Near-threshold structures in inner-shell photoabsorption processes of CH4, NH3, H2O, and HF

Based on the multiple-scattering X alpha method, the photoabsorption spectra near the 1s threshold of CH4, NH3, H2O, and HF with ten electrons, the same as the Ne atom, are calculated. The calculated near-threshold spectra are in good agreement with the experimental spectra. With the calculated results we can provide cleat theoretical assignments for the recent highly resolved experimental spectra. In order to elucidate the attractive effect of the localized inner-shell core hole, we have adopted the ground state self-consistent field potential, the transition state model potential, and the Z + 1 model potential, respectively. The results of the Z + 1 model are in better agreement with the experimental measurements. Along the isoelectronic molecules, thr antibonding orbitals are also the initial orbitals of the corresponding Rydberg series and can be regarded as "precursor orbitals" (i.e., mixed valence and Rydberg orbitals). It is interesting that the sums of the oscillator strengths of such precursor orbitals are approximately proportional to the numbers of the antibonding orbitals.