The electronic states of ethylene oxide studied by photoabsorption and ab initio multireference configuration interaction calculations

The absolute photoabsorption spectrum of ethylene oxide (C ₂H₄O) has been measured between onset and 30 eV, using monochromated synchrotron radiation. Below the ionization threshold (10.56 eV) the spectrum is dominated by sharp peaks related to excitation of Rydberg series converging on the first ionization energy. Above the ionization threshold, valence-excited states, which give rise to broad bands, predominate. Underlying Rydberg states are signalled in weak vibrational structure on the valence bands. Ab initio multireference configuration interaction (MRDCI) calculations have been carried out to assist in assignment of the excited states, both valence and Rydberg. The lowest-lying valence state is electric-dipole forbidden; the first optically allowed valence state lies close to the ionization onset. The spectrum of the oxide is compared with those of cyclopropane (C ₃H₆) and ethylene sulphide (C₂H₄S). © 2008 IOP Publishing Ltd.