ON THE VIBRATIONAL FINE-STRUCTURE IN THE NEAR-THRESHOLD PHOTOFRAGMENTATION SPECTRUM OF THE I-CENTER-DOT-CH3I COMPLEX - SPECTROSCOPIC OBSERVATION OF NONADIABATIC EFFECTS IN ELECTRON-MOLECULE SCATTERING

Photofragmentation of the I-. CH3I ion-molecule complex is observed to accompany photoexcitation in the vicinity of its electron detachment thresholds. The I- photofragment action spectrum displays a vibrationa l progression in the nu(3) (largely C-I stretching) mode of neutral CH 3I, the same mode which is excited upon photodetachment of the complex . The extent of this vibrational activity in the I-. CH3I photoelectro n spectrum is found to strongly depend on the photodetachment energy, becoming very pronounced as the photon energy approaches the detachmen t threshold. This indicates that the vibrational features in the photo electron spectrum arise from non-Franck-Condon effects. These observat ions of selective excitation of nu(3) in both the photoelectron and ph otofragmentation spectra are correlated to nonadiabatic effects arisin g from the repulsive state of the CH3I- anion, which is thought to evo lve into a resonance near the equilibrium separation of neutral CH3I. The I-. CH3I photochemistry is discussed in the context of the mechani sms postulated to govern electron-molecule scattering (i.e., vibration al inelastic and dissociative electron attachment) in bare CH3I. Final ly, we cast the scattering mechanism in a spectroscopic picture and su ggest that the threshold fragmentation in I-. CH3I can be viewed as th e predissociation of a transient or virtual dipole-bound electronic st ate. (C) 1996 American Institute of Physics.