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

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