ALSNews is a biweekly electronic newsletter to keep users and other interested parties informed about developments at the Advanced Light Source, a national user facility located at Lawrence Berkeley National Laboratory, University of California. To be placed on the mailing list, send your name and complete internet address to ALSNews@lbl.gov. We welcome suggestions for topics and content.

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ALSNews           Vol. 78           May 28, 1997

Table of Contents


    1. OPERATIONS UPDATE
    2. SURFACE ALLOY STRUCTURE DETERMINED USING PHOTOELECTRON DIFFRACTION & HOLOGRAPHY
    3. SOLICITING ALS-RELATED WEB PAGES

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

The ALS is in a planned shutdown for equipment installation and maintenance. (For details, see ALSNews Vol. 74, April 2, 1997.) User beamtime is scheduled to resume at 00:00 on June 10 with the following schedule:

Jun 10, 00:00-08:00           User scrubbing & special operations
Jun 10, 08:00- Jun 16, 07:15  1.5-GeV/400-mA/288-bunch user operations

Weekly operations scheduling meetings will resume on Friday, June 6 at 3:30 p.m. in the Building 6 conference room.

2. SURFACE ALLOY STRUCTURE DETERMINED USING PHOTOELECTRON DIFFRACTION & HOLOGRAPHY
(contacts: sylvain@csd.uwm.edu, tonner@csd.uwm.edu)

For modern high-tech materials, the atomic geometries of surfaces and interfaces are frequently a key factor in performance, but measuring these geometries directly is not a simple matter. Most current methods rely on comparing experimental data with calculations from various models, to determine which model matches the observations most closely in general configuration and then to refine details of the model such as bond lengths. X-ray photoelectron diffraction (XPD, described in ALSNews Vol. 45, February 7, 1996) is one means of gathering experimental data for such comparisons. Many scientists hope that XPD will also provide the raw data sets for x-ray photoelectron holography, a method of analyzing data directly (using a Fourier transform) that does not require a model for comparison.

Researchers at the ALS recently used XPD with both these approaches to study a surface alloy of manganese and nickel on a nickel substrate. Their results not only confirm the general characteristics of an unusual earlier model for the alloy structure based on low-energy electron diffraction (LEED) measurements, but also lend hope for x-ray photoelectron holography as a method of direct, quantitative determination of surface structures. The experiments, performed using the ultraESCA endstation on Beamline 7.0.1, use complete photoelectron diffraction sets to create images of atomic positions using a simple Fourier transform, with no need for prior knowledge of the structure.

For surface alloys, as for other solids, the atomic geometry (crystal structure) is usually that with the lowest energy. When a solid crystal cleaves to form a surface, however, the broken bonds change the energetics, and the atoms rearrange themselves into a structure often different from that in the interior (bulk) of the crystal. This rearrangement is called surface reconstruction. In the case of alloys, the composition as well as the structure may change, giving rise to surface alloys that may not exist in the bulk. Furthermore, if one or more of the elements in the alloy is magnetic, then the magnetic ordering (ferromagnetic, antiferromagnetic, etc.) at the surface can also vary and may influence the surface structure.

All of these variables come into play in the manganese-nickel surface alloy, one atomic layer thick, that the experimenters prepared by depositing half an atomic layer of manganese on a nickel substrate in the (001) orientation. Earlier researchers using LEED and other techniques had reported that atoms on the surface were arranged in square patterns with nickel atoms at the four corners and manganese at the center. This surface alloy was one of the two possible structures consistent with the c(2x2) LEED diffraction pattern, the other being an overlayer of manganese atoms. The LEED researchers were also surprised to find that the manganese atoms were 0.25 angstroms farther above the substrate than the nickel atoms, giving rise to a buckled surface.

The research group at the ALS measured x-ray photoelectron diffraction patterns for a large number of x-ray photon energies and then used both comparative techniques and holography to examine the surface buckling in more detail. Their comparison of diffraction patterns with those calculated for model atomic geometries enabled them to determine which of the two structures compatible with the c(2x2) LEED pattern the sample exhibited, confirming the earlier proposed model with surface buckling. They also did a direct holographic transform from their experimental data which showed the buckled structure clearly, albeit with inaccurate interatomic distances.

Based on this study, the researchers hope that they may be able to combine XPD and holography into a new strategy, using holography to provide a basic structure and then refining that structure using simulations with the holographic structure as a starting point. One of their first steps in this direction will be to refine their algorithm and parameters for holographic transforms, using computer simulations and data from experiments like this one. By allowing the rapid acquisition of the large data sets necessary for high-resolution holographic transformations, high-brightness sources such as the ALS play a key role in this work.

This work was conducted by principal investigators Sylvain Ravy (Centre National de la Recherche Scientifique/Universite Paris-Sud) and Brian Tonner (University of Wisconsin-Milwaukee), with Santanu Banerjee, Jonathan Denlinger, and others from the Tonner research group.
Funding: National Science Foundation (grant No. DMR-9413475) and UWM Laboratory for Surface Studies.

3. SOLICITING ALS-RELATED WEB PAGES

The ALS is initiating a new Web page that will give links to user Web pages relating to research done at the ALS. This is meant to facilitate user-to-user communication and provide a quick reference for sites related to the ALS. Any research group at the ALS, or any user who has a page related to work at the ALS, should complete the following information and send it to alsuser@lbl.gov. The information will help us organize the links and make the new page useful for potential and current users.

Title of Group or User Name:
Affiliation(s):
URL:
Related ALS Beamline(s):
Areas of Research:
Basic Content of Site (e.g., experiment schematics, spectra, images,
descriptive text, papers, etc.):
Webmaster or responsible person for this URL (please provide name and email
address to help us maintain our links appropriately):