Diffraction analysis of the photoionisation cross sections of water, ammonia and methane

Theoretical predictions of orbital cross sections and asymmetry parameters have been obtained by the GIPM/D (ground state inversion potential method/diffraction) theory for the valence and inner valence orbitals of water, ammonia and methane. The calculations employ tabulated ground state molecular orbital wavefunctions of double-zeta quality and assume that the total ionisation amplitude is a superposition of atomic ionisation amplitudes. Extensive comparisons with experiment and more sophisticated theories are presented. The agreement is satisfactory for the cross sections in most cases (except for the inner valence orbitals near threshold) but less so for the asymmetry parameters. The growth of interference effects with increasing delocalisation of the molecular orbitals and the ways in which these can reflect orbital type are discussed.