Higher excited states of benzene: Polarized ultraviolet two-photon absorption spectroscopy

Abstract

The Rydberg spectrum of benzene is explored by means of linear and circular polarized ultraviolet two-photon absorption spectroscopy of the jet-cooled molecule. A strong gerade Rydberg series previously designated nRg and all its associated vibronic structure seen in linear polarization is shown to have symmetry A1g, but analysis of spectra obtained under circular polarization demonstrates that E2g origins lie just to lower energies. These combined facts give an unambiguous assignment of this series as nd1 (E2g, A1g, A2g). Four new Rydberg series, each having vibrational structure nearly identical to each other and to the ground state ion, are also identified and have E1g or E2g symmetry. The five Rydberg series converging to the first ionization potential have quantum defects 0.77, 0.06, 0.05, 0.02, and −0.11. Bandwidths indicative of ultrafast radiationless decay processes are observed for low-n vibronic states, but higher states are sharp. In addition, an Eg band system is observed at 68 980 cm−1 (69 330 for C6D6) whose term value, vibrational structure, bandwidth, and isotope shift on deuteration are all consistent with its assignment as the lowest Rydberg state converging to the first excited ionic state (second ionization potential).