Primary product channels in the photodissociation of methane at 121.6 nm

Abstract

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH 4 (CD 4 ) at 121.6 nm. Contrary to the previous consensus view, we find simple C-H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH 3 fragments are formed with very high levels of internal excitation: Some (~25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H + H2 + CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH 4 (A IT 2 ) molecules undergo rapid internal conversion (promoted by the Jahn-Teller distortion of this excited state) to high vibrational levels of the ground state prior to fragmentation. The realization that CH 4 photodissociation at 121.6 nm yields CH 3 (and CH) fragments, rather than methylene radicals, will necessitate some revision ofcurrent models of the hydrocarbon photochemistry prevailing in the atmospheres of the outer planets and some of their moons, notably Titan.2054

Modeling of methane photolysis in the reducing atmospheres of the outer solar system

“…It is also a major reactive component in the atmospheres of many other objects in the outer solar system, notably Titan, the largest moon of Saturn. As CH4 only absorbs ultraviolet radiation short of 145 nm [Mordaunt et al, 1993 [Lee and Chiang, 1983], we could perhaps expect an increase of 30%, or more, in the absorption cross sections of CI-I4 when we consider temperatures as low as in Titan's atmosphere (T<200 K). Clearly, it is imperative that methane's absorption coefficients be measured at low temperatures, at least at the Lyman 0• wavelength, but preferably over the whole range of absorption.…”

Rate coefficients for reactions of ethynyl radical (C2H) with HCN and CH3CN: Implications for the formation of complex nitriles on Titan

“…These models differ from the first post-Voyager model [Yung et al, 1984;Yung, 1987] primarily in the updated methane photodissociation scheme [Mordaunt et al, 1993], better characterizations of the eddy diffusion coefficient, and updated reaction rates. Gladstone et al [1996] also presented a photochemical model for the upper atmosphere of Jupiter based on similar reaction sequences.…”

Photodissociation of Methane at Lyman Alpha (121.6 nm)

“…This value is quite low with respect to direct dissociation. 13 Mordaunt et al 5 also found CH 3 fragments with very high levels of internal excitation. And they suggested that CH 4 molecules prepared in their triplet (S 1 ) A excited state undergo rapid internal conversion to higher vibrational levels of the ground state before dissociation.…”

“…Mordaunt et al 5 pointed out the significance of H-atom elimination in this process, and estimated Φ H = 1.0 ± 0.5. The use of H atom Rydberg time-of-flight technique also allowed them to conclude that a very significant part of the H atom fragments is formed concomitant with methyl radicals.…”