Rupert C. C. Perera

Soft X-ray emission spectroscopy of early transition metal compounds

Abstract Soft X-ray emission studies for several early transition metal compounds, such as Sc, Ti and V compounds, have been studied. Raman scattering was found at the 2p→3d excitation in the transition metal compounds. The elementary excitation of the Raman spectrum is mainly elucidated by the d–d transition within the 3d bands, as well as the interband transition from the O 2p valence band to the conduction band, which consists mainly of transition metal 3d states. The charge transfer transition due to the localized electron picture is dominant in the soft X-ray spectra of Sc compounds. Raman scattering is a powerful tool in the study of the electronic structure of transition metal compounds.

Multilayer-coated mirrors as power filters in synchrotron radiation beamlines

Multilayer-coated mirrors, rather than conventional total-reflection mirrors, have been proposed as a means to reduce power incident on the first optical element of high resolution monochromators. We have designed, fabricated, installed and characterized a multilayer pre-mirror specifically for the 800 to 4000 eV range for the X24-A bending magnet beamline at the National Synchrotron Light Source. Various aspects of this application are discussed, including power and thermal considerations, beamline layout considerations and constraints, choice of multilayer materials and substrate, and techniques to ensure lateral uniformity of the multilayer. Results of a preliminary characterization of a mirror coated with a SiC/V multilayer and installed in the beamline are also discussed. 9 refs., 5 figs.

Molecular x‐ray spectra: S‐Kβ emission and K absorption spectra of thiophene

The high resolution sulfur Kβ emission in fluorescence and sulfur K absorption of thiophene (C4H4S) were measured with a double crystal spectrometer. The sulfur Kβ emission spectrum was analyzed by comparison with complementary spectral data and with MNDO and ab initio (STO‐3G) MO calculations. A tentative assignment of the prominent features in the absorption spectrum was made using MO calculations as a guide. In addition the sulfur LII,III emission spectrum was reinterpreted. The sulfur 1s binding energy of thiophene was estimated as 2477.6 eV.

Cl‐Kβ emission of chloromethanes and comparison with semiempirical and abinitio MO calculations

The chlorine Kβ emission in fluorescence from gas/vapor phase CCl4, CHCl3, CH2Cl2, and CH3Cl were measured with ‘‘good statistics’’ and previously undetected weak x‐ray emission features were observed. Results from ground state minimal and extended basis set ab initio (STO‐3G) and MNDO MO calculations for these molecules are presented. The chlorine Kβ emission spectra were analyzed by comparison with complementary spectral data and the MO calculations. In addition, weak features in Cl‐Kβ emission spectra resulting from self‐absorption in the sample were identified.

Cl Kβ (K-V) emission of CFCl3 excited by synchrotron radiation below and above Cl-1s binding energy: perturbation effects in a highly excited neutral molecule

The Cl Kβ (K-V) fluorescent emission and K absorption spectra from CFCl3 vapor were measured using monochromatic synchrotron radiation. Weak supra-threshold features were observed in the Cl K absorption spectrum. The Cl 1s binding energy of CFCl3 was estimated to be 2829.4eV. A comparison of the Cl Kβ (K-V) emission spectrum excited at 2833.5eV (K + 4eV) photon energy with the Kβ spectrum excited at 2880eV (K + 50eV) photon energy indicated an increase of emission intensity from multiple vacancy transitions. The Cl K absorption has a discrete absorption maximum (or a white-line) at 2822.6eV, the transition of a Cl 1s electron to the first allowed unoccupied MO. For the first time an X-ray emission spectrum was obtained with excitation below the ionization threshold. Features in the 2822.6eV excited (an exciton state, i.e., a highly excited state of the neutral molecule) Cl Kβ (K-V) spectrum were observed to have significant intensity changes and relative energy position shifts from the emission spectrum excited above the Cl 1s threshold. These differences are explained in terms of perturbation effects due to the presence of an electron in the first unoccupied MO.

Optical and spectral characteristics of an insertion device used both as a wiggler and an undulator

Abstract For experiments using the energy region below 4 keV the LBL/EXXON insertion device on Beam Line VI-2 at the Stanford Synchrotron Radiation Laboratory (SSRL) can be reconfigured to work as a source of undulator radiation. For example, with a K of 0.94 rather than its normal value of 5–8 the fourth harmonic of the undulator coincides with the Ar K absorption edge at 3.2 keV. Because the total power is relatively low, carbon foils protecting the beryllium window can be removed. Thus there is a net gain the flux at the Ar edge over that obtainable in wiggler mode with carbon absorbers in place. In addition the beam transmitted by the monochromator has a lower harmonic content and improved energy resolution.

Near-Threshold X-Ray Fluorescence Spectroscopy Of Molecules

The coupling of high-energy-resolution x-ray absorption and emission spectroscopies with a high-intensity and high-resolution synchrotron-radiation beamline has opened up several new avenues of research in inner-shell molecular physics. Of particular importance is the ability to tune the incident photon energy throughout the near-threshold region for the atomic core levels of the molecules. Sub- and supra-threshold excitations have revealed a diversity of phenomena, including screening effects below threshold, satellite-free fluorescence above threshold, and fluorescence polarization in both energy regions. All of these phenomena can be used in a complementary way to uncover some of the underlying molecular dynamics which occur upon inner-shell excitation and ionization. Some of the latest results from the NBS X-Ray Spectroscopy Beamline at NSLS will be presented to highlight these effects.