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(ET) and the anisotropy parameter β(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 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.
ASJC Scopus subject areas
- 物理與天文學 (全部)