Rydberg States of Benzene in Rare-Gas Matrices

Abstract

In this paper we present experimental evidence for the observation of Rydberg states of benzene in solid Ar, Kr, and Xe. On the basis of semiquantitative theoretical evidence we argue that molecular Rydberg-type states of a guest molecule are amenable to experimental observation in rare-gas solids. These host matrices are characterized by a free electronlike conduction band, leading to a manifold of Wannier-type states. The electron–atom interaction in rare-gas solids is relatively weak so that the line broadening of these Rydberg-type states will not be excessive. The identification of these “Rydberg-type” states in a rare-gas solid is based on matrix shifts, vibrational structure, isotope effects, linewidths, and site splittings. Two Rydberg-type transitions of the benzene molecule were identified. From our analysis we conclude that: (a) The lowest (n = 1) molecular Rydberg state in the rare-gas matrix can be correlated with the lowest Rydberg state in the gas phase, but is appreciably blue-shifted. The blue shifts are: 6000 cm−1 for Ar, 3150 cm−1 for Kr, and 1610 cm−1 for Xe. (b) The benzene Rydberg levels in the rare-gas matrix reveal some evidence for Jahn–Teller coupling effects, as is evident from the appearance of a nontotally symmetric vibrational component. (c) The linewidths observed for the Rydberg levels in the rare-gas matrix are qualitatively consistent with theory. (d) The second Rydberg transition in the rare-gas matrix is tentatively identified as corresponding to a n = 2 Wannier state, which has no relation to the molecular levels of the isolated molecule. This assignment makes possible an estimate of the ionization potential of the molecule in rare-gas solids. (e) Our results provide evidence that the lowest molecular Rydberg state and molecular impurity Wannier states are amenable to experimental observation in rare-gas solids.