VIBRONIC EFFECTS IN THE PHOTON ENERGY-DEPENDENT PHOTOELECTRON-SPECTRAOF THE CH3CN- DIPOLE-BOUND ANION

Photoelectron spectra are reported for the ''dipole-bound'' CH3CN- neg ative ion at three photodetachment energies (1.165, 2.331, and 3.496 e V), where the anion is prepared by photodissociation of the I-. CH3CN ion-molecule complex. While all three spectra are dominated by a singl e 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 in dicates that the vibrational excitation is not exclusively due to dist ortion between the ion and neutral, but also involves non-''Franck-Con don'' effects, The origin of the energy dependence is traced to excita tion 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 acetonitr ile, suggesting that even this excitation may not result from intramol ecular distortions. We develop a simple model to illustrate that vibra tional excitation can occur upon photodetachment of a dipole-bound ele ctron due to the perturbation of the weakly bound electron by the fluc tuating dipole moment of the vibrating neutral molecule. We treat this effect in a Herzberg-Teller interaction picture where the dipole-boun d state is mixed with the low lying electron continuum through a dipol ar interaction with the neutral molecule. (C) 1996 American Institute of Physics.