A 9 eV superexcited state of 1,3-cyclohexadiene revealed by double resonance ionization photoelectron spectroscopy

Photoelectron spectra of 1,3-cyclohexadiene (CHD) have been obtained by three-photon, double resonance ionization via the ultrashort lived 1B2 (S2) state, and a previously unknown superexcited valence state at 9.0–9.5 eV. Upon two-photon excitation to the superexcited state there is an ultrafast internal conversion leading to a set of vibrationally excited Rydberg states. Ionization out of these latter states by a third photon results in photoelectron spectra that show two prominent Rydberg series with quantum defects of δ=0.58±0.04 and δ=0.98±0.02, respectively.