A simple model of electron scattering resonances near electronic excitation thresholds is discussed. The model consists of a single discrete electronic state coupled to several electronic continua. The vibrational dynamics in the resonance state is treated, taking proper account of non-Born-Oppenheimer effects in near-threshold electron-molecule scattering. The effect of long-range potentials is included via the threshold expansion of the partial decay widths of the resonance. The analytic properties of the fixed-nuclei $S$ matrix are analyzed in detail for two special cases ($s$-wave scattering in the absence of long-range potentials and scattering from a strongly polar target molecule). The dipole potential is shown to lead to a qualitatively new behavior of the trajectories of resonance poles near excitation thresholds. The model yields a qualitative description of the measured excitation function of the ${B}^{1}{\ensuremath{\Sigma}}^{+}$ state of the CO molecule, where a strong and narrow threshold peak is observed.
The valence photoemission spectra of NiCO, PdCO and PtCO are calculated by the ab‐initio Green’s function method using an extended basis set. The present ab initio many‐body approaches give a reasonably good description of the valence photoemission spectra of CO adsorbed on Ni, Pd and Pt metal surfaces. A detailed study of the dynamics of valence photoemission of CO adsorbed on different substrates is made. For the 1π level of NiCO and PtCO the quasi‐particle picture (Q.P.P.) breaks down, however, for PdCO the Q.P.P. is still valid. For the 4σ and 5σ levels of NiCO, the lowest energy state is the main line single hole (1h) state not the CT (charge transfer) 2h1p (two hole one particle) shakedown state. However, for PdCO and PtCO, the σ (π) to π* metal–ligand CT 2h 1p shakedown state of a non‐negligible intensity becomes the lowest energy state. The main line state is still 1h state and the Q.P.P. is valid. In PdCO and PtCO where the π bonding is weaker than in NiCO, the excitation from the σ metal orbital becomes substantial because of a weaker π bonding.
The valence photoemission spectra of PdCO and PtCO are calculated by the ab initio third-order algebraic-diagrammatic-construction Green's-function method using an extended basis set. The overall agreement with the experimental spectra of CO on a Pd(100) and a Pt(111) metal surface is good. A comparison of the spectral features among NiCO, PdCO, and PtCO is made. For the 5\ensuremath{\sigma} and 4\ensuremath{\sigma} levels of PdCO and PtCO, the quasiparticle picture (QPP) is valid as in the case of NiCO. However, in PtCO and PdCO, the lowest-energy state is not the main-line 1h (one-hole) state but the 2h-1p (two-hole, one-particle) metal-ligand charge-transfer (CT) shakedown state which has a substantial intensity (0.02--0.29 for PdCO and 0.01--0.13 for PtCO). This state is induced by the hole-hopping dynamical relaxation. The second-lowest-energy state of a large intensity (0.51--0.76 for PdCO and 0.62--0.74 for PtCO) is the main-line 1h state. For the 5\ensuremath{\sigma} and 4\ensuremath{\sigma} ionization, the 2h-1p CT shakedown state becomes more stable than the 1h state because of a weaker \ensuremath{\pi} donation in the ground state in PtCO and PdCO in comparison to NiCO. For the 1\ensuremath{\pi} excitation, the QPP is still valid for PdCO in contrast to NiCO for which the QPP breaks down due to the metal-ligand CT static relaxation. The lowest-energy main-line state of a large intensity (0.62) is still the 1h state. For PtCO this main line loses a large intensity to the first satellite and the intensity of the two lines becomes nearly comparable.
Nine L x-ray emission lines of xenon (Z=54) in the gas phase were measured in fluorescence. The spectra were obtained on a double-crystal spectrometer using a conventional x-ray tube for primary excitation. The binding energies of the L subshells were obtained using the experimental x-ray photoelectron-spectroscopy (XPS) ionization energies of the final states and found to be in agreement with earlier XPS measurement. The K-shell binding energy was also determined. The transition energies, level energies, and widths were analyzed with respect to available experimental and theoretical data. Discrepancies between experiment and theory are discussed in terms of dynamical relaxation and decay. It is suggested that this discrepancy can be mended by using the many-body approach such as the Green's-function method, which treats such strong correlations in a consistent way.
