Photoinduced Rydberg ionization spectroscopy of phenol: The structure and assignment of the B̃-state of the cation

Abstract

The newly developed technique of photoinduced Rydberg ionization (PIRI) spectroscopy has been successfully applied to study the B←-X̃ transition in the phenol and phenol-d6 cations. Vibrationally resolved spectra have been obtained for the B state in phenol ion via the origin and the v6, and v12 vibrations of the ground ionic state. Similarly, vibrationally resolved spectra for the B state in phenol-d6 ion have been obtained via the origin and v6 vibration. Calculations to date have suggested the character of the half-filled orbital is π type, and experimental evidence for the b̃-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 B̃ state has the OH group rotated 90° from the planar ground state. Therefore, the symmetry for the B̃ state in phenol cation is assigned to be 2pσ instead of π, 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 σ 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 B̃←X̃ 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. © 1997 American Institute of Physics.