Photoinduced Rydberg ionization spectroscopy of phenol: The structure and assignment of the (B)over-tilde-state of the cation

The newly developed technique of photoinduced Rydberg ionization (PIRI) spectroscopy has been successfully applied to study the (B) over tilde<--(X) over tilde transition in the phenol and phenol-d(6) cations. Vibrationally resolved spectra have been obtained for the (B) over tilde state in phenol ion via the origin and the nu(6), and nu(12) vibrations of the ground ionic state. Similarly, vibrationally resolved spectra for the (B) over tilde state in phenol-d(6) ion have been obtained via the origin and nu(6) vibration. Calculations to date have suggested the character of the half-filled orbital is pi type, and experimental evidence for the (B) over tilde-state assignment so far has been inconclusive. In contrast to previous featureless photoelectron spectra, the main feature in all of the spectra presented here is the presence of several long, low frequency Frank-Condon progressions, suggestive of a large geometry change in the transition. Configuration interaction singles 6-31G calculations, allowing full geometry optimization, show that the first excited a state has the OH group rotated 90 degrees from the planar ground state. Therefore, the symmetry for the (B) over tilde state in phenol cation is assigned to be 2p sigma instead of pi, corresponding to that of benzene and several other monosubstituted benzenes. Further support for this assignment is found in a calculation of the normal mode vibrations, based on the geometry optimized for the excited a state. These show three low frequency normal modes having a large amount of OH torsion, one of which has a ring motion identical to one of the two normal modes that induces the (B) over tilde<--(X) over tilde transition in the benzene cation. This calculated normal mode is, therefore, assigned to the most intense and most extensive progression observed in the photoinduced Rydberg ionization spectra. (C) 1997 American Institute of Physics.