Molecular photoelectron spectroscopy at 132.3 eV. The second-row hydrides

Photoemission spectra of the second-row hydrides CH4, NH3, H2O, and HF, as well as Ne, obtained with ultrasoft (132.3 eV) x rays from the yttrium Mζ line and with soft x rays are compared and discussed. The 2s-derived 2a1 or 2σ orbitals show large relaxation energies, as do the 2s orbitals in the free atoms. The high binding energies of the 2a1 orbitals in CH4 and NH3 indicate that much of the bond energy resides in these orbitals. Bond energies estimated from changes in the average valence-electron binding energies from atoms to hydrides show rough agreement with literature values. Relative molecular orbital peak intensities show dramatic changes from 132.2 eV to 1253.6 eV photon energies, with the atomic cross-section ratio 2p/2s near unity at 132.3 eV and near 0.1 at 1253.6 eV. This difference allows peaks to be assigned to molecular orbitals in some cases by visual inspection, on the basis of atomic orbital composition. Comparison with theoretical intensities based on plane-wave or OPW continum final states shows qualitative agreement at best, and clear disagreement in some cases. Quantitative agreement will require a more sophisticated theory. Values of σ (2p)/σ (2s) for atomic C, N, O, F, and Ne were derived from the spectra at both photon energies.