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ALSNews           Vol. 16           March 14, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. BEAMLINE 8.0 RESEARCH ON PROBING BAND STRUCTURE
    3. VISITING RESEARCHERS -- JOSEPH NORDGREN
    4. SPECIAL MEETING ON SHORT AND LONG-TERM OPERATIONS SCHEDULE

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

Beam availability last week was 96.6% overall and 96.3% during user shifts.

Operations summary for March 14-29
1.5-GeV, 400-mA, 320-bunch operations for users:
    March 15, 16:00-23:15
    March 16-19, 08:00-23:15
    March 22, 16:00-23:15
    March 23-24, 08:00-23:15
    March 25-26, 08:00-16:00
    March 29, 16:00-23:15
1.5-GeV, 1-mA, 320-bunch operations for U8/gas filter characterization
at 9.0.2:
    March 25-26, 16:00-23:15
Maintenance:
    March 20 & 27, 08:00-16:00, with startup 16:00-23:15
Accelerator Physics:
    March 14, 21, & 28, 08:00-23:15
    March 15, 22, & 29, 08:00-16:00

Weekly Scheduling Meetings:
    March 20 at 11:00 in the Building 6 Conference Room.
    March 27 at 11:00 in Building 2-100B followed by discussion of short- and
long-term operations schedule (see #4 below for more information). 

2. BEAMLINE 8.0 RESEARCH ON PROBING BAND STRUCTURE
(contact: jcarlisle@cms1.llnl.gov)

Research results from a group working at ALS Beamline 8.0 are featured in the February 13 issue of Physical Review Letters: "Probing the Graphite Band Structure with Resonant Soft-X-Ray Fluorescence" (Phys. Rev. Lett., Vol. 74, No. 7, p. 1234, 13 February 1995). The September 1994 ALS Report newsletter featured this work after its submission to the journal. The authors are J.A. Carlisle, Eric L. Shirley, E.A. Hudson and L.J. Terminello of Lawrence Livermore National Laboratory, T.A. Callcott and J.J. Jia of the University of Tennessee, D.L. Ederer of Tulane University, R.C.C. Perera of LBL, and F.J. Himpsel of IBM.

The group built on a discovery made by Y. Ma and coworkers at Brookhaven National Laboratory that certain features in the resonant fluorescence spectra of some materials (in this case, highly oriented pyrolytic graphite) arise from a one-step (coherent) fluorescence process, a discovery that has revealed a new way to probe band structure. Band structure, a key predictor of a material's behavior, is the relationship between the two most important parameters for describing electronic states in a solid: energy and crystal momentum (a description of electron momentum defined in the context of crystalline structures).

In soft x-ray fluorescence (SXF), photons are used to excite an electron from an atom's core energy level to a higher energy level, whereupon a second electron from an intermediate energy level drops to fill the hole in the core energy level. The energy lost by the second electron is radiated as a fluorescent photon with an energy determined by the atom and its molecular environment. SXF is an inefficient process because many elements, especially light ones such as carbon, are much more likely to emit electrons than photons when their core energy levels are filled. Therefore, the successful use of SXF as an experimental tool requires a highly efficient spectrometer (like the University of Tennessee endstation used in this experiment) coupled with an intense, bright source (such as the ALS).

The defining characteristic of resonant fluorescence is that the energies of the incoming photons are less than the minimum energy required to remove core electrons from the solid (called the core ionization threshold), rather than considerably above the ionization threshold as in conventional fluorescence work. In this resonant energy regime the photon absorption and emission processes (generally thought of as two separate events) are a single coupled, or coherent, event.

The graphite experiment used the result that for coherent fluorescence, the state to which the first electron is excited and the state from which the second electron drops must have the same crystal momentum (i.e., crystal momentum is conserved). By making small (0.5 eV) variations in the incoming photon energies, the Beamline 8.0 researchers excited electrons to states with differing crystal momenta and observed corresponding energy shifts (dispersive features) in the peaks of the fluorescence spectra. The shifts were in excellent agreement with the graphite band structure predicted by theory, thus demonstrating the potential of resonant SXF for examining the momentum-resolved electronic structure of complex materials, including those for which calculated band structures are not available.

3. VISITING RESEARCHERS -- JOSEPH NORDGREN
(contact: joseph@fysik.uu.se)

Joseph Nordgren, of Uppsala University in Sweden, is at the ALS to pursue soft x-ray fluorescence (SXRF) studies, many of them made feasible only by the high brightness of a third-generation synchrotron source such as the ALS. SXRF using synchrotron radiation allows the study of bulk electronic properties as well as surfaces and interfaces. For instance, the bonding of buried layers in a solid can be studied because x-ray photons (unlike electrons) can penetrate many layers of a solid without losing the information they carry to small changes in kinetic energy. SXRF, being a photon-in photon-out spectroscopy, can also be used to study gaseous molecules and their interaction with surfaces at high pressure, whereas electron spectroscopy studies require a much higher vacuum. The high energy resolution of photons at beamlines like 7.0 and 8.0 provides another advantage by allowing the separation of fluorescence contributions from chemically shifted atomic species in a molecule, and the study of resonant inelastic scattering phenomena is also an important area of research.

In other projects, Nordgren and his co-workers will study the detailed electronic structures of high-temperature superconductors and "simple" transition metal oxides using selectively excited, angle-resolved soft x-ray emission. In collaboration with the Center for X-ray Optics at LBL, he will also explore the possibilities of applying a second refocusing stage to the SXRF station at ALS Beamline 7.0, to allow excitation and high-resolution recording of soft x-ray fluorescence from a few-micron-size spot. This improvement would allow the study of minute samples such as tiny single crystals of high-temperature superconductors. It would also increase detection efficiency (perhaps by a factor of 3-4) by better matching the acceptance of the detector and the emittance of the x-ray emission, and it may make imaging SXRF feasible via scanning methods.

Nordgren spent much of his 1.5-year sabbatical, which ended last December, at the ALS. He now spends about one to two weeks every six weeks at the ALS. His Ph.D. students are at the ALS on a more continuous basis to do experimental work in his absence. Nordgren sees a shortage in the total available beamtime, but he hopes that there will still be sufficient time to do several of his more demanding experiments, which cannot readily be done elsewhere, as well as some experiments which can be performed in shorter times due to the high brightness of the ALS.

4. SPECIAL MEETING ON SHORT AND LONG-TERM OPERATIONS SCHEDULE
(contact: fred_schlachter@lbl.gov)

An open discussion to get feedback from users on three issues related to the short- and long- term operations schedule is planned for March 27. All ALS users are invited; ALS management will be attending to hear your comments and suggestions and to answer questions. The issues to be discussed are:

1) The new ALS weekly operating schedule which began a three-month trial period on February 22. Users are invited to make a brief presentation on whether they wish to continue the trial schedule beyond the initial three month period or to revert to the old schedule.

2) The timing for the two shutdowns which need to be planned for the coming twelve months: one to install the U10 undulator and move the U8 from sector 9 to sector 12, the other to install the wiggler for protein crystallography. We are soliciting your suggestions/comments on the timing of these shutdowns.

3) A possible change to the six-month schedule which began February 22. In response to a user request, we will discuss whether either of two possible schedule changes are feasible and/or desirable. Plan one would interchange the two weeks of 1.9-GeV operation scheduled for the weeks of June 6 and 14 with the two weeks of 1.5-GeV operation scheduled for the weeks of August 9 and 16. Plan two would split the 1.5- and 1.9-GeV operation for these same four weeks on a daily basis, e.g., 1.5-GeV operation during day shift and 1.9-GeV operation on swing shift during user operations during the weeks of June 6 and 14, and August 9 and 16. This schedule is potentially useful for exposing samples to photons over a wide energy range. User input will be solicited on whether to adopt either of these plans or to leave the schedule unchanged.

The meeting will be in Room 100B of Building 2 from 11:00 to 12:00 on March 27 following the usual weekly planning meeting (which will also be held in Room 100B of Building 2). Contact Fred Schlachter (fred_schlachter@lbl.gov, Tel: 510-486-4892) for more information.