On the vibrational fine structure in the near-threshold photofragmentation spectrum of the I−⋅CH3I complex: Spectroscopic observation of nonadiabatic effects in electron-molecule scattering

Abstract

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 vibrational progression in the ν₃ (largely C–I stretching) mode of neutral CH₃I, the same mode which is excited upon photodetachment of the complex. The extent of this vibrational activity in the I⁻⋅CH₃I photoelectron spectrum is found to strongly depend on the photodetachment energy, becoming very pronounced as the photon energy approaches the detachment threshold. This indicates that the vibrational features in the photoelectron spectrum arise from non‐Franck–Condon effects. These observations of selective excitation of ν₃ in both the photoelectron and photofragmentation spectra are correlated to nonadiabatic effects arising from the repulsive state of the CH₃I⁻ anion, which is thought to evolve into a resonance near the equilibrium separation of neutral CH₃I. The I⁻⋅CH₃I photochemistry is discussed in the context of the mechanisms postulated to govern electron‐molecule scattering (i.e., vibrational inelastic and dissociative electron attachment) in bare CH₃I. Finally, we cast the scattering mechanism in a spectroscopic picture and suggest that the threshold fragmentation in I⁻⋅CH₃I can be viewed as the predissociation of a transient or virtual dipole‐bound electronic state.