Excitation and ionization of the monohaloethylenes (C 2H 3X, X = F, Cl, Br, I). I. Inner-shell excitation by electron energy loss spectroscopy

Abstract

High-resolution electron energy loss spectra of all inner-shell excitations of the monohaloethylenes (C 2H 3X, X = F, Cl, Br and I) below 740 eV have been obtained under conditions of high impact energy (in the range 2.0-3.7 keV ) and zero-degree scattering angle. The spectra are interpreted, in a unified manner, as excitations to virtual valence and Rydberg orbitals, as well as to continuum-shape resonances. The halogen inner-shell excitation spectra are dominated by transitions to σ* (CX) levels, while transitions to π* (CC) levels are apparently absent or of very low intensity and unresolved. The term values corresponding to various types of transitions observed in the C 1s excitation spectra correlate very well with the electronegativity of the halogen substituent. Based on the correlation analysis, substitutional effects are found to be most prominent in the case of σ* transitions, while the π* and Rydberg levels are less influenced. A linear correlation between the C-X bond strength and the term values corresponding to the C 1s→σ* (CX) transitions has also been observed.