Theoretical and experimental study of vibrationally resolved partial photoionization cross sections and angular asymmetry parameter β for the 1σg and 1σu shells of N2 molecule in the region of the σ* shape resonance is reported. The measurements were made at the synchrotron radiation facility SPring-8 in Japan. The calculations in the random phase approximation have been performed using the relaxed core Hartree–Fock wavefunctions with the fractional charge of the ion core equal to 0.7. With its help, the role of interchannel coupling between the closely spaced 1σg and 1σu shells was studied. The experiment demonstrates the existence of a correlational maximum in the 1σu shell photoionization cross section induced by the σ* shape resonance in the 1σg shell. This maximum reveals itself even more clearly in the angular asymmetry parameter β for the v′ = 0 and v′ = 1 vibrational states of the ion. The calculation in the random phase approximation gives a consistent interpretation of the experimental data.

We report on an experimental and theoretical investigation of x-ray absorption and resonant Auger electron spectra of gas phase O(2) recorded in the vicinity of the O 1s-->sigma(*) excitation region. Our investigation shows that core excitation takes place in a region with multiple crossings of potential energy curves of the excited states. We find a complete breakdown of the diabatic picture for this part of the x-ray absorption spectrum, which allows us to assign an hitherto unexplained fine structure in this spectral region. The experimental Auger data reveal an extended vibrational progression, for the outermost singly ionized X (2)Pi(g) final state, which exhibits strong changes in spectral shape within a short range of photon energy detuning (0 eV>Omega>-0.7 eV). To explain the experimental resonant Auger electron spectra, we use a mixed adiabatic/diabatic picture selecting crossing points according to the strength of the electronic coupling. Reasonable agreement is found between experiment and theory even though the nonadiabatic couplings are neglected. The resonant Auger electron scattering, which is essentially due to decay from dissociative core-excited states, is accompanied by strong lifetime-vibrational and intermediate electronic state interferences as well as an interference with the direct photoionization channel. The overall agreement between the experimental Auger spectra and the calculated spectra supports the mixed diabatic/adiabatic picture.

By using synchrotron radiation to excite calcium atoms in the 3p-3d resonance region, excited calcium ions are generated. In the first measurement of this kind, we have measured the polarization of the fluorescent photons which result from the decay of these ions, in coincidence with the corresponding angle-resolved photoejected electrons. By combining this with the angular distribution parameter for these same electrons, measured in a separate experiment, we are able to calculate for this specific case, within the assumption of LS coupling, the ratio of the dipole amplitudes for the outgoing waves and their phase difference.

The resonant Auger spectrum from the decay of F 1s-excited CF4 is measured. Several lines exhibit a nondispersive kinetic energy as the exciting photon energy is tuned through the resonance region. The F 1s(-1) atomiclike Auger line is split into two components due to the emission of Auger electrons by a fragment in motion, when electron emission is observed along the polarization vector of the light. This Doppler splitting is direct evidence that the core excitation leads to T(d)-->C(3v) symmetry lowering, by elongation of a specific C-F bond preferentially aligned along the polarization vector of the incident photon.

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The angular correlation between the photoelectron and subsequent polarized fluorescent radiation measured in coincidence is analysed theoretically for atomic photoionization. Using the density matrix and statistical tensor formalism the general expression for the correlation function is obtained for an arbitrary polarized incident photon beam. The particular geometry of a recent experiment (Beyer et al., 1995) is considered and the parameters which determine the angular correlation are expressed in terms of photoionization amplitudes. It is demonstrated that the photoelectron- (polarized) fluorescence coincidence measurements reveal strong circular and linear dichroism. As an example the photoionization of Ca atoms near the 4p 2P3/2 threshold is calculated within the framework of multichannel quantum defect theory.

The vibrationally resolved partial cross sections and asymmetry parameters for the photoionization of the NO molecule producing NO+ N (1s−1) 3Π, 1Π ions in the vibrationally excited states v′ are measured for photon energies between 412 and 419 eV, across the σ* shape resonance region above the N K-shell ionization threshold. The position of the maximum of for the N (1s−1) is observed to move towards the N (1s−1) 3Π and 1Π thresholds in both channels with increasing vibrational excitation in the parent ion. The curves, on the other hand, exhibit monotonic increase in the investigated photon energy range. The major features are fairly well reproduced by ab initio vibrationally specific calculations based on the multichannel Schwinger configuration interaction (MCSCI) method for the dipole transition moments, where the effects of vibrational motion on the photoionization cross sections are included using the adiabatic approximation.

Resonant Auger spectra of H2O are recorded with unprecedented resolution, for excitation energies tuned across the O 1s→2b2 resonance. Electron emission spectra for decay to the 1b2-1 final state show a partially resolved vibrational envelope whose overall appearance is photon energy dependent. Comparison with ab initio calculations indicates that the two-dimensional nuclear motion of the Auger final state can be controlled by sampling different portions of the potential curve and changing the nuclear motion in the core-excited state.