Absorption cross sections and solar photodissociation rates of deuterated
isotopomers of methanol
Bing-Ming Chenga
Synchrotron Radiation Research Center, No. 1, R&D Road VI, Hsinchu Science- based
Industrial Park, Hsinchu 300, Taiwan; bmcheng@srrc.gov.tw
Mohammed Bahou and Yuan-Pern Leea, b
Department of Chemistry, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu,
30013, Taiwan; yplee@mx.nthu.edu.tw
L. C. Lee
Department of Electrical and Computer Engineering, San Diego State University, San Diego,
California 92182, U. S. A.
(Received May xx, 2001; revised xxxx xx, 2001; accepted xxxx xx, 2001)
a
Corresponding author. bJointly appointed by the Institute of Atomic and Molecular Sciences,
Academia Sinica, Taipei, Taiwan. Electronic mail: yplee@mx.nthu.edu.tw; FAX:
886-3-5722892.
2
Abstract
The photoabsorption cross sections of CH3OH, CH3OD, CD3OH, and CD3OD are determined
in the spectral region 165-220 nm using synchrotron radiation as a light source. These data are
used to calculate rates of photodissociation of these molecules by the solar flux at 1 AU. These
rates for CH3OD and CD3OD are only about 80% of those for CH3OH and CD3OH, respectively.
Excitation of the methanol isotopomers in this wavelength region breaks mainly the O-H or O-D
bond to form H or D, respectively. These cross section data are applicable for calculations of
photodissociation rates in various regions of space.
3
1. I ntroduction
Because electronic structure varies only slightly among isotopomers, their photoabsorption
cross sections are commonly assumed to be the same. However, for an electronic transition
leading to dissociation, the absorption spectra of various isotopomers, especially near the
threshold region, may vary greatly due to difference in zero-point energy of the ground electronic
state. If the threshold lies in the region in which solar radiant flux decreases with decreasing
wavelength (180-220 nm), variation in rates of dissociation for different isotopomers might be
nonnegligible; hence the photo-induced isotopic fractionation effects (PHIFE) [Miller and Yung,
2000] should be taken into account in modeling photochemistry of these species.
Absorption cross section of H2O was recently determined to exceed those of HOD and D2O
near the threshold region at wavelengths greater than 175 nm [Cheng et al., 1999]. These data
serve to reconcile a discrepancy between an earlier theoretical model and a recent observation
with the Hubble space telescope of the [D]/[H] ratio in the Martian atmosphere [Krasnopolsky et
al., 1997]; substantial deuterium enrichment of water vapor in the planetary atmospheres of
Earth and Mars is predicted [ Miller and Yung, 2000]. Because such application is important, we
proceeded to test whether photoabsorption cross sections differ among deuterated isotopomers of
methanol.
Methanol has been observed in the atmosphere of Earth [Murad, 1984], comets [Greenberg,
1998; Davies and Kerr, 1999; Yung and DeMore, 1999], interstellar media [Gottlieb et al., 1979;
Jacq et al., 1993], galaxies [Hüttemeister et al., 1997], and massive protostars [Lacy et al., 1998;
Dartois et al., 1999; Kerkhof et al., 1999]. Deuterated methanol has been detected in various
molecular clouds [Gottlieb et al., 1979; Mauersberger et al., 1988; Jacq et al., 1993]. Other than
water, methanol is an abundant molecular species in comets [Greenberg, 1998]. When a comet
4
enters into the solar system, molecules in its tail become subject to dissociation by the solar flux.
Hence absorption cross sections are essential for the calculation of photodestruction rates of
these molecules as well as production rates of H and D atoms in various environments.
Absorption cross section of CH3OH was extensively investigated [Harrison et al., 1959;
Salahub and Sandorfy, 1971; Dickinson and Johnson, Jr., 1974; Nee et al., 1985], but to our
knowledge no data for other isotopomers are reported. We determined photoabsorption cross
sections of these species in the spectral region 165-220 nm, and found that CH3OH has cross
sections much greater than those of CH3OD at wavelengths greater than 195 nm. Similar
phenomena are observed for CD3OH and CD3OD. These data will be useful for understanding
the photochemistry of methanol and for the interpretation of the [D]/[H] ratios observed in
various terrestrial regions of space in which methanol exists.
2. Experiments
The experimental setup is described in a previous paper [Cheng et al., 1999]. Briefly,
vacuum ultraviolet (VUV) light, produced in the synchrotron radiation facility in Taiwan, was
dispersed with a 1-m Seya-Namioka monochromator. A CaF2 window served to eliminate
second-order light. The uncertainty in wavelength measurements is estimated to be less than 0.1
nm.
The absorption cross section was measured with a double-beam apparatus. Before entering
the gas cell, the light source was monitored by light reflected from a CaF2 window placed at 45o
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. The light transmitted through the
5
gas cell was converted and detected in the same way as for the reflected light. Both reflected and
transmitted beams passed similar optical components so that their ratio is independent of the
intensity of the light source, which fluctuates with the intensity of the electron beam in the
storage ring.
The absorption cross section is determined from the absorbance,
ln (Io/I) = n l,
(1)
in which Io and I are the reflected and transmitted light intensities, respectively; n is the gas
density,  is the absorption cross section, and l is the length of absorption path through the gas.
The absorbance was adjusted to zero with the gas cell empty. The absorbance is limited to a
value less than 2 to avoid saturation effects. The gas density at room temperature was determined
from gas pressure monitored with a capacitance manometer (MKS-Baratron). At each
wavelength, the absorption cross section was determined with a linear fit of at least 8 absorbance
values measured at varied pressures. At 210 nm, the gas pressure was increased up to 110 Torr; at
such a pressure the absorbance is still linearly dependent on pressure, implying that the measured
cross section is little affected by dimers or polymers.
CH3OH of purity 99.9% was purchased from Merck. CH3OD, CD3OH, and CD3OD with
listed D purities of 99%, 99.8%, and 99.96%, respectively, were purchased from Isotec, Inc.
Each sample was further purified on evacuating the liquid sample at room temperature five times
with duration of three minutes each time to remove volatile impurities, and then with trapping at
about –75o C and degassing for 5 minutes. After each purification cycle, the sample lost about
40%.
6
3. Photoabsorption cross sections
Cross sections of photoabsorption of CH3OH, CH3OD, CD3OH, and CD3OD in the 165-220
nm region are shown in Figure 1. The spectral resolution is 0.1 nm. Values of cross sections in
units of 10-19 cm2 at 1 nm intervals are listed in Table 1. A complete list of values at 0.1 nm for
all four isotopomers is available at http://ams-bmc.srrc.gov.tw. The uncertainty in cross section is
estimated to be 5% of each given value. The current data agree satisfactorily with previous
measurements [Harrison et al., 1959; Salahub and Sandorfy, 1971; Nee et al., 1985]. For
example, at 185 nm the current cross section is 0.61 Mb (1 Mb = 10-18 cm2 ), comparable with
previously reported values 0.57 Mb [Harrison et al., 1959], 0.65 Mb [Salahub and Sandorfy,
1971], and 0.61 Mb [Nee et al., 1985].
As shown in Figure 1, the absorption features of CH3OH and CD3OH have similar
wavelengths as maxima, whereas shifts to smaller wavelength occur with deuteration to CH3OD
and CD3OD. The widths of absorption features of CH3OH and CD3OH are similar, but broader
than those of CH3OD and CD3OD. These results show that the absorption bands involve mostly
excitation of the O-H/D bond, but are little related to the C-O bond. As the absorption contour is
smooth and devoid of structure, photoexcitation in this region causes dissociation to produce H
or D atom. This assertion is supported by measurements of a quantum yield 0.86 ± 0.10 for
production of H from CD3OH at 193 nm [Satyapal et al. 1989].
4 Rates of photodissociation
Rates of photodissociation of methanol isotopomers by solar radiation at 1 AU (distance
7
between the Sun and the Earth) are calculated according to
R =  I  d ,
(2)
in which I is the solar flux,  is the photodissociation cross section, and  is the wavelength. The
solar fluxes in units of 1010 photons cm-2 s-1 nm-1 averaged over 1 nm interval are listed in Table
1. These fluxes are taken from Rottman [1981] for the 165-190 nm region and from Mount and
Rottman [1983] for 190-220 nm. The fluxes are normalized at overlap points so that they join
smoothly. A methanol molecule excited in this band becomes subject to dissociation. The cross
section for photodissociation is thus equal to the cross section for photoabsorption. Rates of
photodissociation calculated at 10-nm intervals are listed in Table 2. The total rates of
photodissociation in the spectral region 165-220 nm are 3.0610-6, 2.450-6, 3.1710-6, and
2.5910-6 s-1 for CH3OH, CH3OD, CD3OH, and CD3OD, respectively.
The rate of solar photodissociation for CH3OD is much smaller than for CH3OH. When a
comet enters into solar system, its molecules become subject to solar dissociation. CH3OH
molecule is thus dissociated with solar light more effectively than CH3OD. The difference is
even more significant in a dense molecular cloud, in which the radiant intensity is biased toward
greater wavelength region. For most molecules cross sections of photoabsorption increase with
decreasing wavelength, so that the solar light penetrates a molecular cloud better at greater
wavelengths. As CH3OH absorbs more strongly at greater wavelengths, it is dissociated to a
greater extent than CH3OD. Because this solar dissociation produces H or D atom, such a
process affects the [D]/[H] ratio.
CH3OD has been detected in the star-forming region Orion-Irc2 with a high degree of
fractionation [Mauersberger et al., 1988]. The [D]/[H] isotopic ratio of methanol in this region is
about ten times that of water and ammonia [Walmsley et al., 1987; Jacq et al., 1988; Gérin et al.,
8
1992]. The dissociation rates of these isotopomers might be important in modeling their
concentrations in this region.
5. Concluding remarks
Methanol is an abundant organic compound in the universe. Photodissociation is an
important process that destroys methanol isotopomers. Because electronic structure varies only
slightly among isotopomers, their photoabsorption cross sections are commonly assumed to be
the same. However, the present data for methanol and previous data for water show that
deuterated isotopomers have a significant variation in their photoabsorption cross sections,
especially in the long wavelength region, such that this variation can not be neglected in
modeling their concentration distributions in various regions of space. This difference is
demonstrated in the photodissociation rates of methanol isotopomers by solar flux at 1 AU
reported here. These data are applicable for calculation of photodissociation rates in other
environments.
Acknowledgments.
L. C. Lee thanks the Institute of Atomic and Molecular Sciences,
Academia Sinica, Taipei, Taiwan for a visiting professorship.
9
References
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10
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Nee, J.B., M. Suto, and L.C. Lee, Photoexcitation processes of CH3OH: Rydberg states and
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11
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12
Table 1. Solar Flux (I/1010 photons cm-2.s-1.nm-1), Photoabsorption Cross Sections ()
of CH3OH, CD3OH, CH3OD, and CD3OD at Wavelength 165.5  /nm  200.5.
 / nm
165.5
166.5
167.5
168.5
169.5
170.5
171.5
172.5
173.5
174.5
175.5
176.5
177.5
178.5
179.5
180.5
181.5
182.5
183.5
184.5
185.5
186.5
187.5
188.5
189.5
190.5
191.5
192.5
193.5
194.5
195.5
196.5
197.5
198.5
 / 10-19cm2
I
4.09
3.09
3.42
4.26
5.64
6.63
6.81
7.32
7.58
9.08
10.66
11.35
13.21
14.28
13.85
16.17
18.47
17.43
17.01
13.89
17.25
20.45
22.13
29.4
36.6
35.6
40.1
44.6
35.1
53.5
57.7
62.9
66.8
64.6
CH3OH
2.14
2.25
2.37
2.58
2.77
3.04
3.32
3.63
3.94
4.27
4.56
4.93
5.17
5.47
5.66
5.87
6.07
6.18
6.18
6.11
6.00
5.87
5.58
5.27
4.91
4.47
3.95
3.55
2.98
2.51
2.06
1.67
1.32
1.01
CH3OD CD3OH CD3OD
2.03
1.54
1.33
2.24
1.74
1.57
2.52
1.99
1.86
2.84
2.15
2.27
3.10
2.51
2.63
3.46
2.80
3.05
3.83
3.08
3.48
4.12
3.48
4.01
4.46
3.94
4.50
4.81
4.35
4.98
5.16
4.75
5.54
5.51
5.22
6.05
5.86
5.58
6.44
6.11
6.00
6.90
6.32
6.33
7.32
6.45
6.68
7.51
6.56
6.89
7.65
6.57
6.98
7.61
6.40
7.08
7.53
6.26
7.04
7.30
5.93
6.88
6.84
5.63
6.64
6.40
5.21
6.19
5.81
4.71
5.81
5.17
4.14
5.27
4.41
3.56
4.72
3.68
2.97
4.16
2.96
2.40
3.50
2.32
1.87
3.02
1.80
1.42
2.47
1.29
1.04
1.99
0.924
0.733
1.55
0.644
0.526
1.20
0.437
0.373
0.907
0.284
13
199.5
200.5
201.5
202.5
203.5
204.5
205.5
206.5
207.5
208.5
209.5
210.5
211.5
212.5
213.5
214.5
215.5
216.5
217.5
218.5
219.5
69.1
75.6
81.9
82.0
95.8
107.0
112.2
115.3
136.3
148.9
214.9
296.7
331.3
359.6
343.9
432.0
417.3
349.2
345.0
505.4
512.7
0.776
0.568
0.414
0.306
0.230
0.165
0.118
0.084
0.059
0.042
0.030
0.021
0.015
0.011
0.0076
0.0056
0.0040
0.0027
0.0019
0.0014
0.0010
0.242
0.158
0.100
0.063
0.040
0.025
0.016
0.010
0.0059
0.0039
0.0024
0.0015
0.693
0.493
0.364
0.254
0.183
0.131
0.094
0.069
0.050
0.035
0.025
0.018
0.013
0.0090
0.0060
0.0045
0.0031
0.0022
0.0016
0.0011
0.0008
0.184
0.119
0.073
0.046
0.029
0.018
0.011
0.0072
0.0045
0.0029
0.0018
0.0011
14
Table 2. Photodissociation rates of methanol isotopomers
in each 10 nm interval in unit of 10-7 s-1.
range / nm
165-175
175-185
185-195
195-205
205-220
165-220
CH3OH
1.88
8.33
13.97
5.73
0.68
30.60
CH3OD
2.10
9.05
11.17
2.14
0.05
CD3OH
1.76
9.31
14.84
5.19
0.56
CD3OD
1.94
10.37
11.76
1.78
0.04
24.52
31.67
25.88