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. 108          August 5, 1998

Table of Contents


    1. MULTI-ATOM RESONANT PHOTOEMISSION DEMONSTRATED
    2. ALS HOSTS VUV-XII CONFERENCE IN SAN FRANCISCO
    3. ALS USERS' MEETING CALL FOR ABSTRACTS
    4. HOUSING AVAILABLE TO ALS USERS
    5. WHO'S IN TOWN: A SAMPLING OF ALS USERS
    6. OPERATIONS UPDATE

1. MULTI-ATOM RESONANT PHOTOEMISSION DEMONSTRATED
(Contact: CSFadley@lbl.gov)

Surface-science techniques based on x rays are justly renowned for their elemental and sometimes chemical specificity, which derives from the excitation of core electrons at characteristic photon energies. However, these techniques do not directly identify the atomic numbers of the atoms neighboring the absorbing atom in multielement samples. A new technique called multi-atom resonant photoemission (MARPE), recently demonstrated at the ALS by a group from the University of California at Davis and Berkeley Lab, promises to provide information on neighboring atoms and possibly on other features of surface structure.

In normal photoemission from a partially occupied energy level, sweeping the incoming photon energy causes a smooth variation in the photoelectron intensity as long as one is not near an absorption edge. Single-atom resonant photoemission (SARPE)--a well established phenomenon--and now MARPE, however, show a sharp increase in the photoelectron signal when the incoming photon energy matches the energy difference between two energy levels in an atom. For SARPE, these two levels are the one from which the photoelectrons emerge and a lower level in the same atom. The incoming photon excites a resonant transition from the lower to the upper of the two levels, followed immediately by a reverse transition back to the lower level, and the energy released in the downward transition excites a photoelectron from the upper level. In MARPE, the resonant transition is between two levels in an atom that is near the photoemitting atom and coupled to it (e.g., by the Coulomb force). Since the resonance occurs at a photon energy that is characteristic of the neighbor atom, the identity of the neighbor is established.

To demonstrate MARPE, the group studied manganese oxide (MnO), iron oxide (Fe2O3), and the compound La0.7Sr0.3MnO3. In MnO, the researchers measured photoelectron spectra from the oxygen 1s core level. Using the area above the background in the photoelectron spectrum as the measure of the photoelectron intensity at each photon energy, they measured an excess signal when the photon energy swept through the energies needed to excite transitions from manganese 2p to manganese 3d levels (2p resonances). This excess, which closely tracked the manganese absorption spectrum, is the signature of the MARPE effect. The peak excess was approximately 43%, but the excess integrated over the relevant spectral region was about 11%, which compares favorably with a value of 2.5% calculated from a simple extension of the theory for SARPE. The researchers found larger effects in the other compounds, with peak values up to 105% for manganese 3d resonances in La0.7Sr0.3MnO3. They also found enhancement of manganese photoemission due to lanthanum 3d resonances, a second nearest-neighbor interaction.

With the MARPE effect firmly established, the next step is to explore its range of applicability. For example, calibrating resonance enhancements in standard compounds may permit estimating the number of neighbor atoms of each type in unknown samples. Moreover, while the researchers used photoelectron detection in the initial experiments, they believe that detection of x-ray fluorescence (providing a deeper probe) or Auger electrons will also reveal the effect. Sensitivities to bond distance, type of bond, chemical shift, and magnetic order (when using circularly polarized synchrotron radiation) are as yet undetermined.

Research conducted by A. Kay , S. Mun, C. S. Fadley, and R. Denecke (University of California, Davis and Berkeley Lab), E. Arenholz and M. A. Van Hove (Berkeley Lab), J. Garcia de Abajo (University of California, Davis and Universidad del Pais Vasco/EHU, Spain), and Z. Hussain (Berkeley Lab) using the Advanced Photoelectron Spectrometer/Diffractometer on Beamline 9.3.2.

Funding: U. S. Department of Energy with additional support from the Miller Institute, the Deutsche Forschungsgemeinschaft, and the Basque government.

2. ALS HOSTS VUV-XII CONFERENCE IN SAN FRANCISCO

The 12th International Conference on Vacuum Ultraviolet Radiation Physics is happening this week at the Grand Hyatt Hotel on Union Square, San Francisco. The conference, which began Monday morning and runs through Friday afternoon, was organized by the ALS and Berkeley Lab. It features poster sessions, vendor exhibits, and invited talks relating to the interaction of ultraviolet and soft x-ray radiation with matter over a photon-energy range of 5 eV to several keV. Detailed information about VUV-XII is available at the conference web site (http://www-als.lbl.gov/als/vuv12/).

3. ALS USERS' MEETING CALL FOR ABSTRACTS
(Contact: w_meyer-ilse@lbl.gov)

The Advanced Light Source Users' Executive Committee invites users, including students and postdocs, to submit abstracts for oral and poster presentations at the annual Users' Meeting, which will be held at Berkeley Lab on October 22-23, 1998. Some abstracts will be selected as highlights for oral presentation; other submissions will be presented as posters. A special student poster competition with a cash award will also be held at this year's meeting. The poster sessions will be held in conjunction with the vendor exhibits on Thursday, October 22, from 12 to 3 p.m., and Friday, October 23, from 12 to 2 p.m.

To propose a presentation, submit a one-page abstract by Thursday, August 20, 1998 to

Ruth Pepe
Advanced Light Source, MS 6-2100
Berkeley Lab
Berkeley, CA 94720
Fax: 510-486-4773
Email: alsum@lbl.gov

The annual Users' Meeting, sponsored by the Users' Executive Committee, will include new results from the ALS, reports from ALS management, and updates on funding prospects. If you have not received a copy of the meeting's first announcement, which was mailed two weeks ago, and wish to be added to the ALS mailing list, please send your name, affiliation, and complete postal address by email to alsuser@lbl.gov. Meeting information is also available on the web at the address noted above.

4. HOUSING AVAILABLE TO ALS USERS

The Laboratory Directorate has finalized the lease of four 2-bedroom apartments for use by ALS users. The apartments will be rented on a weekly basis, Monday to Sunday, at a rate of $245.00 per person. Each bedroom has two beds, so the maximum number of people per apartment would be four. The apartments are fully equipped with microwave oven, refrigerator, and other amenities. Cleaning has been arranged with the apartment manager. The apartments, which will become available August 15, are located along the Berkeley Lab shuttle bus route, near the corner of Oxford and Hearst streets, and will be rented on a first-come, first-served basis. Reservations can be made with the User Services Office at (510) 486-7745. More information is available on the Web at http://www-als.lbl.gov/als/quickguide/housing.html.

5. WHO'S IN TOWN: A SAMPLING OF ALS USERS

To highlight the richness of our user community and help introduce recent arrivals, we offer this listing of some of the experimenters who will be collecting data during the next two weeks at the ALS.

Beamline 1.4.1: Joel Ager, Wei Shan, Eugene Haller (all of the Materials Sciences Division, Berkeley Lab, and University of California, Berkeley) will be commissioning the UV Photoluminescence endstation.

Beamline 1.4.3: Regina Goth-Goldstein (Environmental Energy Technologies Division, Berkeley Lab) and Hoi-Ying Holman (Center for Environmental Biotechnology, Berkeley Lab) will study identifying markers for human exposure to polyaromatic hydrocarbons.

Beamline 3.3.2: Dean Wiberg of the Jet Propulsion Laboratory (JPL) is continuing work on x-ray collimating grids for the High Energy Solar Spectroscopic Imager (HESSI) in collaboration with the Space Sciences Laboratory. JPL is also beginning preliminary work on a high-aspect-ratio structure that will function as a dichroic filter at microwave frequencies.

Beamline 8.0.1: Manfred Neumann (University of Osnabrueck, Germany) will investigate the band structure of ferroelectric perovskites by x-ray resonant inelastic scattering. Also conducting materials science experiments on the beamline will be Thomas Calcott (University of Tennessee) and David Ederer (Tulane University), John Carlisle (Virginia Commonwealth University), and Franz Himpsel (University of Wisconsin-Madison) and Lou Terminello (Lawrence Livermore National Laboratory).

Beamline 9.0.2.2: Gary Jarvis (Chemical Sciences Division, Berkeley Lab) will be studying pulsed-field ionization of water.

Beamline 10.3.2: Maya Kiskinova and Kevin Prince will be visiting from the ELETTRA storage ring in Trieste, Italy. In one experiment, they will investigate gold-copper ores to determine the efficiencies of gold-extraction processes. In another experiment, a collaboration with Richard Lambert (Cambridge University), they will examine artificially alkali-enriched thin films of Pt. The films apparently have the same selectivity and reactivity for the catalytic reduction of NO, but neither the precise chemical associations nor the distributions are understood. Also, Richard Reeder (State University of New York, Stony Brook) will be looking at complex cation ordering in certain sedimentary materials.

6. OPERATIONS UPDATE
(Contact: rmmiller@lbl.gov)

Beam reliability for the last two weeks was 97.4% overall and 97.3% for user shifts. All outages were of short duration.

Long-term and weekly operations schedules are available on the Web (http://www-als.lbl.gov/als/accelinfo.html). Weekly operations scheduling meetings are held on Fridays at 3:30 p.m. in the Building 6 conference room. The Accelerator Status Hotline at (510) 486-6766 (ext. 6766 from Lab phones) features a recorded message giving up-to-date information on the operational status of the accelerator.

** GLOBAL ORBIT FEEDBACK TEST PROPOSED**
(Contact: GJPortmann@lbl.gov)

In order to further improve the long-term stability of the electron beam, the Accelerator Physics group proposes to implement a slow, global orbit feedback system during the week of August 11. The system should correct a slow orbit drift caused mainly by temperature variations in the storage ring. Users with questions or concerns about implementing the new system the week of August 11 are urged to contact Greg Portmann (ext. 5924) directly. If no users express concerns, the system will be implemented as scheduled.