Argon matrix photolysis and photoionization studies of benzene. Absorption spectrum of benzene cation and benzene dimer cation

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Abstract
Argon/benzene samples subjected to argon resonance photoionization during condensation at 21±1 K revealed a sharp new 552.5 nm product absorption, sharp C − 2 bands and a broad 900 nm absorption. The 552.5 and 900 nm absorptions were destroyed and the C − 2 bands decreased by mercury arc photolysis, which suggests that the 552.5 and 900 nm absorptions are due to molecular cations. The broad 900 nm absorption is in agreement with an earlier assignment to the intervalence transition in (C6H6)2 +. Near agreement between the 552.5 nm absorption energy and the difference between the sharp origins for the first two benzene photoelectron bands invites consideration of the 552.5 nm absorption for C6H6 +. A tentative assignment of the sharp, weak 552.5 nm band to the vibronically allowed 2 E 2g +e 2u ←2 E 1g transition of C6H6 + is in accord with deuterium isotopic shifts and predictions from the gas phase fluorescence spectra of fluorine substituted benzene cations. A strong 310 nm band system is attributed to the C6H7 radical which decreases on photolysis to give 288 and 534 nm absorptions that are assigned to the C6H5 radical.