VUV PHOTOCHEMISTRY OF CH4 AND ISOTOPOMERS - I - DYNAMICS AND DISSOCIATION PATHWAY OF THE H D-ATOM ELIMINATION CHANNEL/

The Doppler-selected time-of-flight technique was used to study the fo rmation of H and D in the photolysis of CH4 and its isotopomers. The c ombination of measurements for the photofragment kinetic energy releas e P(E-T) and the anisotropy parameter P(ET) distributions allows us to differentiate, for the first time, three distinct pathways which are involved in C-H (C-D) bond fission. In conjunction with a recent ab in itio theoretical investigation, the mechanisms for this complicated mu ltichannel dissociation process are proposed. In particular, two disti nct dissociation pathways are elucidated for the two-fragments channel CH3((X) over tilde (2)A '')+H. One pathway invokes a perpendicular-ty pe transition in absorption, which subsequently undergoes intersystem crossing to the triplet surface and then dissociates. The fragmentatio n via this route yields fast CH3 +H with a negative beta parameter. Al ternatively, a parallel-type excitation is involved, followed by inter nal conversion to the ground-state surface on which dissociation occur s. This pathway results in less kinetic energy release and yields a po sitive beta parameter. An intriguing isotope effect is revealed, which calls for further theoretical investigations. (C) 1998 American Insti tute of Physics. [S0021-9606(98)00941-6].