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 stat e of each molecular ion being stable to dissociation. Product branchin g ratios and kinetic energy release upon dissociation were investigate d using He I photoelectron-photoion coincidence (PEPICO) measurements. The principal fragmentation of the excited states is a C-X bond cleav age. Both excited states of the ion CH3Br+ follow essentially statisti cal routes in this channel, though small deviations from a fully rando mized distribution are noted and these become more marked for vibratio nally excited A-state parent. Simultaneously, a minor competing channe l, the loss of H atom(clearly established here), is suppressed. The A state of CH3Cl+ dissociates via a single, highly nonstatistical, chann el leading to CH3+. There is no competing H loss, but this second chan nel opens once the B state is accessed and the still dominant CH3+ cha nnel then quickly becomes statistical in character.