Spectroscopic observation of vibrational Feshbach resonances in near-threshold photoexcitation of X− ·CH3NO2 (X−=I− and Br−)

Abstract

We report the observation of resonance structure in the absorption and X⁻/NO₂⁻ photofragment action spectra of the X⁻·CH₃NO₂ (X⁻=I⁻ and Br⁻) complexes in the region above the electron detachment threshold. The resonance structure corresponds to peaks which appear at the onsets for vibrational excitation of the –NO₂ wag, scissors, and stretch modes of neutral CH₃NO₂, the modes which most strongly distort upon electron capture into its π* lowest unoccupied molecular orbital. We attribute the peaks to excitation of vibrational Feshbach resonances of the CH₃NO₂⁻ transient negative ion, where near-threshold excitation of X⁻·CH₃NO₂ spectroscopically accesses states of the free electron–CH₃NO₂ system.