Bing-Ming Cheng
Synchrotron Radiation Research Center, No. 1, R&D Road VI, Hsinchu Science-
Based Industrial Park, Hsinchu 300, Taiwan
Mohammed Bahou, Chao-Yu Chung, and Yuan-Pern Lee
Department of Chemistry, National Tsing Hua University, No. 101, Sec. 2, Kuang
Fu Road, Hsinchu, 30013, Taiwan
Yuk L. Yung
Division of Geological and Planetary Sciences, California Institute of Technology
150-21, Pasadena, CA 91125
L. C. Lee
Department of Electrical and Computer Engineering, San Diego State University,
San Diego, California 92182
Abstract
Cross sections for photoabsorption of HCl and DCl are determined in the spectral region 135-232 nm using radiation from a synchrotron light source. At wavelengths near the onset of absorption (

>200 nm), cross sections of HCl is approximately 5-10 times greater than that of DCl. These data are used to calculate rates of photodissociation of HCl and DCl by solar flux at the top of the Venusian atmosphere.
For the entire wavelength region measured, the rate of photodissociation of DCl is only
78% that of HCl. At wavelengths greater than 200 nm, the rate of photodissociation of
HCl is about seven times that of DCl. The difference in rates of photodissociation might contribute to the enhancement of the [D]/[H] ratio that is exceptionally large in the
Venusian atmosphere.

HCl is a minor constituent of the Venusian atmosphere, but it plays an important role in the photochemistry and atmospheric evolution [Prinn, 1971, 1985; McElory et al., 1973; Yung and DeMore, 1982, 1999]. Photodissociation of HCl by solar light produces H and Cl atoms that lead to the odd-hydrogen cycle ( OH and HO
2
) and the reactive chlorine cycle ( ClO, ClCO, and ClCO
3
) [Yung and DeMore, 1982]. HCl distributes mainly at altitudes between 60-70 km, below the CO
2
cloud [von Zahn et al.,
1983]. Because CO
2
absorbs strongly solar radiation at wavelengths less than 200 nm,
HCl is dissociated by solar light practically only at wavelengths greater than 200 nm.
It is generally difficult to measure photoabsorption cross sections at large wavelengths accurately, because the values are small and easily susceptible to distortion from impurities that absorbs more strongly in that region. To remove impurities from the sample gas deserves a special attention. As Inn [1975] pointed out, the data of Myer and Samson [1970] are distorted by impurities in this way so that their absorption cross sections are much greater than those of Inn at wavelengths greater than 180 nm. We expended a special effort to assure the purity of the sample and extended accurate measurements to 232 nm.
No photoabsorption cross section of DCl is reported in the vacuum ultraviolet region. The threshold wavelength for DCl absorption is expected to be smaller than that for HCl, because the zero-point energy of DCl is less than that for HCl. A shift to shorter wavelength upon deuteration is commonly observed for many molecules, such as water [Cheng et al.,1999], methane and ethane [Lee et al., 2001], and methanol
[Cheng et al., 2001]. However, taking into account of the singlet-triplet mixing effect,
Lambert et al. predicted that the photoabsorption cross section of DCl has a maximum at a wavelength greater than that of HCl [Lambert et al., 1998].
The [D]/[H] ratio in the Venusian atmosphere has a value about 100 times as large as those for terrestrial and other astronomical bodies [Donahue et al., 1982; Prinn,
1985; Yung and Dissly, 1992; Yung and Demore, 1999]. An understanding of such a large isotopic enhancement attracts current interest. Water vapor in the Venusian atmosphere is trapped below 60 km mainly by H
2
SO
4
. The major species that carries H to the upper atmosphere is HCl, not H
2
O. An H atom produced from photodissociation of HCl carries much kinetic energy to enable its subsequent escape from the atmosphere. In contrast, DCl that is less dissociated can exchange its D atom with other hydrogen-containing molecules, such as H
2
, H
2
O, H
2
S, H
2
SO
4
etc., so that the D atom is preserved. If variation of rates of photodissociation of HCl and DCl is nonnegligible, this selective dissociation by solar light might contribute to the enhancement of the
[D]/[H] ratio in the Venusian atmosphere. Hence, it is important to determine accurately

absorption cross sections of HCl and DCl, especially near the absorption onset.
The experimental setup is described in a previous paper [Cheng et al., 1999].
Briefly, vacuum ultraviolet (VUV) produced in the synchrotron radiation facility in
Taiwan is dispersed with a 1-m Seya-Namioka monochromator with a grating blazed at
140 nm (600 grooves/mm). Two slits of 4

2 mm
2
are installed along the beam path before entering the absorption cell to reduce scattered light. The beam size is 3.5

1 mm
2
at the entrance of the absorption cell and increases to 4.5

1.5 mm
2
at the exit window. Both entrance and exit windows are CaF
2
, which also served to eliminate second-order light. Spectra were recorded at 0.1-nm intervals; uncertainty of wavelength measurement is limited by the scan step.
The absorption cross section was measured with a double-beam apparatus. The light was monitored before the gas cell by reflection from a LiF plate placed at 45 o from the beam line. The VUV light was converted to visible light with sodium salicylate coated on a glass window, and detected with a photomultiplier tube
(Hamamatsu R955) in a photon-counting mode. The absorption cross section is determined from the absorbance A ,
A = ln ( I o
/ I ) = n

l , (1) in which I o
is reflected light intensity, I is transmitted light intensity, n is gas density ,
 is absorption cross section, and l = 385.73 mm is the length of absorbing path through the gas sample. The absorbance was adjusted to zero with the gas cell evacuated to less than 5

10
-7
Torr. The density of gas sample at room temperature was determined from its pressure monitored with a capacitance manometer (MKS-Baratron). A reservoir of
600 cm
3
in volume was connected to the sample cell so as to reduce possible interference resulting from surface desorption, photofragmentation, and variation in gas temperature. At each wavelength, the absorption cross section was determined from a linear fit of five to thirteen absorbance values measured at varied pressures. The gas pressures, 0.097 – 900 Torr for HCl and 0.169 – 217 Torr for DCl, are chosen such that maximum absorbance was less than 2 to avoid saturation effect. At 232 nm the absorbance of HCl is linearly dependent on gas pressure up to 900 Torr, implying that dimers and polymers do not contribute to the cross section measured.
HCl of purity 99.999% was purchased from AGA Inc. DCl with a listed isotopic purity 99% was from Cambridge Isotope Laboratories. Each sample was further purified by a freeze-pump-thaw procedure two times; the sample was frozen at 77 K and pumped for 20 minutes each time. DCl was further purified by vacuum distillation from 168 to 77 K. The sample gas was introduced into the cell while maintaining the

sample cylinder ~190 K. After this purification procedure absorption of possible impurities such as O
2
and H
2
O became undetectable in the VUV spectrum.
Photoabsorption cross sections of HCl and DCl in the spectral region 135-232 nm are determined at a spectral resolution of 0.1 nm, as shown in Fig. 1. The uncertainty is estimated to be 5% of each measurement. Data at 2.5-nm intervals are listed in Table 1 for comparison with those of Inn [1975] and of Nee et al. [1985]. A complete listing of absorption cross sections at 0.1-nm resolution is available at http://ams-bmc.srrc.gov.tw
.
As earlier data of Romand [1949] and of Myer and Samson [1970] were compared by
Inn [1975], they are not listed here. Our values are generally smaller than values of Inn
[1975], but slightly greater than those of Nee et al. [1986], but they agree within experimental uncertainties. Values for
 
215 nm of Inn [1975] are likely erroneous.
According to absorption coefficients of HCl shown in Fig. 1 of Inn’s paper, these values should be ten times as large. Our data agree better with these corrected values. There are no experimental data for comparison with our DCl measurements, but Lambert et al.
[1998] reported theoretical values; for example, calculated values are 3.5, 1.2, 0.05, and
0.0003 Mb (1 Mb = 10
-18
cm
2
) at 150, 170, 190, and 210 nm, respectively, comparable with our measurements listed in Table 1.
As absorption features shown in Fig. 1 are broad and continuous, photoexcitation into this region results to dissociation. The onset of absorption by HCl occurs at a wavelength ~10 nm greater than for DCl, as expected from consideration of relative zero-point energies. Such a phenomenon is observed for many molecules upon deuteration [Cheng et al. 1999, 2001; Lee et al., 2001]. For the same reason, the absorption maximum of DCl is expected to shift to a wavelength smaller than that of
HCl. However, this expectation is not observed in our data as shown in Fig. 1: the absorption maximum lies near 154 nm for HCl and 156 nm for DCl. Furthermore, in contrast to HCl, the absorption band contour for DCl is symmetric, shown in Fig. 1,
Without the singlet-triplet interaction, the absorption feature of HCl is expected to be symmetric as calculated by van Dishoeck et al. [1982]. The observed asymmetry was previously suspected to be a measurement error [van Dishoeck et al., 1982], but sufficient experimental measurements exist to establish that asymmetry is real.
This difference likely reflects a mixing of the ground ( X
1  +
) state of HCl with the
3 
0
state through spin-orbit coupling; therefore HCl has a component of transition to the repulsive excited state, a
3 
0
, which lies near the predominant state A
1 
1
[Givertz and Balint-Kurti, 1986; Lambert et al., 1998]. This singlet-triplet mixture serves to explain the asymmetric absorption band contour of HCl [Givertz and Balint-Kurti, 1986]

and the unusual direction of shift of absorption maximum upon deuteration.
Our observation might imply that the singlet ground state of DCl does not mix with the triplet state as much as for HCl. This assertion is supported with observations that the photodissociation yield of excited Cl
*
(
2
P
1/2
) atom from DCl is less than that of
HCl. In the spectral region 190-220 nm, the quantum yield for Cl
*
(
2
P
1/2
) is about 0.41 for HCl [Zhang et al., 1997; Lambert et al., 1998], but 0.28 for DCl [Ascenzi et al.,
1999]. Although theoretical calculations indicate that Cl
*
(
2
P
1/2
) is dominantly produced through spin-orbit interaction at long range, its yield might be affected by the adiabatic excitation process; that is, initial excitation to the triplet state produces excited Cl
* atoms. A smaller yield of Cl
*
indicates that DCl has a smaller component of the triplet state than HCl.
The rate of photodissociation R is calculated from the photodissociation cross section (

) and the solar flux ( I ), at the Venus by
R =

I

d

, (2) with

being wavelength. The solar flux at the top of the Venusian atmosphere is calculated from the solar flux at 1 AU corrected for the distance from the sun to Venus,
0.7233 AU. The solar fluxes at 1 AU measured at the top of the terrestrial atmosphere are taken from Rottman [1981] for the region 135-190 nm and from Mount and
Rottman [1983] for 190-220 nm. The fluxes are normalized at the overlap region so that they join smoothly. We assume that the cross sections for photodissociation is equal to the cross section for photoabsorption. Rates of photodissociation calculated for every
5-nm interval are listed in Table 2. The total rates of photodissociation in the range
135-232 nm, which are also listed in Table 2, are 6.28

10
-6
and 4.93

10
-6
s
-1
for HCl and DCl, respectively.
The rate of solar photodissociation of DCl is smaller than that of HCl, for which the ratio is also listed in Table 2 for each wavelength intervals. The ratio over the entire wavelength region is 0.78. At wavelengths greater than 200 nm in which HCl and DCl are most exposed to solar light in the Venusian atmosphere, the rate of dissociation of
HCl is almost seven times as large as that for DCl. DCl practically ceases to absorb beyond 220 nm, but HCl retains non-negligible absorption at such wavelengths. This result shows that in the Venusian atmosphere DCl is much less readily photodissociated by solar light than HCl. DCl can exchange its D atom with the H atom in other molecules, such as H
2
O, so that the D atom is preserved for a period longer than H in the Venusian atmosphere.
In addition to being less well dissociated by solar light, D escapes less than H

from the Venusian atmosphere because its initial velocity is less than that of H when it is produced from solar irradiation. For instance, when HCl is dissociated at 200 nm, the
H atom carries most of the excess energy, 1.77 eV., because it is much lighter than Cl.
The excess energy is derived by subtracting the dissociation energy [Huber and
Herzberg, 1979] from the photon energy. The H atom has an initial velocity 18 km s
-1
, which much exceeds the escape velocity of 10.4 km s
-1
[Yung and DeMore, 1999]. The initial velocity of the D atom is predicted to be only 12.5 km s
-1
, because its mass is twice that of H and the dissociation energy of DCl is slightly greater than that of HCl because of smaller zero-point energy [Huber and Herzberg, 1979]. This additional factor makes the rate of loss of D due to solar dissociation of DCl smaller than that of H; hence, solar dissociation of HCl and DCl contributes to enhancement of the [D]/[H] ratio observed in the Venusian atmosphere.
Photoabsorption cross sections of HCl and DCl are determined in the region
135-232 nm. Onset of absorption of HCl occurs at a wavelength ~10 nm greater than that of DCl. The rate of photodissociation of HCl by solar light is much greater than that of DCl; especially at wavelengths greater than 200 nm, the former rate is seven times the latter rate. A H atom produced on solar dissociation of HCl has an initial velocity that much exceeds the escape velocity of a H atom from Venus; hence solar dissociation increases the loss of H relative to D, and in turn enhances the [D]/[H] ratio.
The current data are useful for detailed modeling of the photochemistry and evolution of the Venusian atmosphere.
Acknowledgments. L. C. Lee thanks the Institute of Atomic and Molecular Sciences,
Academia Sinica, Taipei, Taiwan, for a visiting professorship.

Ascenzi, D., P. M. Regan, and A. J. Orr-Ewing, The ultraviolet photodissociation of
DCl: H/D isotope effects on the Cl (
2
P) atom spin-orbit branching, Chem. Phys. Lett.
,
310, 477-484, 1999.
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Photo-induced fractionation of water isotopomers in the Martian atmosphere, Geophys.
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Givertz, S. C. and G. G. Balint-Kurti, Prediction of the Cl(
2
P
3/2
)/Cl(
2
P
1/2
) branching ratio in the photodissociation of HCl, J. Chem. Soc. Faraday Trans. 2 , 82, 1231-1242,
1986.
Huber, K. P. and G. Herzberg, Molecular spectra and molecular structure IV. Constants of diatomic molecules (van Nostrand-Reinhold, New York, 1979).
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, 32,
2375-2377, 1975.
Lambert, H. M., P. J. Dagdigian, and M. H. Alexander, Spin-orbit branching in the photofragmentation of HCl at long wavelength, J. Chem. Phys ., 108, 4460-4466, 1998.
Lee, A. Y. T., Y. L. Yung, B.-M. Cheng, M. Bahou, C. Y. Chung, and Y.-P. Lee,
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Atmos. Sci ., 30, 1437-1447, 1973.
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May 17, 1982, J. Geophys. Res . 88, 5403-5410, 1983.
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ICN between 1050 and 2100 Å, J. Chem. Phys.
52, 266-271, 1970.
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85, 719-724, 1986.
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Venus, J. Atmos. Sci .., 28, 1058-1068, 1971.
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Atmosphere, Earth, the other Planets, and Comets , J. S. Levine, ed. (Academic Press,
Orlando, 1985).
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Rottman, G. J., Rocket measurements of the solar spectral irradiance during solar minimum, 1972-1977, J. Geophys. Res.., 86, 6697-6705, 1981. van Dishoeck, E. F., M. C. van Hemert, and A. Dalgarno, Photodissociation processes in the HCl molecule, J. Chem. Phys . 77, 3693-3702, 1982. von Zahn, U., S. Kumar, H. Niemann, and R. Prinn, Composition of the Venus atmosphere, in Venus , D. Hunten, L. Colin, T. Donahue, and V. Moroz, eds, pp.
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Table 1. Absorption cross sections (10
-18
cm
2
) of HCl and DCl in the region 135-230 nm.
175.0
177.5
180.0
182.5
185.0
187.5
190.0
195.0
200.0
205.0
210.0
215.0
220.0
225.0
230.0
Wavelength

/ nm
135.0
137.5
140.0
142.5
145.0
147.5
150.0
152.5
155.0
157.5
160.0
162.5
165.0
167.5
170.0
172.5 a likely to be ten times of listed value; see text.
1.06
0.796
0.589
0.423
0.294
0.203
0.138
0.0596
0.0239
0.00903
0.00310
0.00101
0.00030
0.00010
0.000034
3.34
3.42
3.43
3.27
3.06
2.73
2.40
1.99
1.63
1.36
HCl DCl
This work This work
1.23 0.455
1.52
2.05
2.38
2.79
3.11
0.759
1.20
1.84
2.48
3.08
3.64
3.93
4.15
4.07
3.67
3.21
2.67
2.11
1.66
1.19
0.871
0.589
0.385
0.239
0.145
0.0880
0.0493
0.0167
0.00485
0.00136
0.00040
0.00011
0.000027
1.09
0.808
0.588
0.432
0.313
0.215
0.145
0.0618
0.0256
0.00983
0.00395
0.000137
a
0.000048
a
3.45
3.72
3.82
3.47
3.32
2.97
2.48
2.04
1.63
1.31
HCl HCl
Inn [1975] Nee et al.
[1986]
1.2
2.11
2.51
2.81
3.24
1.6
2.0
2.4
2.7
2.9
3.1
3.2
3.3
3.2
3.1
2.7
2.3
1.9
1.6
1.3
1.0
0.7
0.5

Table 2. Rates (10
-6
s
-1
) of photodissociation of HCl and DCl by the solar flux at the top of the Venusian atmosphere in every 5-nm interval.
Wavelength
/ nm
135-140
140-145
145-150
150-155
155-160
160-165
165-170
170-175
175-180
180-185
185-190
190-195
195-200
200-205
205-210
210-215
215-220
220-225
225-232
135-232
200-232
HCl
/10
-6
s
-1
0.689
0.471
0.362
0.238
0.124
0.072
0.061
0.023
0.010
0.004
6.28
0.294
0.045
0.081
0.161
0.342
0.444
0.501
0.776
0.962
0.960
DCl
/10
-6
s
-1
0.395
0.201
0.116
0.059
0.022
0.010
0.007
0.002
4.93
0.041
0.023
0.062
0.160
0.390
0.544
0.590
0.814
0.850
0.707
Ratio of
DCl/HCl
0.57
0.43
0.32
0.25
0.18
0.14
0.11
0.09
0.78
0.14
0.51
0.77
0.99
1.14
1.23
1.18
1.05
0.88
0.74

Fig. 1. Photoabsorption cross sections of HCl and DCl in the region 135-232 nm.