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Using multilevel (161-level) numerical simulations beyond the rotating-wave approximation, we investigate the single-atom response of helium to intense pulses of radiation with properties characteristic of a SASE free-electron laser source, and central wavelengths resonant with 1snp excitation (n=3,4). The results show a complete breakdown of the two-level approximation, and suggest that under typical experimental conditions levels other than the resonant level are significantly populated. We discuss the implications for the subsequent development of superfluorescence (a collective effect) from the excited atomic medium, and compare the results to recent experiments.

Symmetrical fluorescence yield profiles and asymmetrical electron yield profiles of the preresonances at the La N-IV,N-V x-ray absorption edge are experimentally observed in LaPO4 nanoparticles. Theoretical studies show that they are caused by interference effects. The spin-orbit interaction and the giant resonance produce symmetry entangled intermediate states that activate coherent scattering and alter the spectral distribution of the oscillator strength. The scattering amplitudes of the electron and fluorescence decays are further modified by the spin-orbit coupling in the final 5p(5)epsilon l and 5p(5)4f(1) states.

Symmetrical fluorescence yield profiles and asymmetrical electron yield profiles of the preresonances at the La N_{IV,V} x-ray absorption edge are experimentally observed in LaPO_{4} nanoparticles. Theoretical studies show that they are caused by interference effects. The spin-orbit interaction and the giant resonance produce symmetry entangled intermediate states that activate coherent scattering and alter the spectral distribution of the oscillator strength. The scattering amplitudes of the electron and fluorescence decays are further modified by the spin-orbit coupling in the final 5p;{5}epsilonl and 5p;{5}4f;{1} states.

Physical review / A 75, 51406 (2007). doi:10.1103/PhysRevA.75.051406, Published by APS, College Park, Md.

Physical review / A 76(1), 1341 (2007). doi:10.1103/PhysRevA.76.013411, Published by APS, College Park, Md.

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We detect a significant angle-dependence in the core level and valence line shapes of photoelectron spectra of single crystal K3C60. This allows the identification of bulk and surface components in the data, and allows us to explain the anomalous line shapes observed for this system. The states near the Fermi level are associated with the bulk of the sample. There is strong evidence of an insulating surface layer, which we ascribe to intermolecular electron correlations. These results simplify the interpretation of previous, apparently conflicting observations.

This article reviews our recent work on photo-stimulated ion desorption (PSID) from molecules condensed at low temperature. We have used electron-ion coincidence (EICO) spectroscopy combined with synchrotron radiation. The history and present status of the EICO apparatus is described, as well as our recent investigations of condensed H₂O, NH₃, CH₃CN, and CF₃CH₃. Auger electron photoion coincidence (AEPICO) spectra of condensed H₂O at the O:1s ionization showed that H⁺ desorption was stimulated by O:KVV Auger processes leading to two - hole states (normal- Auger stimulated ion desorption (ASID) mechanism). The driving forces for H⁺ desorption were attributed to the electron missing in the O - H bonding orbitals and the effective hole-hole Coulomb repulsion. The normal ASID mechanism was also demonstrated for condensed NH₃. The H⁺ desorption at the 4a₁ ← O(N):1s resonance of both condensed H₂O and condensed NH₃ was found to be greatly enhanced. Based on the AEPICO spectra the following four-step mechanism was proposed: (1) the 4a₁ ← 1s transition, (2) extension of the HO - H (H₂N - H) distance within the lifetime of the (1s)⁻¹(4a1)¹ state, (3) spectator Auger transitions leading to (valence)⁻²(4a₁)¹ states, and (4) H⁺ desorption. The enhancement of the H⁺ desorption yield was attributed to the repulsive potential surface of the (1s) - 1(4a₁)¹ state. At the 3p ← O:1s resonance of condensed H₂O, on the other hand, the H⁺ yield was found to be decreased. The AEPICO spectra showed that the H⁺ desorption was stimulated by spectator Auger transitions leading to (valence)⁻²(3p)¹ states. The decrease in the H⁺ yield was attributed to a reduction in the effective hole-hole Coulomb repulsion due to shielding by the 3p electron. Photoelectron photoion coincidence (PEPICO) spectra of condensed H₂O showed that the core level of the surface H₂O responsible for the H⁺ desorption was shifted by 0.7 eV from that of the bulk H₂O. The H⁺ desorption from condensed CH₃CN was also investigated. In a study of condensed CF₃CH₃ using PEPICO spectroscopy, site-specific ion desorption was directly verified; that is, H⁺ and CH₃⁺ desorption was predominant for the C:1s photoionization at the -CH₃ site, while C₂Hn⁺, CFCHm⁺, and CF₃⁺ desorption was predominantly induced by the C:1s photoionization at the -CF₃ site. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a powerful tool for studying PSID of molecules condensed at low temperature.