Dennis W. Lindle

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.

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.

Oxygen transport studies in nanocrystalline ceria films

Oxygen uptake and conductivity were measured by nuclear reaction analysis and ac impedance technique at the intermediate temperature range on sol-gel grown nanocrystalline ceria films with average grain-sizes 7 nm and 38 nm synthesized at 723 K and 1173 K, respectively. Higher oxygen uptake and lower ionic conductivity are observed in ceria films with {approx}7 nm grain-size. High permeation-assisted oxygen diffusion in nanocrystallites combined with oxygen trapping in the disordered region contributes in higher oxygen uptake. However the lower ionic conductivity in the film results from absence of long-range lattice ordering. Relationship between oxygen uptake and conductivity in ceria is discussed in details by considering grain-size dependent defect density, related surface area and enhanced oxygen mobility.

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.

100 % site-selective fragmentation in core-hole-photoexcited methanol by anion-yield spectroscopy

Anion-yield spectroscopy is shown to provide the first experimental demonstration of 100% site selectivity in small-molecule photofragmentation processes. In methanol, CH3OH, creation of the anionic OH- fragment occurs only via resonant excitation below the carbon K edge; the fragment is completely absent near the oxygen K edge. This observation and comparison of all anion and cation partial yields confirm the existence of at least two mechanisms for the creation of anions near the carbon K edge.

Breakdown of the dipole approximation in soft-X-ray photoemission

Abstract Although breakdowns in the dipole approximation in the soft-X-ray photon-energy range ( h ν≤5 keV) were first observed 30 years ago and have been studied theoretically for many years, their significance at low energies has remained generally unappreciated within the broader photoemission community. Advances in gas-phase photoemission experiments using synchrotron radiation have recently highlighted nondipole effects at relatively low energies while probing the limits of the dipole approximation. Breakdowns in this approximation are manifested primarily as deviations from dipolar angular distributions of photoelectrons. Detailed new results demonstrate nondipolar angular-distribution effects are easily observable in atomic gases at energies well below 1 keV, and, in molecules, a previously unexpected phenomenon greatly enhances the breakdown of the dipole approximation just above core-level ionization thresholds. A progress report on this newly burgeoning area from an experimental perspective is presented here, including a brief history, a description of recent advances, graphical representations of nondipolar angular distributions, a re-evaluation of the classic first experiment in the soft-X-ray range and a look to the future.