The Iodine Lamp: A Light Source for Selective Excitation of C O
B y
P.
HARTECK,
R . R . REEVES, JR.,
and
B. A .
THOMPSON
Rensselaer Polytechnic Institute, T r o y , N e w Y o r k , U . S . A .
( Z . Naturforschg. 19 a, 2 — 6 [ 1 9 6 4 ] ; eingegangen am 15. O k t o b e r 1963)
Dedicated
Dr. W . GROTH on his sixtieth
to Prof.
birthday
A light source has been developed f o r photochemical studies which emits the 2 0 6 2 A line of
iodine. This lamp has been used to study reactions of C O excited in the a 3 Z7 level. Carbon
m o n o x i d e was irradiated in an experimental l a m p i n c o r p o r a t i n g an arrangement of concentric
cylinders. T h e output of this lamp was 2 - 1 0 1 8 quanta per s e c o n d . T h e C O a 3 Z7 m o l e c u l e s react to
form C 0 2 with a quantum efficiency between a f e w tenths and unity.
A series of lamps exists for photochemical studies
in the near and far ultraviolet. Most commonly used
are the mercury lamps with the two resonance lines
at 2537 and 1849 Ä. These lamps are convenient
in many cases but suffer from the disadvantage that
the gases used must be extremely clean with respect
to mercury to prevent interference with the overall
process by reactions with excited mercury atoms.
Many years ago the xenon resonance lamp with
its two resonance lines at 1470 and 1295 Ä was
developed 1 . Such lamps were used for many interesting investigations in H A B E R ' S institute, in Hamburg 2 , and in Bonn [see, for example, references 3
and 4 ] , and are today extensively used by many experimenters. Later on the krypton lamp 5 and the
hydrogen lamp using L Y M A N alpha radiation were
developed along similar lines. All these lamps are
very useful for investigations in the far ultraviolet;
however, except for N2 and CO which absorb only
to a minor extent at these wavelengths, all gases
have very high absorption coefficients and selective
experiments cannot be performed.
There is a special interest in the photochemistry
of CO because it has been shown by radiation chemistry 6 and by the work of V O N W E Y S S E N H O F F , H A R T ECK, and D O N D E S 7 as well as the recent work of
G R O T I I , PESSARA, and R O M M E L 8 that CO if excited,
1
may react with another CO forming C0 2 and carbon
suboxide:
CO + CO* —> a) C0 2 + C
b) C 2 0 + 0
Such reactions are of interest in themselves but
are of further interest in connection with the chemosphere of Venus where CO may be expected to be
present as a result of photodissociation of C0 2 9 .
In our recent investigation of ultraviolet absorption
spectra 9 ' 10 we became aware that the transition of
C O from its ground state to the a 3 /7 level (CAMERON
Bands) has an average absorption coefficient of
about 0.01 c m - 1 . Previous work 6 - 8 has been concerned with excitation of CO to higher levels where
both reactions a) and b) could occur. Excited a377
molecules, however, should undergo only reaction a).
On the basis of energetic considerations reaction
b) should not be expected to occur. In discussing
all possible light sources for producing CO a 3 /7 molecules, we realized that a certain very strong transition in the iodine system 5p 5 2 P 1 /2->6s 2Ps/2 fell
just within the (0,0) band of this CO transition. We
have therefore developed an iodine lamp for the
special purpose of exciting the a 3 /7 level of the CO
system which thus enables us to investigate the behavior of this metastable excited CO species. The
77
0
See, for example, W . GROTH, Z . Phys. Chem. B 3 7 , 3 0 7 , 3 1 5
[ 1 9 3 7 ] ; ibid. B 3 8 . 3 6 6 [ 1 9 3 7 ] .
7
W . GROTH a n d H . SCHIERHOLZ, J . C h e m . P h y s . 2 7 , 9 7 3
8
P . HARTECK a n d
F . OPPENHEIMER, Z .
Phys.
Chem.
B
16,
[1932].
2
3
[1957];
5
W . GROTH
and
H . VON WEYSSENHOFF, P . HARTECK, a n d S . DONDES, t o b e
W . GROTH. W . PESSARA, a n d
N. F. 32, 192
H . v o x WEYSSENHOFF,
Naturwiss.
[ 1 9 5 7 ] ; A n n . Phys., Lpz. (7) 4, 69
[1959].
W . GROTII, Z . E l e k t i s c h e m . 3 8 , 7 5 2
[1954],
44,
510
Reinhold
pub-
lished.
Planetary S p a c e Sei. 1, 3 3 3 [ 1 9 5 9 ] .
4
See S. C. LIND. Radiation Chemistry of Gases,
P u b . C o r p . , N e w Y o r k 1961, p p . 1 1 2 - 1 1 6 .
9
P . HARTECK, R . R . REEVES, a n d
D-1984
,0
I I . J . ROMMEL, Z . P h y s .
Chem.,
[1962].
B . A . THOMPSON, N A S A
TN
[1963].
B . A . THOMPSON, P . HARTECK, a n d R . R . REEVES, J .
Res., in press.
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Geophvs.
absorption coefficient of CO for the 2062 Ä radiation emitted by this lamp is 0.0075 c m - 1 which is
quite adequate because at one atmosphere the energy
absorbed by CO in a reaction vessel of few centimeters in depth will be rather constant. Absorption
coefficients of other gases which may be present,
C0 2 , 0 2 , N 2 , H 2 0, are all less than 10~ 4 cm" 1 at
this wavelength. It is therefore possible to study reaction of this excited species without interference.
c m
Theory
The emission spectrum of atomic iodine contains
many strong lines in the ultraviolet extending from
2062 Ä to shorter wavelengths. The spectrum has
been analyzed in detail by K I E S S and C O R L I S S
and the energy levels have also been tabulated by
M O O R E 12 . Fig. 1 shows an energy level diagram for
iodine constructed from these tabulations. Many of
11
- 1
8 4 , 3 4 0
80,000
7 5 , 0 0 0
7 0 , 0 0 0
6 5 , 0 0 0
60,000
5 5 , 0 0 0
5 0 , 0 0 0
4 5 , 0 0 0
4 0 , 0 0 0
3 5 , 0 0 0
3 0 , 0 0 0
2 5 , 0 0 0
20,000
15,000
10,000
5 , 0 0 0
0
F i g . 1.
11
C . C . KIESS a n d
A,
1
[1959].
C . H . CORLISS, J . R e s .
Nat.
E n e r g y level d i a g r a m for iodine.
Bur. Stand. 63
12
C . E . MOORE,
Atomic
Stand. Circ. 567,
Energy
Levels,
Vol.
Ill,
Nat.
1958.
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Bur.
the close packed d and f levels are not resolved and
only a few of the strongest transitions are shown.
A detailed tabulation of all the observed transitions
is given by K I E S S and CORLISS. From the diagram
it can be seen that the intense line at 2062 Ä corresponds to a transition between the 6s
level
and the 5p5 2 P 1/2 rather than the ground state
5p 5 2Pä2 a n d thus is not a resonance line. It is also
clear from the spacing of the levels that at wavelengths longer than 2062 Ä there will be a large gap
in the spectrum with no lines being observed. In
fact, K I E S S and C O R L I S S report no lines between
2062 and 3552 Ä. In addition, it is of interest that
the next lines below 2062 Ä are at 1876, 1844, and
1830 Ä. Thus, by proper selection of window material, it should be possible to prepare a source of
virtually monochromatic ultraviolet light.
A further advantage of the use of iodine is that
its vapor pressure at room temperature is just under
a millimeter wrhich is ideal for use in a discharge.
Apparatus
T w o types of l a m p s h a v e b e e n u s e d f o r this w o r k .
T h e first was a small d i s c h a r g e t u b e a b o u t 6 " l o n g with
a quartz w i n d o w at o n e e n d . B o t h e l e c t r o d e and electrodeless discharges c o u l d b e a p p l i e d to this tube. C. P .
i o d i n e was i n t r o d u c e d into a side a r m and was purified
b y f r a c t i o n a l distillation. T h e m o s t persistent impurity
was w r ater v a p o r , which not o n l y c a u s e d h i g h pressure
in the l a m p and m a d e o p e r a t i o n erratic, but also resulted in strong emission of the O H b a n d s . W h e n this
had b e e n r e m o v e d , the l a m p was quite constant in its
output and the O H b a n d s w e r e n o l o n g e r o b s e r v e d . T h i s
small l a m p was used p r i n c i p a l l y f o r the determination
of absorption c o e f f i c i e n t s discussed b e l o w .
P h o t o c h e m i c a l e x p e r i m e n t s w e r e c a r r i e d out using
a second type of l a m p s h o w n in F i g . 2. T h i s consisted
of an arrangement of c o n c e n t r i c c y l i n d e r s a b o u t 1 0 0 c m
l o n g , the inner o n e b e i n g m a d e of quartz. Quartz wind o w s were p r o v i d e d at each end f o r p u r p o s e s of measuring the s p e c t r u m a n d m o n i t o r i n g the output intensity.
T h e outer P y r e x jacket c o u l d b e filled with any desired
gas f o r p h o t o c h e m i c a l studies. A r g o n was passed c o n tinuously through the inner c y l i n d e r and served the
dual p u r p o s e of p r o v i d i n g a stable d i s c h a r g e f o r excitation of the i o d i n e and acting as a b u f f e r to prevent
chemical attack of the e l e c t r o d e s b y i o d i n e vapor. Earlier e x p e r i m e n t s had b e e n c a r r i e d out u s i n g a static
system without a d d i t i o n of a r g o n , but the f l o w i n g system proved to b e much m o r e satisfactory.
T h e emission s p e c t r u m of these l a m p s was m e a s u r e d
with a J a r r e l - A s h r e c o r d i n g m o n o c h r o m a t o r and
was f o u n d to consist a l m o s t entirely of 2 0 6 2 A radiation
with the 1 8 7 6 A line and the various c o n t i n u a near
3 0 0 0 A being w e a k l y visible. T h e intensity of the
2 0 6 2 Ä line was a b o u t thirty times that of the 1 8 7 6 Ä
line.
Absorption Coefficients
Absorption coefficients of CO, NH 3 , N 2 0 for the
2062 Ä line were determined using the small lamp
mentioned above. The path length in each case
was 94 cm. For CO the iodine emission at 2062 Ä
falls just within the 2059.6 Ä (0,0) band of the
X 1 - T ^ - a 3 i 7 transition ( C A M E R O N Bands). The ab-
ARGON
U.V.
MONCHROMATOR
QUARTZ
QUARTZ INNER TUBE SECTION
WINDOW
Fig. 2. Schematic diagram of iodine lamp.
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sorption coefficient for CO for the 2062 Ä line was
found to be 0.0076 c m - 1 . Those determined for
NH3 and N 2 0 were 23.8 and 0.374 c m - 1 respectively. All those values are in good agreement with
those reported elsewhere 10 ' 13 . For CO the 1876 Ä
line, present to a minor extent, is not absorbed since
it falls between the (3,0) and (2,0) C A M E R O N bands.
Photochemistry
to provide an accurate reference value. The difference between the two concentrations thus represented the C0 2 formed by the action of the iodine radiation. Fig. 3 shows a typical plot of C0 2 formed as
a function of time. Clearly the C0 2 formation is
linear with time. Knowing the volume contained in
the lamp and the number of quanta absorbed, the
quantum efficiency for C0 2 production can, in principle, be readily calculated. However, the rate of
C0 2 production was found to be very sensitive to
changes in temperature. This indicates that the ab-
Calibration of Lamp Output
The output of the lamp shown in Fig. 2 wTas calibrated using the decomposition of ammonia as an
actinometer. The quantum yield for this process has
been determined at various pressures by W I I G
using zinc radiation of 2100 Ä wavelength. The reaction products are reported to be only nitrogen
and hydrogen in a 3 : 1 ratio. Since the absorption
coefficient of NH 3 is less than 1 0 - 4 c m - 1 at wavelengths longer than 2400 Ä, the weak iodine continua near 3000 Ä will not interfere with the calibration.
1 4
The calibration was carried out by filling the
outer jacket of the lamp with purified NH3 to a
pressure of 150 mm. The lamp was then operated
under normal conditions for 5 minutes. The intensitv of the 2062 Ä line was monitored continuously
with the recording monochromator. At the end of a
run a sample of the gas was analyzed mass spectrometrically and the increase in N 2 concentration was
determined. From this and the quantum yield data
of W I I G the output of the lamp was found to be
2 x 1018 quanta per second of 2062 Ä radiation.
Irradiation of CO
Carbon monoxide was purified from iron carbonyl, C0 2 , and other impurities by bubbling through
concentrated sodium hydroxide solution followed
by passing through successive cold traps. This purified CO was then introduced into the outer jacket
of the iodine lamp and irradiated for various times.
After irradiation the concentration of C0 2 in the CO
was determined mass spectrometrically. In each case
an analysis was made of the CO before irradiation
13
K . WATANABE,
M . ZELIKOFF,
and
E . C . Y . INN,
Absorption
Fig. 3. Formation of C 0 2 as a function of time.
sorption coefficient changes substantially with temperature, an effect which could be caused by broadening of the iodine line or changes in the relative
population of CO rotational levels or to a combination of these factors. In an effort to evaluate the
magnitude of this effect the absorption coefficient
of CO was measured using light from the lamp
shown in Fig. 2, thus providing a "high temperature" iodine line. The absorption coefficient varied
somewhat with the operating power level of the
lamp, but was about half that previously obtained
at room temperature.
These results show that changes in temperature of
the lamp can lead to substantial changes in absorption by CO. Because the operating temperature of
the lamp shown in Fig. 2 cannot be precisely con14
E. O. WIIG, J. Amer. Chem. Soc. 59, 827 [ 1 9 3 7 ] .
Coefficients of Several Atmospheric Gases, A F C R C Tech.
Rpt. 53-23,
1953.
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Conclusions
trolled, we can only state that the quantum efficiency
lies between a few tenths and unity. Experiments
currently planned using a new lamp design should
permit a more precise definition of this efficiency.
Irradiations wTere carried out at a variety of pressures between 200 and 700 mm and, as anticipated,
no pressure effect was observed since in this region
the absorption is linear and the collision lifetime is
many of orders of magnitude shorter than the lifetime for light emission.
In this work other reaction products such as carbon and carbon suboxide polymer, which should
also have been formed, were not investigated. In the
static system shown in Fig. 2 such products would
have accumulated on the vessel walls and would have
been very difficult to detect under the experimental
conditions employed.
The iodine lamp has been shown to be very useful
for the photochemical study of excited CO (a 3 /7)
molecules. The lamp provides specific excitation to
the a 3 /7 level in its lowest vibrational level. The
high quantum efficiency for reaction to form C0 2
provides an explanation for the fact that the C A M E RON bands are never observed in emission except at
very low CO pressures.
Acknowledgement
T h e authors w o u l d like to thank M r . R . W . W A L D R O N
f o r experimental assistance. T h i s w o r k was c a r r i e d out
under Research Grant N s G - 2 6 1 - 6 2 f r o m the N a t i o n a l
A e r o n a u t i c s and S p a c e A d m i n i s t r a t i o n and R e s e a r c h
Grant A F - A F O S R - 1 7 4 - 6 3 f r o m the A i r F o r c e O f f i c e
of Scientific Research.
Production of the O x y g e n 5577 A Emission
by Polonium-210 Alpha Radiation*
B y
S . DONDES,
P . HARTECK,
and
C.
KUNZ
Rensselaer Polytechnic Institute, Troy, N. Y.
( Z . N a t u r f o r s d i g . 19 a, 6 — 1 2 [ 1 9 6 4 ] ; eingegangen am 15. Oktober 1963)
Dedicated, to Prof. Dr. W . GROTH on his sixtieth
birthday
The production of the oxygen 5577 Ä emission in purified nitrogen at atmospheric pressure by
radiation with Po-210 alpha was studied spectroscopically. When the concentration of oxygen in
the nitrogen was one part in ten thousand, the most intense emission observed was that of the
forbidden atomic oxygen ('S —> X D) transition. This line emission at 5577 A was seen to be associated with a continuum that extended from approximately 5600 to 5400 A. To determine the reaction
mechanism producing this emission, the effects of an electric field, temperature, and concentration
of oxygen were examined. Several possible mechanisms are considered. The reaction producing
oxygen atoms excited to the *S state which we found most favorable is shown below.
N+ + 0
2
- *
An understanding of the primary processes induced by ionizing radiation and of the reactions of
the ions, atoms, and excited species formed are of
paramount importance in radiation chemistry. An
effective experimental approach to the problems in
this field is to examine the emission spectra obtained
by the irradiation of gaseous systems with the ionizing radiation of Po-210 alpha particles. It is the
purpose of this paper to discuss an interesting observation which was made in an investigation of this
type, namely, the presence of the forbidden auroral
green line of atomic oxygen ( 1 S - > 1 D ) at 5577 Ä
N 0
+
+
0(!S).
in the spectra resulting from the irradiation of purified nitrogen containing small amounts of oxygen.
This auroral green line is observed when the
oxygen concentration is about one part in ten thousand. The intensity of this line plus its associated
continuum exceeds the intensity of any other spectra
emitted under these conditions. Other than the associated presence of the 2972 Ä ( 1 S - > 3 P ) line, no
other lines of atomic oxygen (or atomic nitrogen)
are seen.
The 5577 auroral green line is well-known from
auroral and airglow studies in the upper atmosphere.
* This is an excerpt from a longer article to be published
in Nukleonik, Springer-Verlag.
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