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

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 nR_g and all its associated vibronic structure seen in linear polarization is shown to have symmetry A_1g, but analysis of spectra obtained under circular polarization demonstrates that E_2g origins lie just to lower energies. These combined facts give an unambiguous assignment of this series as nd₁ (E_2g, A_1g, A_2g). Four new Rydberg series, each having vibrational structure nearly identical to each other and to the ground state ion, are also identified and have E_1g or E_2g 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 C₆D ₆) 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).