PRIMARY PRODUCT CHANNELS IN THE PHOTODISSOCIATION OF METHANE AT 121.6 NM

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH4(CD4) 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 CH3 fragments are formed with very high levels of internal excitation: Some (approximately 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 H-2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H+H-2+CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH4(A 1T2) 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 CH4 photodissociation at 121.6 nm yields CH3 (and CH) fragments, rather than methylene radicals, will necessitate some revision of current models of the hydrocarbon photochemistry prevailing in the atmospheres of the outer planets and some of their moons, notably Titan.