Ultraviolet two-photon spectroscopy of benzene: A new gerade Rydberg series and evidence for the 1 ¹E_2g valence state

In the first application of continuously tunable ultraviolet two-photon absorption spectroscopy to a study of the higher excited states of a polyatomic molecule, we have obtained the two-photon absorption spectrum of jet-cooled benzene in the energy region from 55 000 to 75 000 cm^-1. The strongest features of this spectrum, seen as two-photon resonance structure in three-photon ionization, are assigned to members of a new Rydberg series. Vibronic elements of these Rydberg states show evidence of dynamical effects associated with a Jahn-Teller instability in the ²E_1g ionic core toward which the series converges. Notably, the magnitude of vibronic coupling terms in these states appears quite comparable to that present in C₆H₆⁺ and the sym-halobenzene cation ground states, while differing significantly from theoretical calculations predicting v₈ (ring stretch) as the major distorting mode. In addition, we observe two other new band systems: The first, with an origin at 60 800 cm ^-1, has vibronic structure, isotope shifts, and dramatically increased diffuseness upon deuteration, which taken together suggest its assignment as a highly valenized 3d Rydberg state or a degenerate valence state, most likely the lower of the two ¹E_2g states predicted by molecular orbital theory. The second, lying in the 55 000-58 000 cm ^-1 region, can be at least partially assigned to the two-photon forbidden 3p_x,y Rydberg states.