Wayne C. Stolte

Precision measurements of the total photoionization cross-sections of He, Ne, Ar, Kr, and Xe

Abstract Absolute photoionization cross-sections for the rare gases have been measured from threshold to 125 eV with an accuracy of ±1 to 3% and are presented in both tabular and graphical form. These data are compared with the optical oscillator strength measurements of Brion and co-workers, who used the Dipole (e, e) collision technique.

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.

Anion and cation formation following core-level photoexcitation of CO2

Mass-resolved anion and cation partial-yield spectra following photoexcitation of CO2 have been recorded in the vicinity of the C 1s and O 1s thresholds. Anion production was found to be site specific: a weak production of O− was observed on the π* resonance near the C K edge, whereas both C− and O− fragments were observed near the O K edge. Due to suppression of the shape resonances that dominate the cross section above the core thresholds, the anion-partial-yield spectra at the O K edge reveal several previously unobserved electronic states. In addition, several cations such as CO2+ and O2+ were observed for the first time.

Design and performance of a curved-crystal x-ray emission spectrometer

A curved-crystal x-ray emission spectrometer has been designed and built to measure 2-5 keV x-ray fluorescence resulting from a core-level excitation of gas phase species. The spectrometer can rotate 180 degrees, allowing detection of emitted x rays with variable polarization angles, and is capable of collecting spectra over a wide energy range (20 eV wide with 0.5 eV resolution at the Cl K edge) simultaneously. In addition, the entire experimental chamber can be rotated about the incident-radiation axis by nearly 360 degrees while maintaining vacuum, permitting measurements of angular distributions of emitted x rays.

Anionic and cationic photofragmentation of core-excited N2O

We have measured all detectable cationic and anionic fragments in singlechannel mode from N2O as a function of photon energy in the vicinity of the nitrogen 1s core-level threshold. Due to the high degree of localization of the core electrons, the two excitations Nt1s → 3π∗ and Nc1s → 3π∗ show high levels of site-selective behaviour. The observed partial ion yield for the sole anionic fragment,O−, in conjunctionwith the partial cation yields,confirms our previous demonstration of anion-yield spectroscopy as a unique tool to identify core-level shape resonances.

DISSOCIATIVE PHOTOIONIZATION CROSS SECTIONS OF N2AND O2FROM 100 TO 800 eV

We report measured partial cross sections for the fragments from dissociative photoionization of N2 and O2 over the photon energy range of 100 to 800 eV. The partial cross sections were found to follow a power law of the form (hn), where hn is the incident photon energy. The exponent, a, is shown to be near 2.7 for N 2 and for all N2 partial cross sections above the nitrogen K-edge. An exponent of 3.0 was found for O 2 and for all O2 partial cross sections above the oxygen Kedge. q 1998 Academic Press

Yttrium and hydrogen superstructure and correlation of lattice expansion and proton conductivity in the BaZr0.9Y0.1O2.95 proton conductor

Bragg reflections in Y-resonant x-ray diffractograms of BaZr0.9Y0.1O2.95 (BZY10) reveal that Y is organized in a superstructure. Comparison with neutron diffraction superstructure reflections in protonated/deuterated BZY10 suggests that both superstructures are linked, and that protons move in the landscape imposed by the Y. The thermal lattice expansion decreases abruptly for protonated BZY10 at T≥648±20 K, coinciding with the onset of lateral proton diffusion and suggesting a correlation of structural changes and proton conductivity. The chemical shift in the Y L1-shell x-ray absorption spectra reveals a reduction from Y3+ toward Y2+ upon protonation.

Fragmentation processes following core excitation in acetylene and ethylene by partial ion yield spectroscopy

Partial ion yield spectroscopy provides a very detailed picture of fragmentation processes following core excitation in isolated molecules. We exploit this potential in the analysis of decay processes following C1s→π and C1s→Rydberg excitations in ethylene and acetylene. We show that the relative intensity of spectral features related to the excitation to empty molecular orbitals or to Rydberg states is a function of the time variation of the fragmentation process. Namely, we see an intensity increase in the Rydberg states compared to the molecular orbitals as the fragmentation process becomes more extensive, a result attributable to the diffuse nature of the Rydberg virtual orbitals, which makes spectator decay more likely than participator decay. Therefore, the number of dissociative final states that can be reached from Rydberg excitation is higher than for excitation to empty molecular orbitals. In acetylene, we obtain the first direct evidence of the presence of a σ* excitation embedded in the Rydber...

A new method to derive electronegativity from resonant inelastic x-ray scattering

Electronegativity is a well-known property of atoms and substituent groups. Because there is no direct way to measure it, establishing a useful scale for electronegativity often entails correlating it to another chemical parameter; a wide variety of methods have been proposed over the past 80 years to do just that. This work reports a new approach that connects electronegativity to a spectroscopic parameter derived from resonant inelastic x-ray scattering. The new method is demonstrated using a series of chlorine-containing compounds, focusing on the Cl 2p(-1)LUMO(1) electronic states reached after Cl 1s → LUMO core excitation and subsequent KL radiative decay. Based on an electron-density analysis of the LUMOs, the relative weights of the Cl 2p(z) atomic orbital contributing to the Cl 2p(3/2) molecular spin-orbit components are shown to yield a linear electronegativity scale consistent with previous approaches.

Fragmentation of methyl chloride studied by partial positive and negative ion-yield spectroscopy

The authors present partial-ion-yield experiments on the methyl chloride molecule excited in the vicinity of the Cl2p and C1s inner shells. A large number of fragments, cations produced by dissociation or recombination processes, as well as anionic species, have been detected. Although the spectra exhibit different intensity distributions depending on the core-excited atom, general observations include strong site-selective fragmentation along the C-Cl bond axis and a strong intensity dependence of transitions involving Rydberg series on fragment size.