Absorption Cross Sections of CH4 and CH3D: Implication of the D/H Ratio
and the Evolution of Titan's Atmosphere
Chao-Yu Chung2 (鍾昭宇), Bing-Ming Cheng1 (鄭炳銘) , Mohammed Bahou2 (巴穆罕),
Yuan-Pern Lee2 (李遠鵬)
1
2
Synchrotron Radiation Research Center, Hsinchu, Taiwan
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan
Titan’s atmosphere is enriched in deuterium by a factor of  3 relative to that reported for
Jupiter and Saturn atmospheres [1]. The ratio [CH3D]/[CH4] = 6.7 10-5 and 8.710-5 for Saturn
and Jupiter are within the mean interstellar medium of the relative abundance of D/H [2]. CH 3D is
the major reservoir of deuterium in Titan. The gravity is low enough to permit thermal escape of
light gases H and H2. Moreover, the organic compounds that are photosynthesized in the
atmosphere of Titan are deposited on its cold surface, implying an irreversible chemichal evolution
of the atmosphere [2]. A likely mechanism proposed recently to explain the variation in isotopic
fractionation is called Photo Induced Fractionation Effect (PHIFE). The key idea is that rate of
photolysis differs greatly for various isotopomers near the threshold region because of differences
in vibrational zero energy. If the photodissociation threshold region is within (130-240 nm) where
the solar flux decreases drastically, photolysis yield varies greatly with various isotopomers. To
understand if PHIFE theory works for the Titan's atmosphere, knowledge of accurate absorption
cross sections of methane and its isotopomers is required.
The experimental setup is similar to that described in previous paper [3]. The light source
was a synchrotron radiation dispersed by 1-m Seya Namioka monochromator with a grating blazed
at 140 nm (600 g/mm). The slit width was set to 0.05 mm corresponding to a spectral resolution
~0.1nm. The monochromator was scanned in 0.1-nm step with signal averaging period of 2 s at
each step. The beam passed through a beam-splitting LiF plate that constitutes one end of the
absorption cell. The transmitted light passes through the gas cell and irradiated on to a glass
window coated with sodium salicylate. The fluorescence was subsequently detected with a
photomultiplier (Hamamatsu R955) in a photon counting mode. The reflected beam passes
through one additional LiF plate and irradiated onto a glass window coated with sodium salicylate.
The fluorescence was detected similarly and the signal was used for normalization
The resultant absorption cross sections of CH4 and CH3D in the spectral region 105-153 nm
are recorded. Cross section of CH4 is nearly identical to that of Mount et al [4] in the spectral
range 105-150 nm. That of CH3D is reported for the first time. In the spectral range 105-140 nm
absorption cross section of CH3D is unaffected by the presence of minor O2 impurities.
Absorption of oxygen (~300 ppm) Shumann-Runge continuum was subtracted to yield a reliable
cross section of CH3D in the range 140 –149 nm.
The ratio of the cross section of CH4 to CH3D increases from ~1.3 at 135 nm to ~3.1 at 149 nm.
Currently, Yung and coworkers at Caltech are modeling the Titan's atmosphere with these new data.
Preliminary results indicate excellent agreement with the PHIFE mechanism.
References:
[1] J. P. Pinto, J. I. Lunine, S.-J. Kim, and Y. L. Yung, Nature 319, 388 (1986).
[2] Y. L. Yung and W. D. DeMore, Photochemistry of Atmospheres, Oxford University Press
(1999)
[3] B.-M. Cheng, E. P. Chew, C.-P. Liu, M. Bahou. Y.-P. Lee, Y. L. Yung, and M. F. Gerstell,
Geophys. Res. Lett. 26, 3657 (1999).
[4] G. H. Mount, E. S. Warden, and H. W. Moos, Astrophys. J. 214, L47 (1977).