Probable location of a 1E 2g state of the benzene molecule

Photoexcitation spectra of benzene in rare gas matrices show a previously unreported transition near 46000 cm ^-1. The observed bands are not explicable in terms of site splittings, impurity states, aggregation effects, intermediate radius states of the matrix, triplet states, excimer states, exciplex states or σ-π ^∗ transitions. The vibronic spacings in these spectra could be those expected for a ¹E _2g ← ¹A _1g transition and on this and other evidence we argue that the ordering of origins of the first four spin allowed intravalence states of benzene is ¹B _2u (38086 cm ^-1), ¹E _2g (near 46400 cm ^-1), ¹B _1u (48450 cm ^-1) and ¹E _1u (55430 cm ^-1). Our data also show that the transition ¹B _1u ← ¹A _1g accounts for most of the intensity of the 210 nm absorption band system. Our ordering of the spin allowed states permits interpretation of experimental data of others, confirms certain semi-empirical and ab initio SCF MO CI calculations in which account is taken of higher excitations and illustrates the necessity of including such higher excitations. The intensity of the ¹E _2g ← ¹A _1g transition is at least an order of magnitude less than previously calculated indicative of the difficulty of choosing suitable wavefunctions for the ¹E _2g state and of calculating "forbidden" transition probabilities.