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

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 pi* 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.