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ALSNews           Vol. 41           November 29, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. RESEARCHERS TEST MANY-BODY THEORY WITH HOLLOW LITHIUM
    3. SYNCHROTRON-BASED SCIENCE AS A CAREER
    4. ALS ACTIVITY REPORT

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

Beam availability for the last two weeks was 83% overall and 82% during user shifts. Causes of lost beamtime included water flow interlock trips in storage ring sectors 2, 7 and 9; the klystron focus power supply, cathode air flow, and power trips in the rf system; and more frequent refills due to shorter beam lifetimes compared with before the September shutdown possibly due to better vertical beam emittance. Every attempt will be made to operate with the longitudinal feedback system on during all user shifts November 29-December 4.

Operations Summary for November 29 - December 18

Nov 29, 00:00- Dec 04, 08:00  1.9-GeV/260-mA/320-bunch user operations
Dec 04, 08:00-24:00           Maintenance & Startup
Dec 05, 00:00-24:00           Accelerator Physics
Dec 06, 00:00-08:00           Scrubbing & Tests
Dec 06, 08:00- Dec 11, 08:00  1.5-GeV/400-mA/320-bunch user operations
Dec 11, 08:00-24:00           Maintenance & Startup
Dec 12, 00:00-24:00           Accelerator Physics
Dec 13, 00:00-08:00           Scrubbing & Tests
Dec 13, 08:00- Dec 18, 08:00  1.3-GeV/400-mA/320-bunch user operations

2. RESEARCHERS TEST MANY-BODY THEORY WITH HOLLOW LITHIUM
(contact: francois.wuilleumier@lsai.u-psud.fr)

How can you describe the interactions between four objects that exert forces on each other? Worse yet, how can you describe them if you think of the objects as waves that obey the rules of quantum mechanics? If you don't know, don't worry. Even three-object systems are so complex that they are considered mathematically unsolvable. Thus, quantum mechanics, in its attempt to understand such "many-body" systems, has advanced for the last half century on the strength of approximations, and physicists are still looking for good approximations for four-body systems. Photoelectron spectroscopy of "hollow" lithium at the ALS is helping to test new approximations of how many-body systems behave by studying how lithium's three electrons interact with each other and with the atom's nucleus.

In hollow lithium, all three electrons, which normally reside in the innermost (K and L) shells, have been excited (by synchrotron light, for example) so that they are in higher-lying shells, farther from the nucleus, leaving the K shell empty. An inherently unstable state, hollow lithium decays quickly into a lithium ion (Li+ or Li++), ridding itself of its extra energy by ejecting one or two electrons. The resulting ion can have a variety of different internal energies, depending upon which orbitals (subshells) the electrons end up in, leaving the ion in one of several final states. Since the excited electrons in lithium can jump up into any of a handful of electron orbitals to form the hollow atom, and each of the resulting configurations can decay into any of several final states, the possible energy changes within the system are numerous. Making sense of all this requires excellent resolution of the final states, which are studied by measuring the electron energy distribution as the hollow lithium decays. The ALS is providing the brightness that scientists need to observe these decay processes in detail.

Francois Wuilleumier, University of Paris-South, and his co-workers began working on hollow lithium at SuperACO in Orsay, France. There the team developed a cylindrical mirror analyzer-a photoelectron spectrometer that is uniquely suited to the study of hollow lithium. The instrument is an electron-energy analyzer with a lithium vapor source at its center. Its cylindrical shape makes it a highly efficient collector of ejected electrons. With the electron energy information, and a knowledge of the relationship between electron energies and final states in lithium, the researchers can determine the probability (partial cross-section) of producing each final state.

After working with modest spectral (photon energy) resolutions at Orsay (0.5 eV on a bend magnet beamline and 0.23 eV on an undulator), the researchers moved their equipment to Berkeley in order to take advantage of the brightness of ALS undulator light. The high brightness of Beamline 9.0.1 has allowed the scientists to work with 6 to 12 times the spectral resolution (40 meV and even 20 meV when time permitted) and twice the electron-energy resolution and still have enough flux to count at twice the rate - a 48-fold improvement in performance.

Previous work by others on hollow lithium involved photoabsorption and photoion spectroscopy, which provided information about the hollow states but left the final states in question. Now, armed with detailed information about the lithium final states, the researchers at the ALS have been able to test theoretical calculations made by L. VoKy (Physical Review Letters, in press) that predict the behavior of lithium's electrons. The calculations have been found to be highly accurate in predicting both the hollow lithium states produced and the final states to which the hollow lithium decays (ALS and SuperACO spectra).

This experiment was performed by F. J. Wuilleumier (principal investigator), S. Diehl, D. Cubaynes, and J.-M. Bizau (Universite Paris-Sud and Centre National de la Recherche Scientifique, France); E. Kennedy (Dublin City University); C. Blancard (DRIF, Centre d'Etudes de Limeil Valenton); T. Morgan (Wesleyan University); N. Berrah (Western Michigan University); and J. Bozek and A.S. Schlachter (Berkeley Lab).
Funding: Universite Paris-Sud; Centre National de la Recherche Scientifique, France; DAM, Centre d'Etudes de Limeil-Valenton, France.

Publications about this experiment:
S. Diehl, D. Cubaynes, et al. Phys. Rev. Lett. 76, 3915 (1996).
D. Cubaynes, S. Diehl, et al. Phys. Rev. Lett. 77, 2194 (1996).
S. Diehl, D. Cubaynes, et al. Phys. Rev. Lett. 79, 1241 (1997).
S. Diehl, D. Cubaynes, et al. Phys. Rev. A, Rapid Communications, 56, R1071 (1997).
S. Diehl, D. Cubaynes, et al. J. Phys. B: At. Mol. Opt. Phys. 30, L595 (1997).
F. J. Wuilleumier, S. Diehl, et al. in "X-Ray and Inner-Shell Processes", AIP Conf. Proc. 389, edited by R. L. Johnson, H. Schmidt-Boecking, and B. F. Sonntag (Am. Inst. Phys., New York, 1997), pp. 625-645.

3. SYNCHROTRON-BASED SCIENCE AS A CAREER

Francois Wuilleumier recently used a break in his experiment schedule to discuss the pros and cons of a career in synchrotron radiation. He defines the pros easily: the challenge of doing new and difficult science, the excitement of using fascinating machines, and the thrill of getting dazzling results. Atomic physicists, according to Wuilleumier, need a long time to see interesting results, but each new generation of machine has brought new experiments within reach, and the intrigue draws scientists on. Drawbacks include working on an experiment around the clock for several consecutive days (though this can also be part of the excitement), the stress of one's schedule on one's family, and separation from home while experimenting at a light source halfway around the world (where one is too busy and focused to sight-see during the experiment, and too exhausted and homesick to sight-see afterward).

When students apply to work with Wuilleumier, he warns them of the conditions they will endure during their three-year Ph.D. program, but many are unable to resist the work's magnetic attraction (his pun intended). For his current experiment program, the group includes four students from the University of Paris, two senior scientists having permanent positions in his laboratory and responsibility for key aspects of the group's experimental work, and five collaborators from various institutions around the world. Wuilleumier considers collaborators and senior scientists necessary for complex experiments because "experience counts." For this reason he values the French academic system, which allows the best students to get permanent, full-time research positions after their Ph.D.'s and pursue their research without teaching responsibilities.

A note on the practical aspects of round-the-world experiments: shipping expenses can be quite significant! Wuilleumier's group had two tons of equipment, including an electron spectrometer, a laser, pumps for vacuum chambers, and a lithium oven. Professionals packed the more delicate half of the equipment and handled its customs forms; the group packed the rest. Packing took two weeks, and shipping a few days. Total costs: shipping $500,000 worth of equipment by air cargo cost $14,000 round trip.

4. ALS ACTIVITY REPORT
(contact: alsuser@lbl.gov)

Copies of the ALS Activity Report are now available (many of you may have already received a copy at the Users' Meeting or through the mail). The Activity Report is designed to share the breadth, variety, and interest of the scientific program and ongoing R&D efforts in a way that is accessible to a broad audience, and to serve as a reference for the beamlines scheduled for completion between 1995-1996. Recent research results from the ALS are presented within four large areas of scientific investigation: atomic physics and chemistry, health and environment, materials and surface science, and lithography . The results highlight major research programs and demonstrate the capabilities of the ALS in these areas, rather than giving a comprehensive review of 1994 experiments. Anyone who would like to receive a copy should send their name and complete mailing address via email to alsuser@lbl.gov with "Send Activity Report" in the subject line.