Excitation and ionization of the monohaloethylenes (C2H3X, X = F, Cl, Br, I). II. Photoionization by He(I) and He(II) photoelectron spectroscopy and valence-shell excitation by electron energy loss spectroscopy

Abstract

The He (I) and He (II) photoelectron spectra and the valence-shell electron energy loss spectra for C2H3X (X = F, Cl, Br and I) have been recorded. The valence-shell electron energy loss spectra were obtained at an impact energy of 3000 eV and a zero degree scattering angle. In contrast to previous studies, the HOMO orbital in C2H3I is reassigned with dominant iodine 5p· character rather than the π(CC) character observed for the HOMO orbitals of C2H3F, C2H3Cl and C2H3Br. This reordering of the assignment in C2H3I is based on considerations of peak shape, vibrational profile and relative intensity change between He(I) and He (II) photoelectron spectra as well as observations from the valence shell electron energy loss spectra. The valence shell electron energy loss spectrum of each of the monohaloethylenes is tentatively assigned as being due to transitions from the various occupied valence orbitals to common manifolds of π*(CC), σ-*(CX) and Rydberg final orbitals. A linear correlation between the CX bond strength and the term values of the 2a″→σ* (CX) transition is also demonstrated. © 1989.