VUV absorption spectroscopy of acetonitrile between 7 and 20 eV: A revisionist study

The VUV absorption spectrum of acetonitrile between 7 and 20 eV was obtained at high resolution using monochromatised Kr lamp radiation and synchrotron radiation as photon excitation sources. Absorption cross sections were measured over the spectral range studied. We have reinterpreted or assigned for the first time the bands of four valence transitions in the 7-12 eV region, aided by M.O. calculations of transition energies and oscillator strengths. These assignments also required consideration of vibronic symmetry conditions in electronic transitions not previously discussed in the context of the interpretation of acetonitrile VUV absorption spectra. Considerable revision of previously assigned Rydberg transitions converging to the ground 1(2)E and first excited 1(2)A(1) states of the cation in the 8.9-13.2 eV region was also effected. The vibrational modes and frequencies in the valence and Rydberg states and their relation to bonding characteristics of the molecular orbitals are discussed in detail. Absorption features in the 14.2-15.4 eV higher energy region, probably involve Rydberg transitions converging to the 2(2)E ion state at 15.133 eV. (c) 2007 Elsevier B.V. All rights reserved.