Experimental Study of the Higher π → π* Transitions of Benzene in Low-Temperature Matrices

Abstract

In this paper we present the results of an experimental study of the absorption spectrum of benzene and deuterated benzenes in solid Ar, Kr, Xe, and N2 in the spectral region 2800–1700 Å, with special reference to the 2100- and to the 1850-Å transitions. Our main results are: (a) On the basis of the observed vibrational structure the second excited singlet state of the benzene molecule is assigned to the A1g1 → 1B1u rather than to the A1g1 → 1E2g excitation. (b) Theoretical calculations of the dynamic electronic–vibrational coupling between the B1u1 and the E1u1 states support the B1u1 assignment of the 2100-Å transition. (c) The vibrational structure of the 1850-Å A1g1 → 1E1u transition was resolved. (d) No experimental evidence for Jahn–Teller coupling in the π → π* 1E1u state was observed, in agreement with theoretical analysis. (e) Information on site splittings for the higher π → π* excitation of benzene in rare-gas solids has accumulated. (f) Analysis of matrix shifts for the A1g1 → 1B1u and A1g1 → 1E1u transitions indicates that the “solvent effect” is dominated by dispersion interactions. (g) Information on deuteration effects on the B1u1 and E1u1 energy levels was obtained. (h) Qualitative information on intramolecular radiationless decay in the two higher π → π* excited states of the benzene molecule has been inferred from the linewidths in the absorption spectrum.