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 formation of H and D in the photolysis of CH₄ and its isotopomers. The combination of measurements for the photofragment kinetic energy release P(E_T) and the anisotropy parameter β(E_T) 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 initio theoretical investigation, the mechanisms for this complicated multichannel dissociation process are proposed. In particular, two distinct dissociation pathways are elucidated for the two-fragments channel CH₃(X̃²A^″)+H. One pathway invokes a perpendicular-type transition in absorption, which subsequently undergoes intersystem crossing to the triplet surface and then dissociates. The fragmentation via this route yields fast CH₃+H with a negative β parameter. Alternatively, a parallel-type excitation is involved, followed by internal conversion to the ground-state surface on which dissociation occurs. This pathway results in less kinetic energy release and yields a positive β parameter. An intriguing isotope effect is revealed, which calls for further theoretical investigations.