Photoionization cross sections in the valence electron approximation. III. Nonlinear molecules (theory)

A theoretical method involving the plane-wave approximation and the use of semiempirical LCAO-SCF-MOs of the CNDO and MINDO type is developed for the calculation of photoionization cross sections of arbitrary molecules consisting of atoms from the first three rows of the Periodic Table. Three qualitative concepts for the interpretation of the formalism are discussed, and the dependence of the cross sections on the photon energy, on the electron density distribution in the initial orbital, and on the different behaviour of s, p, and d electrons is treated explicitly. Examples for the application to HeI and HeII photoelectron spectroscopy are given, too.