1. ANGLE-RESOLVED 2-D SPECTRA HIGHLIGHT SPIN-ORBIT EFFECTS
by Lori Tamura
Contact: NBerrah@lbl.gov

The detailed structure of multielectron atomic systems cannot be calculated exactly because of higher-order interactions between the electrons. Therefore, approximate theoretical methods must be used to describe such systems. In the case of low-lying excitations in light atoms, the system is typically treated in an LS-coupling framework. By using angle-resolved two-dimensional photoelectron spectroscopy, however, researchers at the ALS have found that LS-coupling predictions are violated. In particular, they found that the spin-orbit effects are unexpectedly large and cannot be ignored when studying detailed spectra in atoms as light as neon.

This experiment dealt with processes involving two-electron excitations, in which the absorption of one photon results in the simultaneous excitation of two electrons. This is only possible because of electron-electron interactions. Detailed investigations of these complex multielectron processes can be performed by using high-precision instrumentation and high-resolution photon sources. Such studies have been conducted successfully on simple atoms such as helium and lithium. In the case of neon, a more complex system, early photoabsorption experiments demonstrated a departure from the predictions of LS coupling in the fine-structure splittings of the photoionization spectra. While these results established the existence of certain spin-orbit effects, they lacked additional information that would be provided by angle- and energy-resolved measurements. Furthermore, only small spin-orbit effects were observed.

The ALS researchers performed more complex, detailed studies of the photoexcitation and decay mechanisms of doubly excited neon in order to probe spin-orbit effects in this relatively light system. Neon is more complicated than helium and lithium because of its occupied 2p subshell. In doubly excited, photoionized neon, one photoelectron is emitted and one electron is boosted to a higher energy level, leaving the ion in one of several available energy states. The dense grouping of these states results in photoionization spectra with a complex structure. These states reveal higher-order spin-orbit effects and provide a stringent testing ground for the accuracy of existing computational methods.

Two-dimensional photoelectron spectra (showing electron yield as a function of photon energy and binding energy) were taken at two angles (0° and 54.7°) simultaneously in order to efficiently observe any effects or features. The apparatus consisted of two highly efficient time-of-flight electron-energy analyzers. The measurements were performed on Beamline 9.0.1, an undulator beamline for high-resolution atomic and molecular studies, on the high-resolution time-of-flight endstation. The resulting spectra can be seen on the World Wide Web at http://www-als.lbl.gov/als/science/sci_archive/spinorbt.html.

Several features of the spectra indicate violations of LS-coupling predictions. For example, in this study, according to LS coupling, signals in some cases should vanish at 0°, so the observation of any signal at 0° in those cases is an immediate indication of the breakdown of LS coupling.

The research group's ability to view the data in two dimensions coupled with the ALS's ability to provide high-brightness light with a very high degree of linear polarization made this experiment possible. The observation of these spin-orbit effects has been corroborated by state-of-the-art ab-initio calculations. The important implication of the detection of prominent spin-orbit effects is that it is not safe to assume the validity of LS coupling, even for low-lying excitations in a system as light as neon.

Research conducted by N. Berrah (principal investigator), A.A. Wills, T.W. Gorczyca, O. Nayandin, and M. Alshehri (Western Michigan University); B. Langer (Fritz-Haber-Institut); Z. Felfli (Clark Atlanta University); E. Kukk (Western Michigan University and Berkeley Lab); and J. D. Bozek (Berkeley Lab).

Funding: U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this experiment: A.A. Wills et al., Phys. Rev. Lett. 80, 5085 (1998).

2. CALL FOR COMPENDIUM ABSTRACTS
Contact: lstamura@lbl.gov

ALS users: now that 1998 is over, it's time to boil down a year's worth of the effort you spent on your ALS project into four pages or less. On January 22, all ALS users from the past year were mailed a call for abstracts to be published in the ALS Compendium of User Abstracts and Technical Reports. If you performed research at the ALS in 1998 and did not receive a mailing, contact Lori Tamura at lstamura@lbl.gov. We need your help to make the Compendium representative of your work and to convey the breadth, depth, and importance of the ALS scientific program. The deadline for the submission of abstracts is February 26, 1999.

All users or user groups (including users who are also ALS staff members) should submit a 1- to 4-page abstract describing each project worked on at the ALS during 1998 (January 1 to December 31), whether published, unpublished, or in progress. As it was last year, the Compendium will be published electronically on the Web as well as on paper, requiring that we have an electronic file of each abstract. A variety of electronic submission options are available, including FTP (file transfer protocol), web upload, or mailed diskette. Detailed instructions for each option as well as templates and author guidelines can be found online at http://alspubs.lbl.gov/Compendium_old/.

3. 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 (Berkeley Lab) and Joe Orenstein (Univ.of California, Berkeley) will be studying the ultraviolet reflectivity spectra of various manganese oxides.

Beamline 1.4.3: Kevin Wilson and Richard Saykally (Univ. of California, Berkeley) will be studying the surface cooling in microjets of water by Fourier-transform infrared spectromicroscopy. Hoi-Ying Holman and Regine Goth-Goldstein (Berkeley Lab) will continue monitoring reactions to mixed contaminants.

Beamline 7.0.1: Gerry Lapeyre (Montana State Univ.) will conduct photoemission studies of gallium nitride crystal surfaces, and Satish Myneni (Berkeley Lab) will conduct spectromicroscopy studies of bacteria and soils.

Beamline 8.0.1: Yasuji Muramatsu (NTT Information and Energy Systems Laboratories) will carry out soft x-ray emission and absorption spetroscopy of hybrid carbon materials for sensing and electronic devices.

Beamline 10.3.2: Alain Manceau (Univ. of Grenoble) will study speciation and microstructure within contaminant soil micronodules. Richard Reeder (State Univ. of New York, Stonybrook) will study speciation of trace-metal contaminants in Long Island soils.

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

Beam reliability for the last two weeks (February 1-14) was 98.3% overall and 98.5% for user shifts. There were no significant outages.

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.