Vibronic effects in the photon energy-dependent photoelectron spectra of the CH3CN- dipole-bound anion

Photoelectron spectra are reported for the ''dipole-bound'' CH3CN- negative ion at three photodetachment energies (1.165, 2.331, and 3.496 eV), where the anion is prepared by photodissociation of the I-. CH3CN ion-molecule complex. While all three spectra are dominated by a single feature centered near zero electron binding energy, as expected for a dipole-bound anion, vibrational structure is also observed and found to depend strongly on the photodetachment energy. This observation indicates that the vibrational excitation is not exclusively due to distortion between the ion and neutral, but also involves non-''Franck-Condon'' effects, The origin of the energy dependence is traced to excitation of the pi*(CN) shape resonance corresponding to the valence or ''chemical'' anion. The vibrational envelope of the nonresonant spectrum is surprisingly similar to the infrared spectrum of neutral acetonitrile, suggesting that even this excitation may not result from intramolecular distortions. We develop a simple model to illustrate that vibrational excitation can occur upon photodetachment of a dipole-bound electron due to the perturbation of the weakly bound electron by the fluctuating dipole moment of the vibrating neutral molecule. We treat this effect in a Herzberg-Teller interaction picture where the dipole-bound state is mixed with the low lying electron continuum through a dipolar interaction with the neutral molecule. (C) 1996 American Institute of Physics.