Femtosecond Multiphoton Ionization Photoelectron Spectroscopy of the S₂ State of Phenol

We investigated the short-lived S 2 ( 1 A 1 ) state of phenol using two-photon resonance-enhanced photoionization coupled with photoelectron spectroscopy. Spectra were obtained by exciting the S 2 state with femtosecond laser pulses at various wavelengths near 207 nm and ionizing with pulses at 207 or 414 nm. The spectra feature a strong transition to the ground electronic state of the ion (X̃) and a very weak transition to the first excited electronic state of the ion (Ã). In comparison, ionization via the longer-lived S 1 state using 275 nm pulses generates slightly more à ions. We interpret the relative intensities of the à state signals as a reflection of configuration interactions in the intermediate S 2 and S 1 states. Time-delayed ionization experiments, as well as experiments with different laser pulse durations, suggest that the S 2 state has a lifetime between 150 and 350 fs. Ionization via S 1 produces a vibrationally resolved photoelectron spectrum, while ionization via S 2 leads to a highly congested spectrum. The envelope of the latter spectrum extends over more than 1 eV and features a long progression, probably in the 6a vibration. This suggests that the S 2 state has a geometrical structure that is displaced from the ground state along the 6a coordinate. No vibrational structure could be resolved in the à ion spectrum obtained by ionization via either S 2 or S 1 .