Threshold Ionization Spectroscopy of the Low Frequency Vibrational Modes of Styrene and Trans-Stilbene Cations

Abstract

High resolution photoelectron spectra are reported for styrene and stilbene with an emphasis on identifying the low frequency modes which are important in the phenyl–vinyl torsional motion. In the styrene cation three low frequency out-of-plane vibrations are measured, ν₄₂, ν₄₁ and ν₄₀ as well as a number of higher frequency in plane modes. Generally the low frequency modes do not appear with significant intensity but by pumping the appropriate modes in the S₁ intermediate state the intensity for these modes in the threshold ionization spectrum is quite good. In the stilbene cation the analogous phenyl–vinyl torsion, ν₃₇, is also measured as well as the related mode ν₃₆. The ethylene torsional mode in stilbene is not observed. In both molecules the observed frequencies are used to relate the electronic structure of the cation to the S₁ and S₀ states of the molecule.