CHANGEABLE BEHAVIOR IN THE UNIMOLECULAR DECAY CHANNELS OF ELECTRONICALLY EXCITED-STATES OF CH3BR+ AND CH3CL+

Fragmentation of the first and second electronically excited states of methyl chloride and bromide cations has been studied, the ground state of each molecular ion being stable to dissociation. Product branching ratios and kinetic energy release upon dissociation were investigated using He I photoelectron-photoion coincidence (PEPICO) measurements. The principal fragmentation of the excited states is a C-X bond cleavage. Both excited states of the ion CH3Br+ follow essentially statistical routes in this channel, though small deviations from a fully randomized distribution are noted and these become more marked for vibrationally excited A-state parent. Simultaneously, a minor competing channel, the loss of H atom(clearly established here), is suppressed. The A state of CH3Cl+ dissociates via a single, highly nonstatistical, channel leading to CH3+. There is no competing H loss, but this second channel opens once the B state is accessed and the still dominant CH3+ channel then quickly becomes statistical in character.