Peter Glans

High-resolution electron time-of-flight apparatus for the soft x-ray region

A gas-phase time-of-flight (TOF) apparatus, capable of supporting as many as six electron-TOF analyzers viewing the same interaction region, has been developed to measure energy- and angle-resolved electrons with kinetic energies up to 5 keV. Each analyzer includes a newly designed lens system that can retard electrons to about 2% of their initial kinetic energy without significant loss of transmission; the analyzers can thus achieve a resolving power (E/ΔE) greater than 104 over a wide kinetic-energy range. Such high resolving power is comparable to the photon energy resolution of state-of-the-art synchrotron–radiation beamlines in the soft x-ray range, opening the TOF technique to numerous high-resolution applications. In addition, the angular placement of the analyzers, by design, permits detailed studies of nondipolar angular distribution effects in gas-phase photoemission.

Electric-octupole and pure-electric-quadrupole effects in soft-X-Ray photoemission

Second-order [ O(k(2)), k = omega/c] nondipole effects in soft-x-ray photoemission are demonstrated via an experimental and a theoretical study of angular distributions of neon valence photoelectrons in the 100-1200 eV photon-energy range. A newly derived theoretical expression for nondipolar angular distributions characterizes the second-order effects using four new parameters with primary contributions from pure-quadrupole and octupole-dipole interference terms. Independent-particle calculations of these parameters account for a significant portion of the existing discrepancy between experiment and theory for Ne 2p first-order nondipole parameters.

Resonant x-ray scattering beyond the Born-Oppenheimer approximation: Symmetry breaking in the oxygen resonant x-ray emission spectrum of carbon dioxide

Although resonant x-ray scattering of molecules fulfills strict electronic symmetry selection rules, as now firmly proven by spectra of diatomic molecules, the accumulated body of data for polyatomic molecules indicates that an apparent breaking of these rules represents the common situation rather than the exception. The CO2 molecule provides a good example of symmetry breaking, with the oxygen x-ray emission spectra showing strong intensity for transitions that are forbidden by the parity selection rule. We present time-independent and time-dependent theories for frequency-dependent resonant x-ray scattering beyond the Born–Oppenheimer approximation in order to explore under what circumstances one can anticipate symmetry breaking in the spectra of polyatomic molecules. The theory starts out from the Kramers–Heisenberg dispersion relation and is generalized for vibrational degrees of freedom and for nonadiabatic coupling of the electronic (vibronic) states, including the frequency dependency of the scatt...

THE K-SHELL AUGER DECAY OF ATOMIC OXYGEN

Relative photoionization cross sections for and produced by the Auger decay of a 1s hole in atomic oxygen were measured by using synchrotron radiation between 525 and 553 eV. Energies and quantum defects of the members of the two Rydberg series converging to and ionization thresholds were determined. In addition, the and ionization thresholds were calculated from the two Rydberg series. The 182 meV resolution of the monochromator allowed a detailed study over both thresholds revealing evidence for post-collision interaction and allowing a comparison of the ionization continuum above both and thresholds with that of the ionization continuum above the Ar edges. This comparison indicates that the lifetimes of the Ar(2p) and O(1s) hole states are approximately the same.

Beyond the dipole approximation: angular-distribution effects in valence photoemission

Angular distributions of valence photoelectrons showing effects due to higher-multipole photon interactions have been measured for the first time. Neon 2s and 2p photoemission exhibits effects beyond the dipole approximation throughout the 250 - 1200 eV photon-energy range studied. The results suggest that any photoemission experiment, on any sample, can be affected at relatively low photon energies, pointing to a general need for caution in interpreting angle-resolved-photoemission measurements.

Selectively excited X-ray emission spectra of N2

Abstract Resonant and non-resonant X-ray emission spectra of N2 are presented. The role of the parity selection rule in the case of homonuclear diatomic molecules is discussed thoroughly and illustrated by the X-ray emission spectra. We also discuss how the parity selection rule and the angular anisotropy of the resonant X-ray emission may affect intensities in a fluorescence yield measurement of N2. Simulated vibrational band profiles calculated using the lifetime–vibrational interference formula are found to be in excellent agreement with the experimental emission bands.

Competition between decay and dissociation of core-excited carbonyl sulfide studied by x-ray scattering

We show evidence of dissociation during resonant inelastic soft x-ray scattering. Carbon and oxygen K-shell and sulfur L-shell resonant and nonresonant x-ray emission spectra were measured using mo ...

Photon energy dependence of ionization-excitation in helium at medium energies

We have measured the photoionization-excitation-to-photoionization ratio for (n = 2 - 6) at several photon energies from 90 to 900 eV. By extrapolating these values we could determine the asymptotic high-energy ratios for (n = 2 - 5) which agree with theoretical predictions. We show that the satellite-to-main-line ratios are consistent with experimental double-to-single photoionization ratios and agree well with recent measurements.

Electron-ion recombination of Be-like C, N, and O

The absolute total recombination reaction rate coefficients for Be-like C, N, and O have been measured using the CRYRING storage ring and compared with the results from distorted-wave theory. For the theory results, it is found that shifts to NIST energy values for the core excited energies of the recombining system are not sufficient to accurately match all of the resonance positions and heights at lower energies. These theory results represent the quality of most archived theory DR data. The accurate calculation of these low energy resonances still presents a significant challenge to theory. In addition, trielectronic recombination resonances, associated with the formation of triply excited states during recombination, have been observed in the total recombination reaction rate coefficient spectra of N3+ and O4+. Finally, we construct a dielectronic recombination Maxwellian rate coefficient from the experimental results for low n resonances, and from the theoretical results for high n resonances. In the case of O4+, the trielectronic recombination resonances have a strong influence on the low temperature Maxwellian rate coefficient. Our best hybrid Maxwellian rate coefficient is compared with archived distorted-wave theory data, and is found to be in reasonable agreement, even at the low temperatures.

X-ray emission spectroscopy measurements of fluorine substituted methanes

The authors present the carbon and fluorine X-ray emission spectra of methane and the fluoromethanes (CH4-xFx, x=0, 1, 2, 3 and 4), measured in high resolution. Relative X-ray emission intensities in dipole velocity and length gauges have been derived from the independent particle approximation using Hartree-Fock ground state molecular orbitals. The results of the calculations are compared with the experimental spectra. In the experimental carbon (K-V) emission spectrum of CF4 the authors observe dipole-forbidden electronic transitions. They suggest that these transitions arise due to vibronic coupling, i.e. intensity is borrowed from the dipole-allowed transitions.