Dennis W. Lindle

Shake-off on inner-shell resonances of Ar, Kr and Xe

Synchrotron radiation was used to excite an inner-shell electron into a Rydberg orbital at the Ar 2p to ns, nd, Kr 3d to np and Xe 4d to n p resonances. The resonant decay into shake-off channels was studied by three different electron measurements. Firstly, threshold electron scans were obtained over the resonances and thresholds. On the first resonance for each atom, photoelectron spectra were collected. The intensity distribution of low kinetic energy electrons was also determined for a few resonances. Finally, a shake calculation was carried out to compare with the experimental shake-off probabilities. Shake-off is observed to be a strong decay channel for these resonances.

Selecting core-hole localization or delocalization in CS2 by photofragmentation dynamics.

Electronic core levels in molecules are highly localized around one atomic site. However, in single-photon ionization of symmetric molecules, the question of core-hole localization versus delocalization over two equivalent atoms has long been debated as the answer lies at the heart of quantum mechanics. Here, using a joint experimental and theoretical study of core-ionized carbon disulfide (CS2), we demonstrate that it is possible to experimentally select distinct molecular-fragmentation pathways in which the core hole can be considered as either localized on one sulfur atom or delocalized between two indistinguishable sulfur atoms. This feat is accomplished by measuring photoelectron angular distributions within the frame of the molecule, directly probing entanglement or disentanglement of quantum pathways as a function of how the molecule dissociates.

Photofragmentation of third-row hydrides following photoexcitation at deep-core levels

The relaxation dynamics of HCl, DCl, H2S, and D2S following photoexcitation in the vicinities of the Cl and S K-shell thresholds ( ;2.8 keV for Cl, ;2.5 keV for S! were studied by means of ion time-of-flight mass spectroscopy. In all cases, the onset of pre-edge core-shell photoionization precedes the formation on resonance of a significant amount of neutral hydrogen as well as postcollision-interaction effects above threshold. Examination of the width of the H 1 peak in spectra taken with the analyzer parallel and perpendicular to the polarization vector of the incident light indicates that on resonance, the photofragmentation asymmetry parameter, b, is approximately two for HCl, and is clearly positive for H2S. @S1050-2947~98!08211-0# PACS number~s!: 33.80.Gj, 33.80.Eh Time-of-flight mass spectroscopy and coincidence measurements of atoms and molecules are relatively wellunderstood techniques @1‐3#. Coupled with the high bright

Design and performance of the Advanced Light Source double‐crystal monochromator

A new ‘‘Cowan type’’ double‐crystal monochromator, based on the boomerang design used at National Synchrotron Light Source (NSLS) beamline X‐24A, has been developed for beamline 9.3.1 at the Advanced Light Source (ALS), a windowless ultrahigh vacuum beamline covering the 1–6 keV photon‐energy range. Beamline 9.3.1 is designed to simultaneously achieve the goals of high energy resolution, high flux, and high brightness at the sample. The mechanical design of the monochromator has been simplified, and recent developments in technology have been included. Measured mechanical precision of the monochromator shows significant improvement over existing designs. In tests with x‐rays at NSLS beamline X‐23A2, maximum deviations in the intensity of monochromatic light were just 7% during scans of several hundred eV in the vicinity of the Cr K edge (6 keV) with the monochromator operating without intensity feedback. Such precision is essential because of the high brightness of the ALS radiation and the overall length...

Hard-X-ray photoelectron spectroscopy of atoms and molecules

We review here the current status of atomic and molecular HAXPES, both experimentally and theoretically. After the era of the ESCA technique, atomic and molecular HAXPES experienced several decades when the emphasis in gas-phase research was in the soft-X-ray range, mainly due to the lack of suitable experimental conditions. In the last few years, at some of the newer synchrotron radiation sources, hard-X-ray beamlines have been developed which provide state-of-the-art instrumentation for the atomic and molecular community. A substantial impulse to the field has also been recently given by the new X-ray free-electron lasers (FELs). Therefore, the high-brightness and high-resolution sources needed for a thriving gas-phase HAXPES community are now available. We concentrate here on experimental results based on the use of SR and, more recently, FELs as ionizing sources. Available parallel theoretical developments are also included. We define the hard-X-ray photon-energy range as 1 keV or higher. We provide a summary of early theoretical work in atomic and molecular HAXPES, and then an overview on HAXPES performed at first- and second-generation SR sources and at current-generation facilities. Scientific topics of particular interest in the hard-X-ray range include nondipole effects, recoil due to the photoelectron’s momentum, new interference phenomena, ultrafast nuclear motion on the femto- and sub-femtosecond scale, and double-core-hole studies. We conclude with a look at future directions in these areas and a few others of potential interest for HAXPES.

Photoelectron spectroscopy and the dipole approximation

Over the past three decades, the dipole approximation has facilitated a basic understanding of the photoionization process in atoms and molecules. Advances in gas-phase photoemission experiments using synchrotron radiation have recently highlighted nondipole effects at relatively low photon 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 the core-level ionization threshold.

Resonance and threshold effects in photoemission up to 3500 eV

Beam Lines at the Stanford Synchrotron Radiation Laboratory (SSRL) now provide photon beams throughout the entire energy range 5–5000 eV, with a pulse structure very well‐suited to time‐of‐flight (TOF) photoelectron spectroscopy. We have used this facility, together with a TOF spectrometer, to measure photoemission cross sections σ(e) and asymmetry parameters β(e) for several interesting systems. A summary of early results is given.Metal vapors (Ba, Cd, Mn, Hg) were studied using a hightemperature oven. Resonant photoemission was observed in several cases. Both σ(e) and β(e) showed resonant behavior at 21.1 eV for several lines in Cd. The 4d, 5s, and 5p σ(e) line profiles differed dramatically, illustrating the detailed information about continuum states that is available from photoemission.Correlation satellites in photoemission from rare gases have been observed over a very wide energy range, including those seen in the K‐shells of He, Ne and Ar and in the L‐shell of Ne. The structure and preliminary in...

First results from the high brightness x‐ray spectroscopy beamline 9.3.1 at ALS

Beamline 9.3.1 at the Advanced Light Source (ALS) is a windowless beamline, covering the 1–6 keV photon energy range. This beamline is designed to achieve the goal of high brightness at the sample for use in the x‐ray atomic and molecular spectroscopy (XAMS) science, surface and interface science, biology and x‐ray optical development programs at ALS. X‐ray absorption and time‐of‐flight photoemission measurements in 2–5 keV photon energy range along with the flux, resolution, spot size and stability of the beamline will be discussed. Prospects for future XAMS measurements will also be presented.

Resonance photoelectron spectroscopy of 5p hole states in atomic barium

Photoelectron spectra of atomic barium have been recorded at several photon energies in the range 20 beta (6p)> beta (5d). Above 21 eV, 5p ionisation begins to dominate, and the Auger spectra were used to monitor the production of the various 5p hole states. As the photon energy is scanned across the autoionising resonances, these Auger spectra indicate that excitation of 5p hole states decaying by two-step autoionisation is the dominant mode in the production of very-low-energy photoelectrons (<or=2 eV) and their corresponding high-energy Auger electrons. Calculations suggest that a heretofore unobserved 5p56p2 2P32/5d autoionising level is responsible for the Auger distribution measured at hv=28.9 eV.

Design and performance of the ALS double-crystal monochromator

A new ``Cowan type`` double-crystal monochromator, based on the boomerang design used at NSLS beamline X-24A, has been developed for beamline 9.3.1 at the ALS, a windowless UHV beamline covering the 1-6 keV photon-energy range. Beamline 9.3.1 is designed to simultaneously achieve the goals of high energy resolution, high flux, and high brightness at the sample. The mechanical design has been simplified, and recent developments in technology have been included. Measured mechanical precision of the monochromator shows significant improvement over existing designs. In tests with x-rays at NSLS beamline X-A, maximum deviations in the intensity of monochromatic light were just 7% during scans of several hundred eV in the vicinity of the Cr K edge (6 keV) with the monochromator operating without intensity feedback. Such precision is essential because of the high brightness of the ALS radiation and the overall length of beamline 9.3.1 (26 m).