
H2
H2
H
17
55
L~973
H2
9
5852
5888
5975
6122
6161
Ne
55
H2
10
'11
H2
H
.2
7
12
55
Rate constants for reactions between ground state atomic
hydrogen and the reactant molecules, and the extinction
coefficients of the l~eactant gases at 1216~.
Reactant
syecies
o2
Reaction
process
H+0
-~>
NO
Ref.
0 .. 2
79
zero
79
OH+O
2
N2+H2
80
H+NO+M.
H+N 0
2
~
co
Half Life
of React-
5x1o- 22
2
H+H+N
~
Rate
19
6 -8x1o•
3 • 8X 10
2
81
N +0H
2
82
H+CO+M
-> CO+Om
2.8x10
H+S0 +M
2
~
HS0 +M
2
2x10 -31
12
83
high
84
56
O:H emj.ssion was obser·ved with the Jarrell
Ash
spectPometer as shown in figure 20, and with the McPherson spectrometer.
The OH emission disappeared com-
pletely wi t~1 the microwave discharge tuPned off' but
remained, with lower intensity, with the oxygen flow of'!' •
. This pePsistence o:r OH emission vvas thought to be due to
a small impurity of oxygen in the argon flow and some
water' vapouP absorbed on the glass tube near the microwave discharge.
ground
Trace (2) in figure 20 shows the back
ctrum under the same conditions as before
except with the microwave discharge tuPned off; the lamp'
. was turned on and
o2
was flowing.
Prom Pearse and Gaydon 85 the spectrum was iden-
tified as the A
2
~+
-
x 2 ~ transition o:r
OH, showing the
unresolved R and R heads o:r the (o,o) transition and
1
2
the Q and Q heads at 3078R and 30891\..
1
2
rrhe OH spectr·um was observed by the McPherson
spectrometer with the :field effect chopper activated.
Some unmodulated emission of OH was observed originating
from
V
1
:::::
reactions.
i, ShOVVing that 01-p!c Wa~:. ))POdUCed by Secondary
57
~rho
spectrum shovm in f'igure 21 was obtained
using the McPherson spectrometer, as are all further
spectra in this chapter.
Trace (1) recorded with micro-
V/D.Ve discharge on, shows the ( 2, 1) transition of' the
(A 31e- x3 z) band of' NH at 31191\ and 31375{ as well as the
( 2, 2) transit ions at· 327 3J,. and 33361?. and the ( 1,1) and
(0,0) transitions at 33?0.2. and 3360~.
As the spectra-
meter v;asused with wide slits the resolution was not
very good, also the signal to noise ratio in the electronics wus higher than with the Jarrell-Ash spectrometer,
which made it more difficult to distinguish detailed
f'eatur>es.
'Phe unmodulated spectr·um was scanned but no
featur>es were seen othei' than the NH bands already mentioned.
'l1he
(A 31C -
x3 z) band of NH shown in f'igure 22
tJ,e
J
was obtained as modulated emission fPom,._reaction
between
n( 2P) and NO. Trace (1) shows the (o,o), (1,1), (2,2)
and ( 2, 1) pands as observed in ·the Peact ion with N •
2
search for OH emission between 2500)t and 6500.R was
unsuccessf'ul.
A
58
Unmodulatedemission showed the presence of
OH(A 2 ~) but no production of NO in excited electronic
levels.
2
Nitrous oxide plus H( P)
The modulated emission f'rom this reaction is
shown in figure 23.
The tr•ace recorded with the dis.
0
0
0
charge on reveals strong bands at 3360A, 3370A, 3137A
and 3119A identified as the A 3 1C- X 3_~ transition of
NH described earlier.
The modulated emission showed no
f'eatur>es which could be attributed to NO or OH.
A scan of' unmodulated emission figure 24 showed
bands arising
~rom OH (A
apJ.sing fPom the B
2
Crn•bon Monoxide and
2
~) and the
NO
~-syst~m
level of' NO.
1C
B(?YJ.
Initially two bands were obser•ved as
figure 25.
'Phey are the (0,0) band of' OH (A
sho~'/Yl
2
in
~ -X 2?C)
0
ot 306L)A and the (0,0) and (1,1) tl"'ansitions of CH
(A
2
~
- X 2 ?C) at
0
0
31 I.1L1A and 3156A.
Attempts to repeat
the exper:Lment gave only the CH' bands. The OH(A 2z)
was probably produced by a reaction of H( 2P)
with an 0 2 impurity.
Unmodulated spectl"'a revealed the
59
CH bands mentioned above and also hydr•oxyl emission.
Weal\: emission at
0
3064A due to OH(A
detected but was not reproducible.
2
Z) was
It is now thought
to have arisen fr>om oxyr;en impurities in the gases as
with CO.
The gas was purified befor·e use in later runs
0
and only the band at 31 L!-4A was obtained.
This is the
same as observed in the CO reaction and shows the
presence of CH(C
~)
(Figure
26).
Where the field effect chopper was activated the
2
usual OH (A z) emission was observed.
The sti•ong bands shovm in figure 27 bfive features
0
·at 32L~OA,
0
0
3098A and 3060A.
The
32Lj.OA feature is iden-
and Q heads of the (0,0) vibrational
1
2 +
2
0
band of the (A Z -X ?c) transition of' SH. The 3098A
and j06oX features could be emission :from OI-I( 2 z) but it
tified as the R
1
seems more lilwly that they are the Q
and R heads of
2
1
the (1 ,0) vibrational levels of.. the SH transition above.
A sear•ch for modulated emission f'rom SO was unsuccessful.
( 0,0)
!'aN
a....
3ooo
3oso
3200
A t..
2
2
2
OR ( 2- rr) emission ~rom o2 plusH ( P).
Fig. 20
3000
Fig. 21
31 oo
3114
T,i1-I (3n -
3242
3z) emission fl"om
3399
N
2
A
A
plus H ( 2 P)
emission rrom NO plus
3130
3220
331 Q
3.a'l00
Fig. 23
NH (~IT - 3 Z)
emission from N 20
plus H ( 2P).
3000
3100
3200
3300
3400
3soo
t..
A.
-I'<V
J
:r:
3000
0
3160
3250
3350
3..:170
ll
o'
emission f:r·om
3COO
3060
3'16 0
0
3310
A
A
( o. 0
)
Q
( 1, 0 )
3000 '
3240
3370
A
A.
Table
Reactant:
0 .. 24 torr 0 2
0 .. 10 torr N 2
0 .. 10
NO
v.
.
Bands:
OTT
n A2.,.,
z-x 21t.
NH
'li:-X3 :z
A.3
...
v3:z
....
7C-..~
of' H(
Chemiluminescent React
0
3064A
0
3360A
0
3360A
v'
Normalized
Intensitii::
0
i,.O
2,1 '0
2,1 ,o
(0,0)
1.15 (0,0)
0 .. 30
(2,1)
0.9L~
(1,1)
1. 77
(0,0)
)
Process:
(70) H(
)-1-0 2 ( 3 :z) -+ OH( ~) +0( 3P)
->
(31C) +N(4S)
2
2
(73) H( P)+N0( 1C) -+
(31C)+0(3P)
(
) H( 2p) +N 2 ( 1:Z)
0 .. 52 (2,1)
1 .. 42 ( 1 '1 )
0.10 torr N 20
l\TT' •A.3 ?C-X3Z:
.r:n
.
0
3360A
2,1 '0
o•
(0,0)
(
) H(~)
1
) ( A)
->
( 31C)+N0( 2'A)
0 .. 37 (2,1)
0 .. 49 (1,1)
0.20 torr CO
2
2
c :z-x 'A
0
3143A
1.,0
o.
( 0;·0)
(81) H( 2P)+C0( 1z)-+ CH0( 2A)
(82) H(~)+CHO
0 .. 12 torr co 2 cH c 2:z-x 21t.
0 .. 04 torr so 2
A2:Z-X 21C
0
314-3A
0
3237A
1 ,o
o.66 (o,o)
1 ,o
1. 97 ( o, 0)
->
CH(
)+OH
)+C0 2 ( 1:z)....,. CHO + o( 3P)
(8-B) H(~)+S0 ( 1 A. ) -> SH( 2:Z)+0
2 1
2
(86) H(
0\
0
61
The modulated emission features from the various
reactants aPe summai•izod in Table V.
Also shovm in the
table are the intensities of these features relative to
the intensity of the 306L1X OH band arising from the reaction of H( 2P) with 0 • Because of variations in the out2
put of the lamp and the transparency of the lithium
fluoride windows, combined with the likelihood that some
of the reactants absorbed a significant amount of Lyman""'()(
radiation, the relative intensities in Table V are
believed. to be
cor•:~;•ect
only to within a factor of 2.
The
most notable feature of these intensity figures is that
the total number of quanta emitted per reactant molecule
is remarkably constant.
The range of variation of the
median value is less than a factor of three.
This is
surprising,. in vievv of the widely differing natures of
the reactants.
Ta;ble V also summarised the main reac-
tions proposed inthe following discussion to explain the
2P).
chemiluminescence observed in reactions of
H(
Some uncertainty arises in proposing reactiont
which account for the observed luminescence as experiments.! information is obtained about the electronic state
or only one of the products.
In all cases it has been
assumed that the other product is in its ground state.
62
The sugge
ed reaction mechanisms accounting f'or
the observations previously mentioned are:
(1) The reaction with oxygen to produce modulated
OH( 2 ~- 2 ~) emission:
The 5.5eV excess energy is su:Cf'icient to produce
1
1
o( D) or o( s) but modulated emission f'rom these states
is not expected because of their long lifetimes.
The production of unmodulated OH(A ~) would
2
occur by
)
( 71 )
This reaction is
orted by Tiktin, Spindler and
86
-21
3
-1
Schiff
to have a rate of 3 x 10
em -sec , and to
form OH( 3~) with v'
= 1,
as well as v', =
o.
Purcell and Tousey have shown that atomic hydl'ogen
can be detected spectroscopically as low as 110 km in the
atmosphere and that the absorption of solar Lyman-a. occurs
at this altitude during the day.
rrhe strong OH bands
produced by reaction (70) could ther'efore explain the
production of airglow bands intense enough to be observed
on the gr•ound.
Chapter• VII.,)
('l\1is point is discussed further• in
'rhe chemionizat ion reaction
H(
) + 02
~
HO + + e
(69)
2
orted by Tanaka 57 could not be detected With the
tern used here; it is likely that both reac2
tions (69).
(70) compete for H( P) .•
(2) Production of NH (A 3 ~ - x32) bands is possible
2
by the direct reaction of H( P) with rmolecula~~ nitrogen
(67)
as proposed by Tanaka 52 •
Because no N emission was
2
observed the second mechanism he proposed
(65)
(66)
appears unlikely to be the dominant process in NH
for3
The nitrogen atom produced in reaction (67) is
mation..
shovm as being in the (Lj'S) state, since the production
of a nitrogen atom in the
2
D or
2
L\
P state would be endo-
ther·mic.
Efforts to detect the NH bands reported by
iled until the reaction volume
viewed by the spectrometer was increased.
Initially the
64
NH bands were observed only in the unmodulated emission.
'Z
This suggested that NH(A..J''11') vms produced by the ground
state r-eaction
(72)
To confirm this mechanism for NH(A 3?C) emission
ni tPogen and hydx•ogen vtere passed through sepaPa te microvvave discharges and mixed in the reaction chamber shown
in
gure 12.
The two gases were reacted without
iPradiation by the Lyman-a. lamp.
The spectrometer· was
at 3360X and c11anges in the unmodula ted intensity
s
were noted when the two discharges wer·e turned off alte:r·na tely.
Once all oxygen impurities had been removed
from the nitrogen, decreases in intensity were observed
when ei ther• discharge v.ras turned off.
The IJroduct ion
of NH(A3?C) was therefore dependent on the pPesence of
ntomic ni tr•ogen nnd atomic hydi'ogen.
The Pelationship between reaction (72) and the
dissociation ener•gy of NH is discussed by Phillips 56 who
suggests on the bnsis of this experiment thnt pre· t 1011
·
assocJ.a
must occur between N ( L1.S ) and H( 2S ) as the
ground states of these ntoms do not correspond with the
(A
3
?C) state of NH.
This association occurs along the
~-state giving a calculated lower limit to the
65
dissociation energy o:r NH of 94 K cal/mole.
'J.1he rather
low intensity of the emission by comparison with the NO
and N
2
that there may
afterglows i'or example sugge
be an activation energy i'or for•ming NH in the· 5 .z state
at a hiD:h enough energy to cross the A state.
If so then
the calculated lower limit would be too high by an
amount equal to this acti vntion energy.
..
,.
Tllis type of
activctt:ulinverse predissociation has been observed in
shock tube exper·iments with 0 0
87
Recently Seal and
88
1
Gaydon
gave the n;H o:r NH as 74 k cal/mole- ..
c..
•
(3) The Renction of NO with
H(~)
to produce
NH emissions is:
(73)
v
==
2,1,0
As the ionization potential o:r nftric oxide is 9.27eV
Lyman-a. radiation has sufficient enePgy :for photoionisation to occur.
NO + hv
~
NO+ + e + 0.93eV
(74)
The r·eaction
(75)
66
is ondothePrnic by 0.7eV and therefore should be much
slower than reaction (73).
Similarly a reaction between NO+.and ground state
hydrogen cannot explain the pPoduction of unmodulated
emission from NH( 3 ~).
The Penning ionization process
(7.6.)
could also occur undo I' our conditions, but would be
difficult to distinguish fpom direct photoionization.
Another possible primary l)rocess, one that we
have not observed, is
(77)
This reaction is not spin forbidden and it
not obvious
why it should be unimportant.
~.Phc
observation of OH::' in the unmodulated emission
suggests that the reaction
(71)
occurs as a seconoary process.
Notably absent from the spectra, both in the
modulated and unmodulated form was emission from excited
NO.
The absence from the unmodulated emission implies
67
that primary processes producing atomic nitrogen are
not important.
It must also be concluded that absorp-
tion of Lyman-a, by NO produces the ion, by reaction (74)
and does not cause any appreciable photodissociation to
N and 0.
2
(/.i.) 'fhe reactions of N 0 with H( P) to give
2
v 1 ::::2,1,0)
2
Since the region of emission of' OH( z) in this
2
ctrum was obscured by the (2,1) transition of NH( ~)
the observations of the reaction do not entirely rule
out a second primary process
(79)
.•)'.>.:,; ·
j;.his reaction is
.c.\~¢.,~f'\o+
spin forbidden.
Nitric oxide produced by reaction (78) is assumed to be
in the ground state as no modulated emission from NO was
obser·ved.
68
It is possible that some NH( 31C) is produc
by reaction (73), as photolysis of N 0 has been shovm
2
15 16
to produce NO by reaction (22)
'
• However the intensity of bands observed
su~gests
that the reactants
v:ould have higher concentrations than would be the
case for NO produced by N 0 photolysis.
2
2
Unmodulated emission from OH( Z) observed
2
during the reaction of H( P) vvith N 0 pr•esumably occuPs
2
by reaction (71).
'rhe most probablF: mechanism for the production
2
of NO (B 1C) to give the bands observed is
NO (B
2
1C) + M
which has a rate constant 5 x 10- 33 cm
-1 81
sec
•
(80)
6
molecule-
2
(5) The production of CH(~) bands by the
2
reaction of' CO and H( P) is exl)lained by the reaction
( 81)
:followed by
'
(82~
69
or
(82a)
The direct mechanism
is endothermic by 34.6 k cal mole- 1 and would probably
· not occur rapidly enough to explain the intensity observed.
CO is not photolysed at 1216R.
2
Unmodulated emission of OH(A Z) observed in the
CO plus H( 2P) Peaction system is explained by reaction
( 71), the 0(
) would be produced by react ion ( 82)
followed by
(84)
or (82a).
(6) Both of the possible pi•imary reactions with
')
C0
can sive CH(LZ) emission via the mechanism proposed
2
1'or CO; they are
2
H( P) + C0
2
H( P) + co
2
2
1
1
( z)
-;. OH( 27C) +CO ( z')
(85)
1
( :6)
-;. HC0( 2A) + o( 3P) + 5.8eV
(86)
The heat of formation of HCO is given in Gaydon 8 9.
vO
A process analogous to one rrthe reactions of N
could account f'or emission from CH( 2 2~).
It is however
endothermic and is therefore not .likely to be rapid
enough for the CH emission to be detectable.
00
2
is
photolysed at 12161\ to CO; this prov:l:des another way of
auproaching reaction (82) and (82a).
The hydr>oxyl emission detected with the field
effect chopper activated can also be accounted for by
renction (71).
(7) A
por.J~ible
reaction leading to the production
2
of E3H( z) is
(88)
It is nlso possible that photolysis of 80
yroducing relatively long lived
80
2
+ hv
~
·
so
r)
SO + H(""P)
~
occurs
so.
+ 0
and then SO reacts with the H(
2
(89)
) present
(90)
71
2
2
Either of U1ese reactions can account f'or tho SH( Li - 1e)
bands observed, but tho first is favouPed because of the
higher concentration of so
Although no OH(A
2
2
relative to SO.
~) or
SO(
2
:z) ~;vas detected, these
cios may be present as the band heads due to
SH(A
) (1,0) at 30601?. dominated the spectrum and would
have obscured otheP detail.
The reaction
( 91)
could have
pl~oduced
both excited SO and excited OH which
may have been Pesponsible for the poorly-resolved bands
at 3064-~ and 327 i ~.
72.
1'r•apping of IJyman a in an Atomie hydrogen gas.
J:<'rom Chapter II the reciprocal of the number of
t
i.mes a r'esonant photon of energy is captured and
re-emitted is given by the following formulae:
(i) fop an j.nfinitc slab of thickness L with
only Doppler broadening of the spectral line:
c: =
k L
0
(ii) for an infinite cylinder of radius R with
only Doppler broadening:
k R
0
(iii) in the case of' pressure or impact br-oadm1:Lng:
for an infinite slab
and
(iv) for an infinite cylinder with pr-essure
bPondening
g
:.::
1.115 (?C k R)
p
73
where k:D is given by
A. 2 N
k
0
=
}?
2
=
with
f'N
mV/
'
from Furssov and Vlassov
and
= .1.T =
y
90 ,
me
g1
f'
g
A. 2
2
0
(a) For an infinite slab 2 em thick
A. 3N
k
2
0
s-i} gives g1
Theref'ore
g
0
_g
-&:
g1
=2
g 2/g
1
= 6.
=
3
.i
=
( 2kT) 2
2.229 x 10 5 em sec
mh
1
= 1 ·.-2-x-fo-=8'
)
'
k 0 for N
=
10
1L!.
10
15
10
16
10
17
sec
-1
-1
values of'
- -·
atoms em ~ can be cal-
culated and then values of g and
.1.g
obtained, with
7 Lj.
results as in table
vr.
Table VI
Light trm)ping in an atomic hydrogen gas (Doppler broadening) in an infinite slab.
N
k
0
-1
1
g
(Atoms em -3)
(em
1014
4.52
7 .. 88x10
1015
45.2
182
1016
452
5.5 xio- 3
4. Lj.8)Ci 0- 4
3 .. 88x1o- 5
3-46x1o- 6
25800
1017
L~520
1018
45200
(b)
g
)
-2
12.6
2230
28900
For an infinite cylinder radius 1.25 em the
values of g are calculated in a similar manner, witl;I
results as in
Tabl~
VII.
Table VII
l~drogen
Light trapping in an atomic
gas (Doppler Broaden-
ing) in an infinite cylir.J.der.
k
N
(Atoms em
-3
)
(em
0
-g1
g
-1
)
-1
1014
~.•
1015
45.2
7.9 xio- 3
126
1016
452
6 .. 4x10-4
1560
1017
4520
18400
1018
45200
5 .. L1.4x1 0- 5
6
4.84x1o-
1 .. 2 x1 0
52
broadening
From equations (vi) and (vii)
:r_
=
yp
=
w
::
_3m~
me
g
1
1
g·
2
"A.o
2uJ ~
c
2?1:
2
g2
\).
::
1
N
2
1
N
"A.0
2?1:
2
c
-:;:.
8 .. 3
20700
76.
9
.
61C
=
yp
2
N
g1
g2
1
-A.3
substitution in (v) gives
k
;::
p
using (iii) this gives
f'or an infJnite slab, and equation (vi) gives
g
=
f'or an infinite
c~linder.
These give the values in Table VIII
Table VIII
Light tram;ing by hydrog0n gas with pressure broadening
-g1
g
Inf'inite slab
5o2
X
1 0-L~
1 .. 9
X
10
Infinite cylinder
6 .. L~
X
10-'-1.
1. 56
X
10
3
3
The calculated values of 1/g are plotted in
:t'igur·e 28.
'fhe dashed curve in figure 28 represents
4
0)
0
Pressure broadened
----
( slab
( c yJinder )
3
2
II
1 5
14
Pie;. 2e
1 6
Light trarn;ing in an atomic H gas.
r og
NH
77
the transition from the Doppler broadened case to that
of
This is similar to the experi-
ssur>e broaden
mental curve gi von by Alp::n•t et al, and used by Walsh
.
for mercury vapour
65 66
'
•
It vms ho:oed to measure the trapping time of a
Lyman-o< photon in atomic hydPogen and hence obtain an
c.x:perimen~~o.l
valueso
value of g to compare wi
A pulse
the theoretical
Lyman-oc with a decay time an OPder
of' magnitude lower than the trapping time of photons
in the gas (i.e. a decay of 2
used to excite the H atoms.
the decay time of a
Lyman-~
X
1o- 6 sees) was to be
rrhe ei'fect of trapp
on
pulse scattered by atomic
hydroe;en at di:Cfei·ont pressures, was then .to be
moni tared vd.th an oscilloscope, using the
monochromator described in Chapter IIL
~.'lcPhel"'son
Initially it
was hoped that the rapid decay time necessapy c auld be
produced us
320 Hz.
rcguired.
a Bulova tuning f'orl{ chopper, tuned to
This houever did not give the short fall time
Pulses derived from condensed discharges
thPough hydrocen gave
cays of' the order· of 200
r
sec'
and it appeared that the tJ.me constant of the circuit
78
vvas l:Lmi ted by the cspaci tance of the lamJ) and its
lends.
With these relatively long decay times it was
not possible to measure trapping times directly.
Fur-
theP wol:·k using electr•on impact excitation is now
beinr; carried out in this labor-atory to obtain some
experimental tr•apping times for Lyman-IX photons in an
atomic hydrogen gas.
79
Stern-Volmer
the r•eact
quenching of H( 2P) by
ots of t
29..
s named are shown in fi
The
s analy-
stPaight lines were obtained by a least s
sis assuming F = F0 when
0.
From the
s of
d, quenching constants v1ere calculated
slope o1Jta
us
[Q]
the
rn Volmer relation derived
Chapter I I ..
II, were then calculate
In calculating these va
s
2
of k and o the eff'ect of li
tr•apping on the effective lifetime of' H(
), is taken intoaccount ..
natural li
ctive 1
is
rr:/g..
time is only 1.2 x 10
time, in the presence of li
The par·tial pr•essur•es
sec; the
trapping,
atomic h,ydrogen were
measur>ed during the quenching studies vvi th the isothermal calc
values of the
meter pr>obe.
Fr>orn the measured pH'
ing time were calculated, as outlined
in Chapter VI, with r>esults in Table IX.
0
o·- N 0
o
-
A
-NO
co
v -co
0
-
2
2
N
'G
2
0-
s0
-
2
2
0
co 2
1· 7
0
0
NO
2
1· 5
1. 0
0
0.6
0·2
Pq
.
~ue:nchinc;
2
of H ( P) fiuorosence.
(torr)
80
Trapping of' Lyrnan-e< in the ntomie hydrogen gas
Reacto.nt
1
g
21
PH
nH
( toPl")
(x 10 -'15 em -3)
(x 10 "-)
g
-0
sec.
··----
02
0.19
6 .. 1
7. L1.
8.89
X
10- 6
N2
0.18
5·9
7.2
8. L1.
X
10- 6
,r
NO
0 .. 'i 6
5 .. 3
6 .. 6
7-9
X
10- 0
N2 0
0.19
6.1
7 .. L1.
8.9
:x:
10- 6
,r
co
0.19
002
0 .. 16
6 .. 1
7 .. L~
8.9
X
10- 0
5·3
6 .. 6
7-9
X
10- 0
X
10 -o
[
/'
so 2
0.20
6 .. 6
7.7
9-2
To obtain the values o:r g in table IX it was assumed
that the hydrogen vapour :formed an in:fini t e
c~,rl
inder
2.5 em in diameter' extending upstream and downstream
from the windows o:r the react ion vessel in :figure 14
o:r Chapter III.
Holstein's theory vvas used..
It was
assumed that as the measurements were made close to the
transj_tion r·egion in :figur·e 28 the dotted cur•ve applied ..
81.
Results of quenching experiments are summarized
in table
x.
'l1 he values of slope obtained b;i least squares analysis
quenching measurements
F
Reactant
( Fo - 1) /
N2
0.583
NO
o. 851
N2 0i
1 .. 61
co
co 2
0 ...971
so 2
3 .. 12
[Q]
2 .. 03
'rhe Stern Volmer• equation in Chapter II is
F'
0
F
therefor•e
1 + k'C' I
[
Q]
(i)
82
- 1)
[ Q,
where
'1:1
.i
k '1:
J
is given in table IX.. From table X we have for
=
k rt:'
slope
=
3.12 tor-r
-1
Converting to c.g.s. units
[Q J
=
where T
foreign
~".X
0
300 K and Pi\. is the :partial pressure of
s in torr ..
'L'herefore ·
8
=
k
= 6 .. 24
x 10
-1 2
em
3
molecule
-1
sec
-1
Also from Chapter II
F
a
2
( __Q -
2n ...c.
when
n
i)
::::
=
[Q]
1
j27C R'L' <m
1
1
~
+-)
m2
(ii)
83
'J:his gives
2
0
=
'(' X 9 .. 658 X
where 't. 1 = 7. 7 x ·t 0-
.
fi
.
2
0
6
sec: in the case of' so ..
2
::;;
6L~
=
77 • 8
T
=
-1 9cm2
X 10
Results calculated in a similar manner ror the
othel" quenchers are summarized in Table XI.
Table XI
Values or o
2
and k calculated ror the quenchers
NO, N 0, CO, C0 and
2
2
2
0
Heactant
(x 1o 19 cm 2 )
(x 10
12
lc
3
-1
-1
em molecule sec )
20.5
'l. 65
N2
13.5
1.09
NO
18 .. 1
1 .. L1.7
N 0
2
38.7
3.13
2J .. i
1. 87
L:.3 .. 2
3 .. L1.8
77 .. 8
6 .. 24
C0
2
802
N ,
2
so 2
02
co
o2 ,
84
The effective lifetime of the excited state in the
prescmce of several quenchers Q
n
is given by
1
=
(iii)
whe1•e in the case in question k is the PccipJ:'ocal of
0
2
the natural lifeth<:e of H( P) and lc [Q.] is the rate
l
l
of quenching by Q .•
l
'.rable
xr:r
gives values of
whePe F is the intensity of scattered fluorescence at
1216:A,
at differing pressures of Argon and hydx•ogen.
Variation of Quenching of' 0
p0
p
= o. 2
2
with pressure of H and Ar.·
2
torr
2
H2
Pargon
(torP)
(torr)
o. Lj.29
1 .. 289
1. 20
0.379
1. 371
1. 383
1 • Lj.60
1 .. 850
i. 20
0. L124
0.200
0.280
F 0 /F
1 .. 18
1.19
1.18
85
From the first two entries in table XII we find
that a change of' pH
from 0 .. 379 torr· to 0. 429 torr is
2
exactly conwensated by a change of p ar from 1.371 torr
to 1.289 topr. Thus
1\:.,,.
n
2
/kar·
0 .. 082/0.050
1 ..
6
Similarly from the third and fifth entries
~
2
/k ar "' 3. ·j whe:r.'e the greater p:r.-•essure ranges involved
make tl1is estimute more reliable. Taking k,_ = 3kar we
li2
F
_Q
1
find that where
= 1.18 we have PH + ~ 1)~ar equals
F
2
either O.f397 Ol' 0.855, mean value o. 891 while for
1. 20' p
F
H2 + :k par equals either 0.836 oi' 0.859,
mean value 0~848. Extr-apolating to pH = 0
2
=0
= 0.20
torr
=
0.81
'l'his gives us
(
= 0
;:::
Cor·r•ect
P.
1) I
v
[ H2] =
.20 '
y.08_1 - 1 )
0.848
this for the quenching caused by the
presence of 0.2 torr of o 91 we obtain slope for H
2
2
86
q_uenching
==
0.679 torr
-1
..
From the
ope of' 0
2
this
gives
=
1.5
Theref'or-e the results obtained
e:x::perime.nt~lly
in the
pr•esence of' H and Ar must be corrected by mul ti}?lying
2
by the factor
1 +
1g
This has been done for the rate constants and cross
sections in the last two columns o:r 1'able XIII. The
2
2
and aAr are also given in
a)?:pro:x:imate values of aH
2
table XIII.
87
Table XIII
Corrected value of k and a
2
taking into account the
effect of' Ar and H •
2
Correction
a2
lc
Reactant
(x 10+12 em 3sec -1)
factor
(x 10 -r19.cm2)
--
02
1. 230
2.03
25.2
N2
1. 286
1 .. L~2
17 .. 3
NO
1. 222
1. 79
22.1
N 0
2
·j
"253
3-92
48.5
co
1. 261
2. 36'
29 .. '1
co 2
1. 250
Lj.• 35
54 .. 0
802
1 .. 287
8 .. 03
100 .. 1'
35
16 .. 8
... 0 .. 45
. . . 5·6
H
·1 ..
2
Ar
F
The standard dcviation ... in (F0 ~-·1·) / [Q]
eoch reactant is given in table XIV below.
to.[t,e
er:t•ors in k and
a
2
for
The percen-
calculated f.rom these standard
deviations is also given.
88
Table XIV
Standard deviations and err·ors in k and
Standard deviations
F
1 )/[Q]
in ( ..,o
Henctant
.J:I
-
cl
at
;o
e:PI'OP in
2
k and a •
18
02
0.1556
N
0.0529
9 .. 1
NO
Oo3022
35·5
N 0
2
0 .. 255
16
co
0 .. 1844
19
C0 2
0.316
15
so 2
0 .. 363
12
2
'l'he main ex:per·itnento.l errol"' was caused by low signal to noise l'atio when observing the scatter•ed
Lymnn-a.
'l'his made it difficult to obtain accurate
values of (F /P - 1 )/[ Q] especially ·when F /F was close
0
0
to unity ..
Errors in g.
The region of transition from
Doppler broadening to pressure broadening is not
w~ll
89
defined by Holstein or Walsh.
·.r
It was necessary to malce
our measurements at atomic H pressures in the transition
region in order to obtain a reasonable intensity of
As the position of the curve in
scattered Lyman-a.
fi.gur>e 28 cannot be comDletely defined a
20;~
uncertainty
in g is assumed.
Table 18! gives the total percentage err>ors in
values of k and a
2
and the uncertainty in results.
It is Dlanned in f'uture work in this laboratox•y
to meqsure g discreetly for comparison \?i th theory, and
2
.
thus d er
more preclse
va 1 ues or., a ana' 1c.
... --,
',
...
-·
Err•ors in a
2
and k
a
Reactant
02
N2
NO
N0
2
co
co 2
802
H2
Ar
o·l
70
ePror
38
29
55
36
39
36
31
f vrithin a
faetor of
2~
2
2
k
(cm )
X 1019
(em 3sec- 1 )
25 ·I· 9
17 + 5
22 + 12
L!.B + 17
29 ·!· 11
+ 19
100 ·I· 31
'16. 8
5.6
2.0 + 0.8
1 .. L]. + 0.4
1. 7 + 0 .. 9
9 + '1,, 4
2.3 + 0 .. 9
L~. 3 .. 1. 6
8 .. 0 + 2.5
1.35
0.45
X
1012
!~-
90
On the basis of the kinetic theory of gases the
2
collision cross sections (a ) Table XVI can be derj_ved
2
for the quenching species 9 •
Table XVI
Collision cross-sections calculated from viscosity
measurements.
a:-0
ctant
2
a ,2
:Jl__
ao
')
(em'-)
2
a
I
2
02
2
02
13.3
X
10-16
1.00
-1
I
5•3 X 10
N2
14.4
X
10-16
1. 08.
0 .. 68
8.5
X
10 2
NO
1 Lj.• 1
:X:
10
1 .. 06
0.88
6. Lj.
X
10
N20
21.5
X
10-16
1. 62
1. 92
4·5
~(
10 2
co
1Lj.• 5
X
10-16
1. 09
1 .. 16
2 .. 9
X
10
21.0
X
10-16
1. 58
2.12
3·9
X
10
co 9
"-
800"-
-16
4.00
2
2
2
91
lh~om
a comparison of the collision cPoss-sections
and the quenching cross-sections ('I'he last column of'
l't:tble XVI) it can be seen that a l"eactant r.;olecule collides up
2
to 8.5 x 10 times :eor each quenching reaction
1
Cornparing the relative collision cross-sect ions
with the relativequenching cross-sections in Table XV',
we find the triatomic molecules have higher values of
2
2
2 . 2
oQ /o 2 than of aQ /a. 0 • This is presumably due to the
2
presence of' more degrees of :freedom in the triatomic
molecule in which·it may absol'b energy.
Argon is a
weaker guencheP pr>esuma1)ly because of' the small number
of degrees of freedom.
Thcr·e are two bas).c por:.;sible mechanisms involved
in quencLing, pure ener-gy t:vans:Ler>
and a chemical mechanism involving a bond r-upture
Q
!."rom
table XVI it can be seen that of the diatomic molecules
N
2
and NO are less e:Lficient guenchers of' H( 2P) than
.
2
.
o2 •
This suggests that the quenching of' H( P) is a r-eaction
which involves bond fracture in therJe three cases
(dissociation energies,
N2
. 225.1
k cal mole
-1
'
-1
02
118.0
NO
150. 1 k cal mole • Energy
),
J,\.
cal mole
-1
'
92
2
-1
available f'Pom H( P) 235 k cal mole ).
This is in
contrast to the molecule CO where in spite of a largeP
diG soc
1
t j.on ener-gy ( 256 .. 2 k cal mole- ) the quenching
cr-oss-section is laPger, implying that the dominant
mechanism for· quencl1ing is
The CO* may have been formed in the a 3 1C state
'-~'hich emits below 2500~, and V/Ould not have been s eon
in the work reported in Chapter
v.
Tinsely 93 gives protiles of atomic hydrogen in
the atJ:iOsphere above 100 Km.
His models assume a
temper:1ture at infinity of 'l000°K.
He
ves a peak
concentration of H atoms at 100 Km of 107 atoms cm-3.
At this concentration the effect of light trapping
would be small, since the mean free path of photons
8
would be 10 times that in the present reaction vessel,
and thcPefore ther>e would be a low e:f'ficiency for the
production of H(
) at these altitudes.
Also the solar
-2
-1
Lyman-a flux in the atmosphere is only 0.1 erg em
sec
or 6.1 :x. 10 9 quanta em -2 sec -1 51- , measur•ed at the centre
93
of the broad solar emission line.
It therefore seems
unlikely that the chemiluminescent reactions discussed
in Chapter V vvould lead to si[!;nificant OH o·c NH
emissions in the dayglow.
9L~
APPY.iNDIX I
For a medium with absor.P.t ion coefficient lc( v)
and f'requency spectrum P( v) of radiation emitted from
a given volume , t
probability.T(p) of radiation
travelling a distance p is defined by 63
T(p)
=
JP(v)
e-k(v)pdv
(i)
}.ii tchell and Zemansky 94 give exp:r•e ssions f'ol"
the absorption c
1.
icient as follows
Natural absorption.
This type of absorp-
tion occurs in an isolated atom at rest
k( v)
( ii)'
=
wheroe v is the impressed frequency, v
0
the fr•equency
at which k( v) :ls mo.ximurn and y the reciprocal of' the
lifeti;nc.
2.
C is a constant.
Doppler broadening.
1~1is
type of absorption
p:r·evu :Lls vvhen the mot ion of the atoms is large compar>ed
with the natural width of the line.
Mitchell and Zemansky give
95
(iii)
f'or Doppler broadened lines where
=
vo
(2RT.)-&
rn
'"A
lc
=
0
3
0
811.-
N
(iv)
g1
g
.j
--.-I
( v)
1
2
1\.ZV 't'
0
vrhere R is the gas constant per mole, T is the
1
absolute temperntur·e and m the (gP am molecular) weight
of the
g
2
s.
N is tne number of atoms per c.c., g
1
and
the statistical weights of the ground and excited
states.
3.
Pressure or Lorentz broadening arises from
the intersct:Lon between indivldual atoms.
A
Pal
f'ormula used to take account of' both Doppler' and
ssure broadening is
(vi)
\'!hoPe
and x
a
=
=
[( v-v )/y ]c/v •
0
0
0
96
From Furssov and Vlassov
y
p
90
(vii)
=
where f is t11e oscillator strength for' the absor:ptton
of the resonance line.
cation of emiosion spectra involves a
Sped.
· knowledge of the nature of the excitation of the molecule.
Hols·:;ein
P(v)
<x:
has shovm
(viii)
lc(v)
and a;::;suming the :nor'malization JP(v )dv = 1 he shovYed
k(v)
N P( v)
=
=
'1:
l(l?(
(ix)
v)
by substitution of (ix) in (i) he obtained
rr ( p)
(x)
Use of (ix) in (iii) gives
P(x)
(xi)
so for a Doppler broadened line
(xii)
97
vthich by manipulation
T(p)
ves
(xiii)
=
lc p
0
l<'or a di
rsion distribution (i.e9 pressure broadened
distribution) Holstein derives an expression
'l' (p )
v1here k
(xiv)·
=
(xv)
=
d
'J:he intr·oduction by Holstein o:e a probability
K(p )dp that a quantum is captured after• tNlversing a
distance between
p and p + dp made it possible to
define a mean free path for quanta:
.(xvi)
Now
K(p) dp
T(p) - T(p + dp)
:::::
aT
- dpop
(xvii)
from the ltn"!s of })robability, so
A.
=
-
I:
p •.ElT dp
ap
(xviii)
which is found to diverge so that it is not possible
98
to def'ine a mean free path.
Transport excitation of Radiation
An adequate description of tr•ansgort of radiation
was achieved by using a Boltzman type integrodif':f'er•ential equntion.
!_ 1
If G(.:£.
)d.:£. is the probability that a quantum
emitted at .:£. 1 , is absopbed in a volume
around point
E. and if n (£,) is the density of' excited atoms, then the
production of excited atoms in the volume element ,...._
dr in
ven by the contribution of the quanta
time dt can be
emit ted
' in time dt, yn(£.' )d!,' dt, multipl
b;)r
d by
G(r £ 1 )d£, and summed over the volume of the gas.
Then the production of excited atoms in dr in
line dt
is eiven by a
=y
dt
Jn(~ 1 )
G(£ £. 1
)
dr 1 (xix}
and. the decr-ease in number- of excited atoms by
b
==
y dt
n(r)
(xx)
From the law of conservation of particles
dt
~f)·
01
=
y
U.t
dr
J
n(£ 1 ) G(£ £. 1 ) dr·' - ydt
n(r:)
(
)
where the integration is over the volume of the enclosure.
99
In Courant Hilhe1--t 95 Cha:pt eP III this is a stan-
dar·d
t~rpe
(1 -
of' integral equation
B
y.)
Jacz.:
n(.£)
£ 1 ) n(r') dr'
(xxii)
which has solutions of' tho t,ype
G(r'r)
:::::
n(£) e-(3t
:::
G(£
y
.£ 1 )
der-ived that
It can
D.
(xxiii)
n ( r· 1
:::::
)
dr•dr'
(xxiva)
1
( r·) d£.
(xxivb)
0
AT/Qlieation of (xxivb) to (xxiva) using (xxiii) gives
(xxii)
Courant Hilbert Cha}!tex- III shows that a general solution
is possible of the form
=
wher>e f3
n
e -13nt
is a series of ascending :JOSi t i ve numbex•s and
n (s) denotes o. solution corresponding to [3n.
11
(xxv)
100
Then for a long time
(xxvi)
Holstein no1v uced the R
z variational procedure
to find n(r) and ~ (The subscripts are dropped).
1Prom (v"l'vq)
~\.,.~'...
•'-
L<.
(x.xviii)
and the integrols are all over the volume o:t' the enclosure.
It is poscible to a.:proximate n(.£.) to a finite
series.
rn
=
Z a.n.
J..
(r)
J. . J. -
(xxix)
where n. ( r) and a. ai•e known flmct ions obtained by
1
-
,
J.
~·
minimizing
This Gives from (xxii)
=
(xxx)
·1 01
vthere
(£.) n. (£) E (£.) dr
J
(xxxi)
'J')O
obtain the minimum value of (3 diff'ePentiate
(xxx)
m
zj
j
[K .. - fl H .. ] a.
lJ
"( lJ
J
:::
(xxxii)
0
which gives a determinant of coefficient which must be
eo,unl to zei•o, and cons ti tut es an algebraic equation of
the mth ox•der in (3/y.
interest and gives (3/y.
(
The lovrost solution is of
Any
of ( m-'l ) equations fr•;)m
i
= 2, - -
, m.
~ri;is deter-mines n(r:) within a multiplicative const
Decay
or
~xcitation
in a volume of gas.
r;te:Ln cxpPer:;sed T( p) j_n the general asymptotic
'P( p)
(xxxiii)
102
in order to c
culate es
e factors for pressure and
Doppler broadening.
Substitution of (
1
ii) inthe expression
ven
a'.r
op
mo.
=
Us
ions (xxxi) and (xxxii) and inte[';Pnt
e
ever• the volume of the
f'act ors
sure he obtains the es
0
i von in Chapter II.
i.e.
for· a Doppler' bPoadened resonant line in an infinite
ab
=
1
1 • 1 50 ( 7C kpt) -::T.
for Pressure brondeninp in an infinite slab.
'Phe chanee in 0 arising from the structure shows
that '1'( p) may be cons j.dered as the a vero[r,e oi' transmission
103
i:'actoPs rr. ( p), of x individual components each treated
1
as an isolated line.
Then the functions for g are
ctill valid as long as 1c
(1)
0
is r eploc ed by lc
It is likely that
dis~ributions
di spePsion distPi but ion ocmu•.
of' those ts
11
0
/x.
other than
'!'he most imJ)oPtant
stotistical distribution 1196 which for large
( ,,o
v - v), · P(v) is· given by
(2)
Equilibrium between excited states and elec-
trons leads to a modification of equation (xxi).
A
contribut1on to exc1tation due to thepl'esence of elecThis is given by A n (r) where n (r) is
e ·
e
the electron density and A is a constant. Destruction
trans occurs.
of the excited state due to a collision adds a furtheP
ter-m
(3)
Another· pr·oblem arises when one ic notdealing
vt:Lth the idealized cases of' pure DoppleP
broadening.
OP
pressure
Walsh attempted to derive a general ex-
pression for g which included both types of broadening
'104
by lJutting
(xxxvii)
=
where K is of the order of unity.
He obtained a general expression for T(p) from
equations (ii) ancl (iv)
(xxxviii)
T(p)
nhere
rn
.J..d
1
k
0
( ?~
log k p)
0
·1
T
c
=
1
1
cif~)2
0'
and
T
cd
::::
These exp:c-esd.ons reduce to those given by
Holstein when Doppler or pressure broadening is
assumed and they are integrated over the infinite slab.
105
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Tablcs of'
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4th