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. 24           May 9, 1995

Table of Contents


    1. OUTAGE CONTINUES -- UPDATE ON DIAGNOSTIC WORK
    2. OPERATIONS UPDATE
    3. POST-COLLISION INTERACTIONS OBSERVED IN ARGON STUDY
    4. LONG-TERM ALS SCHEDULING MEETING MAY 26
    5. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS

1. OUTAGE CONTINUES -- UPDATE ON DIAGNOSTIC WORK
(contact: rmmiller@lbl.gov)

There has been no operation of the ALS since Tuesday, April 25, when stored beam was suddenly lost. Since then, many ALS staff have been working overtime to diagnose the problem, but these efforts have not yielded a solution.

The ALS is now conducting systematic checks of the storage ring magnets. Mechanical checks are being made to check the integrity of coil isolation and bus-bar interconnections. Loose connections have been found (and tightened), but only one was observed to have an adverse effect on beam behavior (it led to unstable beam accumulation, but not to the severe loss of beam being investigated). With a magnet's power supply running at operating currents, technicians are measuring the voltage drop across the magnet coils, looking for variations from nominal levels which could indicate an electrical short or other problem. All quadrupole magnets have now been checked. Sextupole measurements are in progress; when these are complete, we will try again to store beam. ALS staff have already checked the power supplies for many magnets. A 2% error in a single magnet among approximately 200 magnets in the ALS storage ring could potentially cause the problems we are now experiencing.

The ALS users on site have been patient and supportive, minimizing interruptions to diagnostic work. This cooperation is greatly appreciated.

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

The storage ring has been down since Tuesday, April 25 (see item #1 for details). When operations resume, the ALS will return to the following schedule.

Operations summary for May 9-28
1.5-GeV, 400-mA, 320-bunch operations for users:
    May 10, 08:00-16:00
    May 17, 08:00-16:00
    May 18-21, 08:00-23:15
    May 24, 08:00-16:00
    May 25-28, 08:00-23:15
1.5-GeV, 2-bunch operations for users:
    May 11-14, 08:00-23:15
Maintenance: 
    May 15 & 22, 08:00-16:00, with startup 16:00-23:15
Accelerator Physics:
    May 9, 16 & 23, 08:00-23:15
    May 10, 17 & 24, 16:00-23:15

3. POST-COLLISION INTERACTIONS OBSERVED IN ARGON STUDY
(contact: jsamson@unlinfo.unl.edu)

A group led by James Samson (U. of Nebraska) recently obtained argon ionization spectra at the ALS which significantly extend the data available on post-collision interactions (described below) in argon L-shell ionization. The spectra have a resolution of ~ 50 meV, the highest resolution used to date in studying argon L-shell decay.

In their experiment at Beamline 9.0.1, the researchers took ion-yield spectra for four species of argon ions (Ar+, Ar2+, Ar3+, and Ar4+). To take one such spectrum, they adjusted a simple mass spectrometer, with a mass/charge resolution of about 1 unit in 20, so that it detected only one ion species with high sensitivity. Then they used synchrotron light to ionize argon gas, tuning the incoming photon energy in 2-5 meV steps over the 243-254 eV range and measuring the ion yield at each photon energy to form a full spectrum.

The incoming photons in this experiment ejected electrons from the 2p orbitals (known as L2 and L3 orbitals) of argon atoms in a gas sample. Since the photon energies used are very close to the minimum energy required for such L-shell ionization, the escaping photoelectrons had low kinetic energies and thus slow speeds. When atoms lose one or more core (non-valence) electrons in this way, one possible result is Auger decay, in which one valence electron drops in energy to fill the 2p core hole, transferring its excess energy to another valence electron, which leaves the atom.

This escaping Auger electron has a high kinetic energy compared to the photoelectron, so in classical terms, the Auger electron can overtake the photoelectron as both travel away from the atom. Before the photoelectron is overtaken, it still "sees" the Auger electron as part of the atom, partially shielding (canceling out) the positive charge on the argon nucleus. After it is overtaken, however, the photoelectron "sees" a larger (less shielded) charge on the argon nucleus. Sometimes this larger apparent nuclear charge is sufficient to recapture the photoelectron, pulling it back into an atomic orbital. Such photoelectron recapture is one type of post-collision interaction (PCI), which affects the yield of multiply charged ions (Ar2+ through Ar4+) and thus the shapes of the corresponding spectra; other PCIs can affect the shape of the Auger electron spectrum without actually recapturing the photoelectron.

In previous experiments, PCIs were observed only for the L2 edge in argon, and only for Ar+ and Ar2+. This experiment broke new ground by producing clear observations of PCIs at both L2 and L3 edges for all degrees of ionization (Ar+ through Ar4+). These observations were possible because of the experiment's high energy resolution, about 50 meV, which corresponds to a resolving power of ~5,000.

PCIs are of interest as a class of interactions occurring in many-electron systems. Since the quantum-mechanical equations for atomic and molecular systems containing more than two bodies (a nucleus and one electron) cannot be solved exactly, many-body theory depends on approximations based on two-body equations. Experiments such as this one provide the data necessary to refine these approximations.

Samson's group for this experiment included post-doctoral fellows John He, Jeff Cutler, and Wayne Stolte of U. of Nebraska and John Bozek of the ALS.

4. LONG-TERM ALS SCHEDULING MEETING MAY 26
(contact: fred_schlachter@lbl.gov)

On Friday, May 26, all users are invited to attend a long-term operations scheduling meeting to discuss the period between the end of the September-October 1995 shutdown and the start of the next shutdown (provisionally scheduled for Spring 1996). The meeting will be held at 3:00 p.m.; its location will be announced in the May 23 issue of ALSNews. The ALS Users' Executive Committee, which will be meeting that day, will participate in the scheduling meeting.

5. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS

The ALS has two cycles per year for soliciting proposals from scientists who wish to conduct research at the facility as independent investigators: April-September and October-March. The next submission deadline is June 1, 1995 for beamtime between October 1995 and March 1996.

To request a proposal form, contact:
Elizabeth Saucier, ALS User Administrator
Tel: (510) 486-6166
Fax: (510) 486-4960
Email: alsuser@lbl.gov

For beamline and PRT information, contact:
Fred Schlachter, ALS User Liaison
Tel: (510) 486-4892
Fax: (510) 486-4960
Email: fred_schlachter@lbl.gov