Title
Author(s)
Citation
Issue Date
URL
Emission spectrum of the flame of bromine burning in
hydrogen and mechanism of the reaction
Kitagawa, Tetsuzo
The Review of Physical Chemistry of Japan (1938), 12(3): 135147
1938-12-31
http://hdl.handle.net/2433/46533
Right
Type
Textversion
Departmental Bulletin Paper
publisher
Kyoto University
The
EMISSION
SPECTRUM
BURNING
Review
of Physical
OF THE FLAME
IN HYDROGEN
Chemistry
of Japan
Vol. 12f No.
3(1938)
OF BROMINE
AND MECHANISM
OF THE REACTION*.
By Tcrsuzo Itl'rAcAw:1.
In the preceding paper') it was reported that in the emission spectrum of the
flame of chlorine burning in hydrogen some emission bands were found, which
were ascertained to belong to the excited chlorine molecules, and the mechanism
of the combustion reaction between chlorine and hydrogen was discussed from these
results.
In the combustion of bromine in hydrogen a number of emission bands belonging
to the excited bromine molecules were found. In the present paper the mechanism
of the reaction between bromine and hydrogen will be discussed from this fact.
[I]
The Emission Spectrum of the Flame of Bromine in Hydrogen.
(i) Apparatus
The apparatus used is shown in Fig. I. Purified hydrogen was introduced
through a flow-meter and the side tube
O
texcessive
into
the
quartz
vessel
a.3cm.
in
hydrogen and the hydrogen
diameter
diameter
and to
and
Iocm.
cm. in
in length,
length,
and the
and
the
XKI
V.
1
rl
11
Fig. I.
t. The MainPart of the Apparatus.
Ap
I.
bromide formed by combustion were
expelled through the other side tube d.
i
Bromine vapour was let flow out
through g and /e at a constant velocity,
being kept constant above the
the temperature of the reservoir of liquid bbromine
boiling point of bromine (58.7°C.)
(58.7°C. by n[cans of the electric current on nichrome wire
(B5, No. 33) coiled round it. The ttube g-/i was specially made of quartz.
The
method to light the flame of bromine was the same as that of the preceding report".
of the bromine vapour and only the
No grease was used on the passage
pa
111 quantity
acid.
ground part g was joined with a small
quantity of
of viscous
viscous phosphoric
pho
s
used
as
in
the
preceding
experiment".
The same glass spectrograph wa
The
• This paper is the English translationof the same article publishedin le,v. rrl.:. C1u„r.japa",
10, x (1936).
I) T. Kitagawa,Re•. Ph;'s. C/umt.
Jaftm. 7
4
I
The Review of Physical
136
Chemistry
of Japan
Vol. 12f No. 3(1938)
T. KJTAGAWA
Vol. XII
dispersion of the spectrograph was 12 A/mm at 4500 A and 45 A/mm at d 9900 A.
The Ilford Hypersensitive
(ii) Temperature
The temperature
Panchromatic
Plates were also used.
of the flame
of the flame was measured
with platinum-platinum
by means of a thermoelement
rhodium alloy. which indicated the temperatures
of about
48o°.5oo°C.
There was some reason to believe that the temperature of the
flame was somewhat higher than this. It was proved, however, that the temperature
of the flame of bromine was relatively low compared with the flame of chlorine
which had been about iooo°C.
(iii)
Emission spectrum
of the flame
The flame was visibly of orange colour.
The emission spectrum of the flame
is shown in Plate I, the time of exposure being about i hour.
Table
Band-Rends
Wave length
in
the
Wave
EmLs ion
numl
No.
i
inl.
A, A
I.
Spay rum of I1te Br.miuo-I
er
v, cm-t
In Plate I about
tydrngen-Flame.
Wave length
Wave
number
Imt.
Nn.
A,A
I
6875
14541
0
23
6142
2
6901
14700
x
24
6120
3
6744
[4823
3
25
6097
4
6729
14857
3
26
6072
5
6675
14978
5
27
6o56
6
6667
14995
5
28
6024
7
66o5
15135
7
29
5982
8
6593
15163
30
5957
9
6548
15269
7
8
31
5941
10
6532
15305
S
32
5910
11
6474
15443
x6
33
5867
12
6417
15579
10
34
5831
13
64o6
15605
6
35
5802
14
6360
15719
9
36
5794
15
6342
15763
7
37
5761
16
6313
15836
S
38
5753
17
6zgo
15893
9
39
5724
18
6264
15[)61
6
40
5692
19
6240
16021
8
41
566o
20
6220
16672
3
42
5629
21
6199
16155
7
43
5600
22
6x68
16209
5
v, cm-'
16276
16334
16398
16466
6
x65o8
16596
16712
16751
16827
x6916
17039
17146
17230
17254
17354
17377
17467
17563
17663
17762
17852
2
4
5
4
4
4
3
3
5
5
4
1
3
2
1
2
[
1
6
1
The
No. 3
Review
of Physical
Chemistry
of Japan
Vol. 12f No.
EMISSION SPECTRUM OF TIIE FLAME OF BROMINE
Plate
137
I.
The Emission Spectrum of the Flame (Ref., Fe-arc spectrum).
43 emission bands arc observed in the. range is 688o-sooo A. The fine structure
of each band is obscure. but the band-head is distinctly observed, and all the
bands shade to the red. The band-heads of these emission bands were measured,
being referred to the Fe-arc spectrum- In Table I are given the wave-length 2,
the wave number in vacuum Y and the intensity roughly estimated.
(iv) The carrier of the emission bands
From the fact that the emission band system in Plate I is relatively of simple
structure
it is probable that
the carrier of the spectrum is a diatomic molecule.
The diatomic molecules present in the flame are hydrogen, hydrogen bromide and
bromine.
The band spectrum belonging to hydrogen is known as many lined
spectrum which exhibits innumerableemissionlines and is not of an ordinaryhand
form.
Hydrogen
Wolfl
bromide
and Binkele2)
continuous
has a continuous
found
spectrum
in the
in the
discharge
ultraviolet
ponding
to this could
be found
evident,
therefore,
the emission
that
two kinds of molecules.
will be the bromine
Many
made.
on
and
number
the
numbers
Kuhn2),
coincide
to the conclusion
2)
3)
4)
5)
column
the
The
absorption
wave
Nakamura4j
by Brown
third
but
of bromine
in the
bromide
no emission
flame
it is considered
the absorption
the flame in question.
wave
of hydrogen
do
Weizel,
a
spectrum
corres-
and
hydrogen.
not
belong
that the carrier
wavy
It is
to these
of the bands
molecule.
researches
by
flame
bands
as far as we know.
tube
region,
in the
Accordingly,
Let us compare
measured
spectrum
spectrum
band of bromine
length
and
lately
other
band
these
values
In Table
been
band of
was
II are given the
it is found
within the experimental
that the emission spectrum
have
of bromine
emission band by the author
Comparing
well with each
Browns).
vapour
with the emission
of the absorption
and that of the
resp.
of bromine
in the second
that both wave
error.
of the flame is ascribed
This leads
to the bromine
W. Weizel,•11. W. Wolf u. If. E. Biilkele, 7.. physik. Chem.(B), 10, 459 (1934
It. Kuhn, Z. PGysik,39, 77 (1926),
U. Nakn,nura, Alnu. Coll. Sc. A)41o Imp. Univ. (A), 9, 335 (1926).
\V'. 1:. Brown, P/rvs. R.v., 38, 1179 (1931) 39. 777 (1932).
3(1938)
i
The
I38
Review
of Physical
Chemistry
of Japan
Vol. 12f No.
T. KITAGAWA
Table
(bmparLon
of
the. Absorption
Vol.
II.
Hands of Bromine
with the N mission
Bands
of the Flame.
I
No.
ALsorptionbands
of bromine
(hy G., Brown)
em-t
Vibrational
quantum
number
(J, vt')
Lminsionbands
of the flame
(by the author)
cart
14541
14700
14700
14823
14857
I
2
2,
3
4
5
6
7
8
15169.2
(6,5)
i
9
10
15 311.6
II
15450.0
155855
15624.6
15714.7
15762.6
15845.1
15W.8
15967.2
16029.4
16078.4
16156.4
16214.0
1627&g
16343.4
16401.4
16472.2
16517.5
16594.4
16718.7
16789.1
16831.5
1691z9
17033.7
17150.0
12
13
14
15
i6
17
i8
i9
20
21
22
23
24
25
26
27
28
29
30
31
32
31
34
(7,5)
(s,5)
(9,5)
(7,4)
(10,5)
(8,4)
(11,5)
(9,4)
(12,5)
(10,4)
(8,3)
(11,4)
(9,3)
(12,4)
(10,3)
(13,4)
(11,3)
(14,4)
(12,3)
(13,3)
(11,2)
(14,3)
(12,2)
(13,2)
(14,2)
I
35
36
37
33
391
392
401
40.:
41
42
43
17259.8
173533
17368.4
17472.1
17467.1
17572.0
175794
17667.6
177599
17845.8
(15,2)
(x3,1)
(16,2)
(17,2)
(14,1)
(18,2)
(15,1)
(x9,2)
(20,2)
(21,2)
l
14978
14995
15135
15163
15269
15305
15443
15579
(15605)
15719
15763
15836
15893
15961
16921
16072
16155
16zo9
16276
I6.344
16398
16466
16508
16596
16712
x67Sx
16827
169i6
17039
17146
17230
17254
17354
17377
17467
17467
17563
[175637
17663
17762
17852
Calculated
value
Difference
cm-t
cm-1
14.55x4
14694.4
11
-6
14714.6
15
14832.9
10
14S6o.i
14967.8
3
-10
I5002.I
7
15i4o.6
6
15170.2
7
15275.5
7
15312.2
7
8
15450.7
15585.6
15624.6 .
7
20
x5716.8
15763.1
155443
15898.0
5
8
15969.1
16ozg.2
S
16077.7
16x56.7
6
16212.6
16z81.5
x6343.8
4
6
10
2
16400.5
3
16471.3
x6516.6
5
16596.1
0
16715.1
16788.3
3
7
16831.2
16913.1
17032.1
17148.2
17232.3
17260.4
173513
17368.8
174730
8
4
-3
-7
2
2
6
_3
-8
6
17467.4
0
17573.1
17579.6
la
17
17669.1
17761.0
6
-I
_3
17848.6
XII
3(1938)
The
No...
EMISSION
SPECTRUM
OF
TRR
Review
FLAME
of Physical
OF
Chemistry
BROMINE
of Japan
Vol. 12f No.
139
molecule. This conclusion is also supported by the fact that both the emission
band of the flame and the absorption band of bromine shade to the red region.
(v) On the emission band of bromine
Few studies on the emission band of bromine have been done as compared
with those on its absoption band, and any emission band system of bromine which
coincides well with the absorption has not yet been found) in the discharge tube.
According to Urey and Bates", a faint band spectrum in the range between
2) 4250 and 4675 A was found in the flame of oxygen which contained bromine
burning in hydrogen, and the spectrum was considered to belong to a compound
of oxygen and halogen. In the present experiment, however, any such emission
band was not observed in the wave length range.
According to R. S. Mulliken", the emission hand of bromine in the flame
(Plate I) shoud be due to the electron transition from the Out state to the '-"9+
state, for the normal state of a bromine molecule is
and its excited state On'.
Accordingly, the excited bromine molecules in the Oui' state must be generated
in the flame.
Before we consider how the bromine molecules are excited by the chemical
reaction in the flame, let us calculate from a band-head analysis the vibrational
energy which is possessed by the excited bromine molecule in the Ou'' state.
[II]
Band-Head Analysis and (1(r)-Curves of Br.,.
(i) Band-head analysis of the emission band
The band-heads in the emission spectrum (Table I) were analysed and the
results obtained are given in Table TIT,where the unit is given in cm, and the
intensity is indicated in parentheses. v' and v" in Table III are the vibrational
quantum numbers of the excited and the normal states of the bromine molecule respectively, and these values have been determined in the isotopic displacement
of the absorption bands by Brown.
Front the band-head analysis it is seen that the emission bands of bromine
in the flame have been emitted by the transition from the vibrational states v'=
6)
7)
8)
S1)
V. Uchida and V. (ta, ,JS. J. Phvt., 5, 59 (19^-5).
II. C. Urey and J. R.. Bates, Ph1 . Rev., 34, 1541 (1920R. S Kulliken, Rev. Afod. Phvs., 4, 1 (1932).
W. M. Vaidya has lately observed in the flame of ethyl bromide the emission bands of bromine
which coincide with the present author's (Pne. lnd. Acad. Sot [A), 7, 321 (1935)}
3(1938)
The
140
T.
Band
Review
of Physical
Vol.
Table I11.
of the Emission Bands in the Flame. [Br., Ou+_.IE6+]
(V and 9t' are the vii,rational quantum numbers of the excited
and the normal stales respectively.)
3
2
5
6
7
15163(7)
153o5(8)
15443(10)
15579(10)
14700(1)
14857(3)
14995(5)
15136(7)
15269(8)
14541(0)
14700(1)
14823(3)
4
6
7
8
9
IO
II
16781(3)
16gi6(5)
17039(5)
17230(1)
17354(2)
17467(2)
17563(1)
14
15
Vol. 12f No.
KITAGAWA
5
13
of Japan
Analysis
I
rz
Chemistry
17146(4)
17254(3)
17377(1)
17467(2)
17563(1)
17663(1)
17762(1)
17852(0)
x6
17
i8
19
20
21
16072(3)
16209(5)
16334(4)
16465(4)
16596(4)
16712(4)
15605(6)
15763(7)
15893(9)
16021(8)
16155(7)
16276(6)
16398(5)
16827(3)
16508(2)
lZg+
14978(5)
15719(9)
15836(8)
15961(6)
zz
ou+
U
7t0O
o=
1000
I;
Fit
o+ I6
2.
2$
24
Fig.
2
32
n
r
36
40
U(r)Curves of the Br_
44
molecule.
41
Id %.
XII
3(1938)
TheReview
ofPhysical
Chemistry
ofJapan
Vol.
12fNo.3(1938) '
No.
3
EMISSION SI'ECr'RUM
141
OF THE FLAME OF BROMINE
5-'-21 in the excited state to ,/'=1-.;7 in the normal state. Thus, the vibrational
quantum numbers ,/ and ,/' of the emission bands could be determined as follows:
r/=5. 6, 7........ 21 ;
./'=r, 2, 3........ 7.
(1)
The wave number v of the bromine
bands can be expressed by the following
Table IV.
Simplified
Expression
of the BandAnalysis
formula given by Brown :
of Bromine in the Transition
Ou+._^2•g+.
v=-15831.2+(t 63.81,/
Q O6crved
in the al sorption spectrum
by G. Brown.
- 1-597/2-0-0087"')-(322-7 171"
+ Ol,serned in the emission spectrum of
-f.1 .5?ors) cut-'.
the Br-1 t_tlmne by the author.
(2)
The calculated wave numbers being substituted the values of J and J' (1) in formula
0
given by Morse" thus :
U=E+Djt-eel
~1~TCtn
',
a=, J8, exewep//s cm- 1.
The vibrational energy G(v) of a diatomic molecule whose vibrational quantum
0
0
0
0
0
e
4
I
5
6
7
8
9
10
II
I
12
t
13
i
14
15
0
16
0 e 0
17
0
I8
0
19
0
20
0
21
0
22
(3)
4 15
6
23
0
24
O
48
Q
0
0
0
0
0
0
0
0
0
0
O
O
0
e
e
e
e
e
e
e
e
e
e
e
e
e
E) e
ED e
e
e
+
+
e
e
I
number is v is expressed by
1
G(r)=(rla
+2)-S
(tlel
?/+?) )+)',,,.(V+
2
Z) -...Cin
', (4)
9) W. Jevons, "Re(art
7
3
Brown in the absorption spectrum of bromine
vapour are represented by the sign 0 and
of the emission of the bromine bands let us
draw the U(r)-curves of the bromine molecule. The formula to express the potential
energy U in a diatomic molecule has been
3
2
bets of emission bands of the flame in the
third column.
In Table IV the bands observed by
(ii) U(r)-curves of Br_
In order to make clear the mechanism
2
0
(2) are given in the fourth column of Table
If, which coincide well with the wave nom-
those by the author in the emission spectrum
of the flame by the sign + for simplicity.
I
or land-.Sfectns of Diatomic AfokcWR4" 193%
+
The Review of Physical Chemistry of Japan Vol.12f No. 3(1938)
142
where
T. KITAGAWA'
y eo, is so small
v is
not
Table
V.
so
large".
that
Er,
the
D.,
th ird term
wr,
Vol.. X11
in the right side can be neglected
xam„ re and
a in these
formulae
when
are given
in
Tai ;IL V.
Molecular Constant-. a( the ]{.cited and the Nnrmnl St tes of the Itmmine 111'nlccelct9l_
ti,nstints
Units
cm'-t
ptt+-state
t2g+-state
D
E.
15910.5
-
volt
I D. I W.
kcal.I cm-t
o.462
io.65
1.961
45.zo
3827.3
t6o57-i
enl
is
cn.
'
to-acm
a
ioacmt
t65.39
1.59
z65
1.947
3z3.86
1.15
z.zx
I.655
The molecular constants in Table V have been adopted from Jevons' Report").
The U(r)-curves of the bromine molecule obtained by equation (3) is shown in
Fig. 2, where the potential energy U (cm) is taken as the ordinate and the
internuclear distance r (cm) as the abscissa. The upper curve indicates the excited
state and the lower one the normal state. The calculated vibrational levels are
shown by horizontal lines in the figure. The arrows downwards indicate the
processes of the transition for the emission bands of bromine in the flame.
The wave lengths of the emission bands starting from the vibrational states
such as z/<s are, from Frauck-Condon's principle, so long that they go beyond
the sensitive range of the photographic plates used, and from this reason they
have not been observed in practice. From Table III it is apparent that the intensities of the emission bands belonging to v'>15 are very small. It follows, therefore, that the excited bromine molecules at the Ou* state in the flame arc, in the
main, in such vibrational states as z/=ts-ro.
[III)
Chemical Reaction Mechanism.
(i) Mechanism of the emission bands of the flame
It has been clarified in the preceding section that in the reaction system of
the combustion of bromine in hydrogen there must be formed the bromine molecules which have been excited to the Ou* state. These molecules will be written
as Br..* hereafter. Let us consider how Br..* is formed in the chemical reaction
between bromine and hydrogen from the standpoint of the chemical reaction kinetics.
(t) How much excitation energy is required in the formation of the Br.-.*?
The inner energy of a diatomic molecule contains, in general, electronic, vibrato) In thinTable.D=D,-G(o),,whereG(o)isthe vibrational
energyat theabsolute
zeroo
temperature.
it) 1Y.Jevons,"Aepozd
on Band-.Sf
ctm of Diatomic
Alokade,,"1932,P. 280,
TheReview
of PhysicalChemistry
ofJapanVol.12fNo.3(1938)
No.:1
EMISSION
SI'F(-TRUM
(IF TRF.FLAME.
OF BROMINli
193
tional and rotational energies, of which the quantum of the rotational energy is so
small as to be neglected here.
According to Jevons't), the electronic energy F-,,, required to excite the bromine
molecule from the '.t.'Rstate to the Out state is 45,000 cal. (J°•0f=t5831cm '). Let
is,' and EV' represent the vibrational energies of the molecule in the excited and
the normal states respectively, then the required excitation energy W is expressed
thus:
(5)
The vibrational energies Ez/ and ET/' can be calculated by formula (4), using the
molecular constants given in Table V'%.
(2) The origin of the excitation energy 6V should be brought forth from the
chemical reaction heat Q. If so, the amount of the reaction heat has to be enough
for the excitation energy.
It has been admitted by many experimental results that the reaction :
Br_+ I L,=2 H Br+ 24,300cal.
proceeds according to the following reaction mechanism":
Br.=2Br-45,20ocal.,
Br+H,= ,,HBr+H-t6,2oocal.,
and
1-I+Br.,=HBr+Br+4o,5oocal.
(6)
(7)
(8)
(9)
The bromine atones formed in the primary reaction (7) bring forth successivelythe
secondary reactions (8) and (9).
In the reaction system at higher temperatures such as the flame, it can be
considered that the alternate repetition of the reactions (8) and (9), namely chain
reactions, may take place. In the chain reactions the reaction (9) is only an exothermic one. It follows, therefore, that the excitation energy IV must have
originated from the reaction heat produced in (9). Then, is the reaction heat Q=
40,5o--cal. of reaction (9) really sufficient for IV?
a) If Ez/=o in equation (5), then
l410)= E„n,-EJ',
(10)
where W(o) represents the energy required to excite Br. to the lowest vibrational state of Br,,*. It the reaction heat Q is considered to be not smaller than
12) Rv-hc• C(v)erg., and betweenthe Lot
its of energytherelobesthe following
relation:I volt
=8ro6ern-'=23,o55cn1=x.58X
Io-'erg/moleculeI
t3) K. F. Bonhoeferu. I'. Iiarteek,„Gnradlosnn
derPhotorhemie,"
1933,S. 233•
The Review of Physical Chemistry of Japan Vol. 12f No. 3 (1938)
T.
KITAGAWA
Vol.XII 144
and, therfore,
Substituting
(12),
~4 ,5oocal. which corresponds
~4,5oocal.
in
wehave
E„,a=45,ooo
L•v'
to the energy of the vibrational state of Br_ whose vibrational quantum number
v' is 5. This shows that the Br_ molecule which has been in the vibrational states
higher than v'=5 may be excited to become Br*. And in fact the presence of
the molecules in such vibrational states higher than v'=5 is evident from the
appearance of the absorption bands belonging to these states in the absorption
spectrum of bromine vapour (Table IV). With rising temperature the molecules
can advance to higher vibrational states by gaining vibrational energies, and so it
is apparent that in the case of higher temperatures such as the flame a number
of molecules in the states higher than v'=5 exist.
b) The excitation energy required to excite the molecule to the vibrational
state 7/=i, in the excited state is
YV(
15)=E:+ Fr/ -EV',
(to)
where Ev=6,ooocal, for v'=[5 from calculation. When we assume that the mean
collision energy (EH,=5,ooocal.) possessed at the temperature of the flame participates in the excitation, we have
Q+E >E +EvI-Ev",
F-v">5,500cal.
The vibrational state of Ev"=5.5oocal. corresponds to the vibrational quantum number v"=6.
Therefore, the Bre molecules in the vibrational states such
as v'>6 will be excited to the vibrational state such as v=15 in the Ou+ State.
The excitation of the bromine molecule in the flame can be explained from the
reaction energy Q=4o,5oocal. generated in reaction (9)t"
(3) As at the temperature of the flame (about Boo°K) the mean kinetic
energy of molecules is only 2'3 kcal., it can not be considered that the whole of
the reaction heat 40.5 kcal. turns into the translational energy of the molecules
generated, i.e. I-IBr and Br, but that almost the whole becomes the inner energy
of those molecules. l-IBr or Br, however, has none of the electronic excited state
in the neighbourhood of the energy level as 40.5 kcal. Accordingly, if the rotational energy of HBr is not taken into cosideration, Q is to be accumulated as
14) In the chemical
reactionsystemof the nameIt- and Br-atoms
haveof coursebeengenerated,
butin thespectrum
of theflameneitherlinespectradueto theseatomsnor many-linedd
spectrum
belonging
to the II_ moleculecouldbe found. This may be ascribedto tile fact that the
excitation
energyIVof theseatomsor molecules
is farhigherthanthe reactionheatQ of reaction
(9). For the appearance
of the,emission
spectraof II_, If and Br in the observable
rangethey
require'e
at leastthe following
energies:IT-,318local.:II (Balmere), 276kcal.i
Br,15okcal,
The Review of Physical Chemistry of Japan Vol. 12f No. 3 (1938)
No. 3
EMISSION SPECTRUM -OF TIIE FLAME OF BROMJNE
146
the vibrational energy of f-IBr immediately after reaction (9). Let HBr* represent the hydrogen bromide molecule possessing so much vibrational energy. The
calculation made"' shows that it is possible for the normal hydrogen bromide molecule to have the energy 40.5 kcal. if it is in the vibrational state whose vibrational
quantum number rP is 6. In the studies of the infra-red absorption spectra of
hydrogen bromide the vibrational states such as ;P=2, r, o in the '-Y state have
been observed and in the near future the presence of such a vibrational state as
J'=6 will be ascertained.
(4) Let us assume that the mean life of I-1Br` is considerably long and its
vibrational energy is scarcely lost before collision. Then we can imagine that in
the collision of I-I13r*with Br, the excess energy of HBr* may be transferred to
the colliding Br2 molecule, and the latter may be excited, thus:
Br,('_`,)+IlBr*=Br,*(Ou*)-f-IMBr.
(13)
Br._` formed in (13) will again return to its normal state emitting the band spectrum
in the flame, thus :
Br,*(Ou+)=Br.('_''R)+fiv,
(14)
where v is the frequency of the emission band.
In short, the mechanism of the emission of the band spectrum is explained as
follows: the reaction energy Q produced in 'reaction (9) is accumulated in
I-IBr as the vibrational energy immediately after the reaction, and when it collides
with Br„ this molecule may be excited. The excited Bre molecule presents the
band emission while it returns to its normal state. As the energy of the spectrum
in this case originates from the chemical reaction between bromine and hydrogen,
the emission of the band spectrum of the flame can be regarded as cliemiluminescence.
(ii) Branching mechanism of the chain reaction
It has been admitted that a combustion or explosion reaction has a mechanism
of a branching chain reaction. Let us consider the branching mechanism of the
chain in the combustion of bromine end hydrogen.
In Table V the dissociation energy of Bra is 45.2 kcal., which is approximately equal to its excitation energy. Therefore, when HBr.:* collides with Br„
there occurs not only the excitation (13) but also the decomposition of Br,:
Br,('S)+1-IBr*=2Br+IIBr.
(t5)
r5) The calculation
was done accordingto formula(4) by applyingthe molecular
constantsw.
~z647cm-'andxaw, 44cm-'of HBr(lE).
TheReview
of PhysicalChemistry
ofJapanVol.12fNo.3 (1938)
146
T. KITAGAWA
Vol.XII
The Br atom thus generated becomes the origin of a new chain reaction, and thus
the chain branches.
Other ways of branching of the chain reaction may also be considered. For
example, (A) Br.* has the excess energy just sufficient for the dissociation energy
of the normal By molecule as its excitation energy. Therefore, if Br.* collides
with Br., according to the resonance of energies the latter decomposes thus:
BT.('S'r')+Br.*(Ou*)=aRr+Brz("r*).
(l6)
(B) The dissociation ]teat of 13r._*being to.7kcal. as seen in Table V, it is far
more unstable than the normal molecule, and apt to decompose even by simple
collision, which has been confirmed by W. Jost"1 in his study of the photochemical
reaction between bromine and hydrogen.
In these ways, the chain reaction branches so frequently that the reaction
velocity increases very rapidly and either explosion or combustion occurs. As the concentration of Br.' is extremely
Fig 3schematic Expression
low compared with ,that of I3r_,it seems more probable that
of the Branching
of the
the chain reaction branches according to the scheme as-(8)_
Br_•IL-Chain
Reaction.
t
which is shown in Fig.
Br
Which of the two reactions, (13) or (i 5), has larger probability to take place can not be easily discussed here. If
B`
we assume,
however,
that bothreactions,
(13)and(15),
HBt*\Br
proceed
atthe
same
time,
itisconsidered
that
the
former
J+Br
a
explains the mechanism of the band emission observed in the
flame and the latter that of the branching of the chain
reaction.
B
r
BT
Summary.
i) In the study of the enti>sion spectrum of the flame of bromine burning in
hydrogen, about 43 emission bands have been found in the range between AT.
6875'
and 56ooA, and the wave length of the band heads measured.
2) In comparison of the emission band'of the flame with the absorption band
of bromine vapour, both wave lengths have been quite -in good agreement,
and so it has been clarified that the emission band in the flame belongs to the
bromine molecule and that its electron transition is thus: Ou'
3) From the results obtained by band-head analysis the vibrational quantum
r6) W.Jest, Z.physik.Chem.,134,92(19x3);(13),3. 95 (19x9)•
TheReview
of PhysicalChemistry
ofJapanVol.12fNo.3(1938)
No.
EMISSION
SI'ECCRUM
OF THE FLAMEOF BROMINE
147
numbers belonging to the emission bands of bromine have been determined as
follows :
J=5, 6, 7. ....... 21 ;
v'=1, 2, 3, ......, 7.
It has been assured that the excited bromine molecules in the Ou+state are generated in the chemical reaction system of the flame.
q) The process of the band emission has been explained by the U(r)-curve
of Br, drawn by Morse's function.
5) The mechanism of the generation of the excited bromine molecules during
the combustion reaction has been discussed from the standpoint of the mechanism
of chemical reaction and it has been concluded that the band emission of the flame
is a sort of chemiluminescence.
6) The mechanism of the branching of the chain reaction in the case of the
combustion reaction has been proposed thus:
(8) Br+H_=1TBr+H,
(14) Br.*=Br_+hv,
(9') H+Brs=[-IBr*+13r,
(15) IlBr*+Br.=HBr+zBr,
(13) HBr*+Br.,=HBr+Br.*,
(t6) Br_*+Br.=Br_+zBr.
of these, (8)-(9')-(13)-(I4)
represents the mechanism of the band emission and
or (8)-(9')-(13)-(16)-'the
mechanism of the branching of the
chain reaction.
In conclusion, the author has great pleasure in expressing his sincere thanks
to Professor S. Horiba and Professor M. Kimura for their valuable guidance during
the course of this work. He also acknowledges his indebtedness to the Japan
Society for the Promotion of Scientific Research for a grant-in-aid.
The Laboratoryof I'.iyrfml Chemielna
Kyito Imperial i nieexity.