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ALSNews           Vol. 45           February 7, 1996

Table of Contents


    1. OPERATIONS UPDATE
    2. XPD STUDIES YIELD VOLUMES OF INFORMATION
    3. AN XPD PRIMER

1. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

Beam availability for the last two weeks was 87.6% overall and 88% during user shifts. Causes of lost beamtime included intelligent local controller (ILC) and ILC power supply problems, magnet water flow interlock trips, and storage ring rf and beamline hutch interlock trips. Swing shift on February 4 was lost due to several power dips and equipment failures caused by the weekend storm.

Operations Summary for February 7 - February 26

Feb 07, 00:00-08:00           User Scrubbing & Special Operations
Feb 07, 08:00- Feb 12, 07:15  1.5-GeV/400-mA/320-bunch user operations
Feb 12, 07:30-24:00           Maintenance & Startup
Feb 13, 00:00-24:00           Accelerator Physics
Feb 14, 00:00-08:00           User Scrubbing & Special Operations
Feb 14, 08:00- Feb 18, 23:15  1.5-GeV/400-mA/320-bunch user operations
Feb 19, 00:00- Feb 20, 07:30  Holiday
Feb 20, 07:30-24:00           Maintenance & Startup
Feb 21, 00:00-16:00           Accelerator Physics
Feb 21, 16:00- Feb 26, 07:15  1.5-GeV/400-mA/320-bunch user operations

Weekly operations scheduling meetings: Fridays at 3:30 p.m. in the Building 6 conference room.

** SEPTEMBER SHUTDOWN POSTPONED **
The shutdown which had been tentatively scheduled to begin in mid-September has been postponed to 1997 to provide increased beamtime to users from the end of the April shutdown through the rest of the year. The proposed operations schedule for this period (see next item) includes additional one-day maintenance periods between June and November for various tasks which would have been done during the September shutdown. The extra days allotted to maintenance work will be taken from accelerator-physics time and user time in alternate months.

** DRAFT POST-SHUTDOWN SCHEDULE AVAILABLE **
A draft of the long-term schedule for operations for May 22 to November 3 is now available from the ALS. If you would like a copy of the proposed schedule, send a "please send me a long-term operation schedule" message to alsuser@lbl.gov and provide either your full postal mailing address or your fax number.

Users who have specific comments on the proposed schedule should send them to Fred Schlachter (email: fred_schlachter@lbl.gov; fax: 510-486-6499) by February 19. The final schedule for the period May-November will be issued and appear in ALSNews in early March. The operations schedule for the period after November 3 will be established at a later date after additional discussion with users.

2. XPD STUDIES YIELD VOLUMES OF INFORMATION
(contact: jddenlinger@lbl.gov)

Using x-ray photoelectron diffraction (XPD), researchers at the ALS have generated an exceedingly detailed set of diffraction data for crystalline copper. The experiments on copper (100) were performed by Jonathan Denlinger and Eli Rotenberg of the Beamline 7.0 ultraESCA group. The researchers have also acquired similar data sets for reconstructed surfaces of tungsten (110) and silicon, studying both bulk and surface environments.

The fine detail of the data set provides an opportunity to test holographic methods for transforming XPD data (using a Fourier transform) into a representation of the crystal in real space. Since the structure of copper (100) is already well understood, the data can be used to determine the accuracy of such manipulations and to compare different transformational methods. The copper data set also provides a test for theoretical multiple-scattering simulations that may explain subtle features in the data.

X-ray photoelectron diffraction is a technique for probing crystal structure. Incoming x rays eject electrons from core levels (inner shells) of atoms in the sample. The trajectories of the ejected electrons are influenced by elastic and inelastic scattering (scattering in which energy is not lost to the scatterer and scattering in which energy is lost, respectively) around nearby atoms. The elastic scattering gives rise to variations in intensity throughout the angular distribution of emitted electrons. These variations carry information about orientations and distances between near-neighbor atoms in the crystal. The level of incoming photon energy also affects the intensity variations, because it determines the kinetic energies of ejected electrons. (For more details about XPD, see "An XPD Primer" in this issue.)

Photoelectron diffraction experiments have generally been approached in one of two ways: either by collecting electrons at a single kinetic energy over a broad range of angles (angle-dependent XPD) or by collecting electrons of a single binding energy (from a single orbital) at one angle over a broad range of kinetic energies (energy-dependent XPD). The high flux provided by the ALS at high photon-energy resolutions makes collection times short enough that scientists can now gather data over a broad range of both angles and energies in relatively little time. The new data set for bulk 3p emission from copper (100), acquired in three eight-hour working shifts, reflects data collection at about 480 angles for 75 distinct energies.

The group's XPD data can be represented as a volume data set-a three-dimensional plot of the intensities of collected electrons according to their kinetic energies and angular distribution. It resembles a cylinder of marble with light areas corresponding to high photoelectron intensities. The vertical axis represents kinetic energy, and each horizontal slice corresponds to a circular diffraction pattern (a planar projection of the angle-dependent XPD results) for a specific energy. Stacking up the diffraction patterns into a volume makes it possible to view, for the first time, the continuous variation of intensity oscillations with changes in kinetic energy.

The information in the volume data set is so rich that the researchers have taken to presenting the data as movies. One version steps through all the horizontal slices from the cylinder sequentially, showing three distinct regimes: low kinetic energies with rapidly changing intensity patterns where multiple scattering is predominant, an intermediate kinetic energy region where backscattering oscillations dominate, and higher kinetic energies with relatively constant patterns that arise from forward scattering. Another movie shows vertical cross sections through the cylinder rotated about the kinetic energy axis, highlighting oscillations in backscattering intensities and the continuity of this diffraction structure over the different angles.

This experiment was performed by B. P. Tonner (principal investigator) and J. D. Denlinger (University of Wisconsin-Milwaukee) and Eli Rotenberg and S. D. Kevan (University of Oregon).
Funding: Office of Basic Energy Sciences of the U.S. Department of Energy.

3. AN XPD PRIMER

X-ray photoelectron diffraction (XPD) is a surface-sensitive technique for studying the structures of crystals. X-ray photons are used to excite core-level electrons in a sample enough to eject the electrons from their atoms. Ejected photoelectrons of a single kinetic energy can then be collected over a full angular distribution. Since the kinetic energy of the photoelectron depends on the binding energy that holds it in its atom, and the binding energy in turn depends on the type of atom (the element and its chemical environment), scientists can detect electrons coming from a single type of atom in a sample and thus study the crystal structure around it. This chemical specificity allows researchers to differentiate even between oxidation states of the same element or between bulk and surface atoms in the same sample.

The angles at which electrons emerge from the sample are influenced by scattering. Within a crystal, electrons escape from emitter atoms and scatter from near-neighbor atoms as they pass by them. In the wave view of electrons, the original and scattered electron waves interfere with each other, giving rise to a diffraction pattern. This pattern shows the numbers of electrons detected at various angles with respect to the sample surface; intensity peaks form where the original and scattered electron waves interfere constructively. Peak locations depend on the distances and angular relationships between the atoms in the crystal and also on the incoming photon energy (since this influences the electrons' kinetic energies and thus their scattering behavior).

In a typical XPD experiment, the detector and x-ray source remain in a fixed position, but the sample platform can rotate through different polar and azimuthal angles, allowing detection of emitted electrons over a wide range of directions. Researchers can also obtain an energy scan (spectrum) for a specific emission angle by varying the photon energy and setting the detector to collect core electrons with the corresponding kinetic energies.

The interpretation of XPD data, whether angle-dependent or energy-dependent, focuses on two main types of scattering: forward scattering and backscattering. In forward scattering, the emitted electron wave scatters around an atom lying between the emitter and the detector. Constructive interference between the emitted and scattered waves occurs along interatomic axes, so the diffraction pattern reveals the orientations of the axes in the crystal. Constructive interference also gives secondary intensities whose distances from each other relate to the distance between the two atoms. Forward scattering is especially prominent at higher kinetic energies.

In backscattering, the emitted electron wave scatters off an atom behind the emitter (i.e., the emitter is roughly between the scatterer and the detector). Backscattering intensities, which also result from interference between the emitted and scattered waves, are more pronounced at lower kinetic energies. Over this energy range, the intensities of backscattering peaks oscillate as the interference changes from constructive to destructive. The period of this oscillation is directly related to the distance between the emitter and the scatterer, so researchers can calculate interatomic distances by studying these oscillations.