Photoabsorption and fluorescence cross sections of HCl are investigated in the wavelength region between 105 to 220 nm. The oscillator strengths of discrete structures at wavelengths shorter than 130 nm are measured

Lee, L. C.

Nee, J. B.

Suto, M.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19850021731 2020-03-20T17:51:14+00:00Z
Quantitative Studies for Photoabsorption and
Fluorescence of HCl
J. B. Nee, M. Suto and L. C. Lee
Department of Electrical and Computer Engineering
San Diego State University
San Diego, CA 92182
ABSTRACT
s
Photoabsorption and fluorescence cross sections of HC1 are investigated
in the wavelength region between 105-220 nm. The oscillator strengths of
discrete structures at wavelengths shorter than 130 nm arc measured.
	 q
(NASA-CR-175965) ^UAKITbTiVE STUDIES ICS	 NES-30C43
PHCTOABSORPTIOb AND FLUOREECEKE CF HC1 (Sao
Diego State UBiv., Calif.)	 14 p
HC s021!!F A01	 CSCL G7D	 Unclas
G3/25 240(5
I. INTRODUCTION
The absorption spectrum of HC1 was analyzed first by Price l in the
early thirties. Spectroscopic investigations have been done recently by
	 It
Ginter, Tilford, and their colleagues 2 ' 3 ' 4 and by Doulgas and Greening-5 	
J
Jacques and Barrow  have studied the emission features of HC] from the B(V)lE+
state. Absorption cross sections of HU have been investigated by Myer. and
Samson, 7 Inn, 8 and Smith et al. 9
 The work of Inn was in the wavelength
region of 140-200 nm, which is caused by the absorption continuum of the Airl + X1 E+
tranEition. Smith et al. 9
 carried out the measurement of accurate oscillator
strength of the C 1P. state. Myer and Samson have measured the absorption cross
sections in the 105-210 nm region. Generally speaking, the absorption cross
sections in the 105-130 nm region have not been accurately determined,
especially at wavelenghts below 120 nm region, where spectrum is very complex
as indicated from the works of Refs. 2-5. Assignments of the many absorption
features are still uncertain. A study by combining quantitative measurements 	 1s
of both absorption and emission could give some useful hints for understanding
the nature of higher states of HC1.
In addition, to the interest in spectrosopy, the quantitative data of
	 I
HC1 are important for the studies of planetary atmospheric chemistry and inter-
stellar chemistry. The existence of chlorine atoms in the earth's stratosphere
have generated great concerns in recent years for their relation to t : .e balance
of ozone at altitudes about 25 Km. 10 HC1 is a stable molecule resulted from
the chemical reactions involving species Cl and C10. HC1 has a long chemical
lifetime in the stratosphere, and only chemical reactions (mainly with OH) and
photodissociation can destroy it. In addition to the existence in the earth's
atmosphere, HC1 has also been found in the stratosphere of Venus, whose photo-
1
-2-
chemistry has been discussed. 
10 
In the interstellar medium, HC1 has been
predicted to exist in clouds containing both hydrogen and chlorine species, 11
however, so far the existence of HC1 has not been confirmed by observation. It
is likely that HCl is destroyed by photodissociative process occuring in the
interstellar diffuse clouds. The quantitative photoabsorption and fluorescence
data are thus needed for de r ermining the destruction rate of HU in the inter-
stellar medium.
II. EXPERIMENTAL
The set-up for this experiment is the same as our previous mepsurements.12
In short, synchrotron radiation dispersed by a 1 m monochromator (McPherson
225)entered into a photoabsorption cell through a LiF window. The monochormatic
light intensity was occasionally measured with a copper plate, but more often
monitored by a photomultiplier tube (PMT) with a LiF window exte rnally coated
with sodium salicylate. Fluorescence signals produced from the photoexcitation
of molecules were measured by two photomultiplier tubes, one has a cathode sen-
sitivity {n the VW and another in the UV-visible wavelengths.
HU gas was obtained from Matheson with a purity of at least 99.07. No
further purification was made for the experiment. The ohotoabsorption spectrum
obtained from the measurements did not show any absorption features caused by
the most possible impurities such as 0 2 or CO2 . The gas in the absorption cell
was slowly pumped with a sorption pump at constant flow rate and pressure.
A MKS Baratron (Model j10 CH) was used to measure the pressure. All data
including synchrotron electron beam current, gas pressure, absorption signal,
fluorescence signals were processed by an IBM personal computer:
-3-
ri
13
i
I
III. RESULTS AND DISCUSSION
1. Photoabsorption Cross Section
The photoabsorption cross section of HC1 was measured over the 105-200
nm wavelength region. The absorption cros.3 section was determined by comparing
light intensities with and without gas attenuation. The results are shown in
Figs. 1-3. The previous studies have shown that the absorption spectrum for
X. < 120 nm consists of sharp structures. Since the resolution of the monochro-
mator we used (0.03 nm) is much larger than the line width of the absorption
structure, our measurement only gives an averaged cross section over the
monochromator bandwidth.
The absorbances measured at several absorption peaks do not vary 	 +
linearly with the HC1 pressure. This may be due to saturation of sharp absorp-
tion lines existing within the peak. The absorption cross section of the	 a
sharp lines may be so high that they totally absorb the light even at very low
pressure, and further increase of IIC1 pressure would not attenuate the light
intensity any further. Since a linear dependence of the absorbance on pressure
was observed at the continuum (e.g. 115 nm), the saturation is definitely not
caused by the formation of dimer. To minimize the saturation effect, the
experimental. cross sections were measured at low concentrations, normally
-3( 3 -5)x10 13 cm	 The measured values shown in Figs. 1-3 should be considered
as the lower limits because of the saturation effect. Nevertheless, the
absorption cross sections at the continuum are quite accurate, whose experiment 	 4i
uncertainties are estimated to be within 10% of the given values.
The band origins, vo , of various excited states of HC1 have been given
by Ginter and Ginter 4 and by Douglas and Greening. 5 In general, the positions
of vo do not coincide with the absorption peaks, but usually within 0.3 nm
-4-
-5-
The assignment of the absorption peak shown in Figs. 1-2 3s based on the
v  value that is found nearby the peak and on the absorption strength which is
comparable with the value given by Douglas and Greening. 5
 Only the strong
absorption peaks are labeled in Figs. 1-2 in order to avoid any ambiuity caused
by the mixture of weak absorption peaks.
Oscillator strength can be calculated from absorption cross sections
according to the following formula
F - 113Ioda/a2
0
with X in A and o in Yb(10 -1 cm2 ). Oscillator strengths for the absorption
peaks given in Figs. 1 and 2 are calculated as shown in Table 1. Compared
with the data of Smith et al. 9 , our oscillator strength for the Cl ^(v' = 0) state
is low. However, for the higher v' states the agreement is reasonably good.
The absorption cross sections at wavelengths longer than 135 nm (corre-
sponding to the Al i	 X 1 %+ transition) are shown in Fig. 3. At wavelengths
between 135 and 170 nm, our measured cross sections range from 10 to 20% lower
than Inn 's. 8
 The largest discrepancy occurs at the absorption peak around 155
nm. However, a good agreement is found for wavelengths longer than 170 nm.
The strong dissociative nature of the A1  state is responsible for the most	 a
photodissociative HC1 in the atmospheres of earth and Venus. However, in the
earth's stratosphere, photodissociation of HC1 is not a significant loss process
compared with the reaction of OH + HC1, because of the low absorption cross
section at wavelengths longer than 180 nm where the 0 2
 transmission window
starts.
2. Fluorescence
The emission of HC1 from the electronic state B(or V) 1 £+
 has been reported
by Jacques and Barrow. 6 We have measured the fluorescence cross section of HC1
with a UV-visible PMT. No fluorescence signals have been detected with the
solar blind PMT (response in 115-200 nm). Both the fluorescence and absorption
cross sections were measured simultaneously at a monochromator resolution
of 0.3 nm. The results are shown in Fig. 4.
The fluorescence sigr n is can be expressed analytically as
-n•Q•^.	 1
If 
_	
fC•Q ne	 l+Tkn
where C is the constant relating to geometric factors and detector efficiency,
of is the fluorescence cross section, n is the concentration of HCl molecule,
a is the total absorption cross section, k is the optical. path length from
entrance to the PMT view region, T is the radiative lifetime of the electron
ically excited state of HC1. The term of Tkn is small at low pressure. By
comparing with the OB(A - X) photofragment f^uorescence from photodissociation
of water vapor, the cross section 0  in Eq. 1 can be obtained. The fluorescence
cross sections were measured at HC1 concentrations around (1-3)x10 14 cm- 3.
The fluorescence observed in the ITV-visible region is mostly produced
et excitation wavelengths shorter than 120 nm as shown in Fig. 4. The excited
state may also fluoresce at wavelengths shorter than 115 nm through the
resonance scattering process. However, our VITV PMT has a MgF 2 window which
does not respond at wavelenghts sno-ter than 115 nm. The fluorescence we
detected mainly results from transitions between the excited upper states and
an intermediate excite state (most likely the A 1 T state). For the excitation
wavelengths in the 115-1.20 nm region, the fluorescence is mainly due to the
B(or V) 1 F+ state, whose emission is known to be in the 200-300 nm region.6
Fig. 4 shows several states that emit, these including H 1 E+ and M 1 T.
The quantum yields are low for these two states compared with the V 1 E+ sta*-.
These states are likely to be strongly dissociative.
-6-
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TABLE 1
Oscillator Strength for Strong Absorption Bands of HC1
State X(nm) AX(nm) op(10-18cm2) f f
(ref.	 9)
b 37 1 (v 1-0)' 133.2 0.6 0.0026
v 1= 1 128.6 0.4 0.0068
v l= 2 124.4 0.4 0.0026
v 1-3 120.7 0.4 0.0013
H 1 E+ (v l-0) 112.8 0.5 35 0,0068
111n(v 1=0) 111.6 0.5 61 0.015
M1n+H 1 E(v 1= 1) 110.4 0.5 126 0.027
(unresolvable)
1 3 E-(v 1=0) 111.9 0.2 16 0.0029
N 17(v 1= 0) 111.1 0.2 11 0.0020
1m317	 -0) 111.0 0.1 16 0.0015
j 3 1- (v 1 =1) 109.3 0.4 16 0.0061
r 3 TT(v 1= 0) 109. 0.4 19 0.0072
K 1n(v 1= 1) 108.4 0.3* 18 0.0052
R
1
7(v 1= 0) 108.2 0.3* 22 0.0064
m 3n(v 1= 1) 107.7 0.2* 34 0.0066
H 1 S.+ (v 1= 2) 107.6 0.2* 28 0.0055
11=1)M TT(v 107.3 O.A. 21 0.0041
C 1TT(v 1 =0) 129.0 0.8* 515 0.10 0.185
(v 1 =1) 124.9 0.7 88 0.0184 0.022
(v 1 -2) 121.0 0.3 13 0.003
estimated
,^X: bandwidth, a p : peak absorption cross section, f: oscillator
Strength calculated with f = 113fadX/a2
C
Y
1. W. D. Price, Proc. Roy Soc. (London) A167, 216, (1938).
2. S. G. Tilford, M. L. Ginter and J T. Vanderslice, J. Mol. Spectrosc.
33, 505, (1970).
3. S. G. Tilford and M. L. Ginter, J. Mol. Spectrosc. 40, 568, (197 ).
4. D. S. Ginter and M. L. Ginter. J. Mol. Spectrosc. 90, 177 (1981).
5. A. E. Douglas and F. R. Greening, Can. J. Phys. 57, 1650, (1979).
6. J. K. Jacques and R. F. Barrow, Proc. Phys. Soc. London, 73, 538 (1958).
7. J. A. Myer and J. A. R. Samson, J. Chem. Phys. 52, 266, (1971).
8. E. C. Y. Inn, J. Atmos. Sci. 32, 2375, (1975).
9. P. L. Smith, K. Yoshino, J. H. Black, and W. H. Parkinson, Astrophys. J.,
238, 874 (1980) .
10. Y. L. Yung and W. B. DeMore; ICARUS, 51, 199, (1982).
11. E. F. Van Dishoeck, M. C. Van Hemert, and A. Dalgarno, J.: Chem. Phys. 17,
3693, (1982).
12. M. Suto and L. C. Lee, J. Chem. Phys. 80, 4824, (1984); L. C. Lee, ibid
72, 4334 (1980).
13. M. Bettendorff, S. D. Peyerimhoff, and R. J. Buenker, Chem. Phys. 66,
(1982), 261-279.
1
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FIGURE CAPTIONS
Fig. 1 Photoabsorption Cross Sections of HC1 at :.05-120 nm. The docted
line at 110.4 nm indicates the shape of the absorption band.
Fig. 2 Photoabsorption cross sections of 9C1 at 120-130 nm. The dotted
•
lines at 124.5 and 129 nm indicate the shape of the absorption
bands.
Fig. 3 Photoabsorption cross sections of HC1 at 135-200 nm.
Fig. 4 Spectra of photoabsorption cross section (full line)and and fluores•:ence
cross section (dotted line) of HC1 measured at a monochromator of
0.3 nm.
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Quantitative studies for photoabsorption and fluorescence of HCl

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