Ultraviolet two-photon spectroscopy of benzene: A new gerade Rydberg series and evidence for the 1 1E2g valence state

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Abstract
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 2E1g 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 ν8 (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 1E2g 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 3px,y Rydberg states.