Ultrafast Dynamics in Superexcited States of Phenol

We observe three-photon ionization photoelectron spectra of phenol in a molecular beam. After excitation by the first photon to the S 1 electronic state, a second photon lifts the molecule to a set of superexcited molecular states at an energy of 9 eV, which is 0.5 eV above the ionization energy. Photoelectron spectra are obtained by ionizing the molecule with a third photon. The spectra show prominent Rydberg series with δ = 0.32 and δ = 0.80, as well as several other series. Time-delayed photoelectron spectra are obtained from two-color ionization experiments. These spectra are analyzed by eliminating underlying contributions from competing ionization processes, and by referencing all decays to a set of particularly persistent photoelectron peaks. A kinetic picture emerges that invokes an ultrafast conversion of the initially excited, optically bright, superexcited valence state to a set of vibrationally excited Rydberg states, on a femtosecond time scale. Further dynamical processes among the Rydberg states proceed on a picosecond time scale.