A comparative study of the influence of the chemical bond on the photo-ionization cross section at a distance from the threshold

Abstract

cross sections of molecules is the difficulty of finding the wavefunctions of the continuous spectrum in the field of the molecular ion. In the range at a distance from the threshold, the situation is improved, since the ejected electron can be regarded approximately as free and described by a plane wave. In spite of the fact that the use of a plane wave alters qualitatively the angular distribution of the photo-electrons for levels with I # 0, the total cross section of the photo-effect for high energies of the photo-electrons is described satisfactorily in the plane-wave approximation, which is often called the Born approximation I [i]. The calculation of the photo-ionization of the ~-electrons of ethylene in the Born approximation II showed that for Ep = 25.I, the use of the plane wave increases the total cross section by 15%. Another advantage of calculations in the region at a distance from the threshold is the possibility of using as molecular wavefunctions the wavefunctions of the self-consistent field (SCF) method. This is not at all obvious. The fact is that although single ionization leads to the ejection of one electron from the molecule, interaction of the ionization channels with different molecular orbitals (MO) may take place in the photo-ionization process. This interaction can be described as going beyond the limits of the MO SCF method, which means that it is necessary to take account of electron correlation. These effects, for atoms, were examined by Amus'ya and co-workers [3, 4] and Wendin [5]. Specific calculations for inert atoms showed, however, that the farther from the threshold the ionization process takes place, the smaller the contribution of correlation effects. This makes it possible to carry out the calculations in the approximation of the SCF method.