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

Dessent, Caroline E. H.; Bailey, Christopher G.; Johnson, Mark A.
December 1996
Journal of Chemical Physics;12/15/1996, Vol. 105 Issue 23, p10416
Academic Journal
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. © 1996 American Institute of Physics.


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