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.

Previous Issues are available.


ALSNews           Vol. 92           December 10, 1997

Table of Contents


    1. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS
    2. USERS' EXECUTIVE COMMITTEE INTRODUCES WEB SITE
    3. ALSNEWS AND BERKELEY LAB ON HOLIDAY
    4. THE INSIDE STORY ON OXYGEN
    5. OPERATIONS UPDATE

1. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS

The ALS is now accepting proposals from scientists who wish to conduct research at the facility as independent investigators during June-September 1998. The deadline for proposals is January 15, 1998. Because the period scheduled in our last proposal cycle (December 1997 to April 1998) was short, all proposals that were not granted beam time for that period will automatically roll over to this proposal cycle.

The proposal form for independent investigators is available in Portable Document Format (PDF) on the Web (http://www-als.lbl.gov/als/quickguide/independinvest.html). Information on the proposal process is available at the same location.

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

For information on beamlines available to independent investigators, contact:
Fred Schlachter, ALS Program Support
Tel: (510) 486-4892
Fax: (510) 486-6499
Email: fred_schlachter@lbl.gov

2. USERS' EXECUTIVE COMMITTEE INTRODUCES WEB SITE
(contact: w_meyer-ilse@lbl.gov)

The Users' Executive Committee has established a new Web site to facilitate communication with ALS users. The site includes an electronic form for submitting comments directly to the UEC. Comments can be submitted anonymously if desired. The new site also contains the full text of the official ALS User Policy and the Users' Association Charter. Contact information is provided for all UEC members. Suggestions for additions or improvements to the site are welcome and can be submitted through the comments form on the site itself. The UEC Web pages can be reached from the ALS home page (http://www-als.lbl.gov/), through the Quick Guide for Users (http://www-als.lbl.gov/als/quickguide/), or directly (http://www-als.lbl.gov/als/uec/).

3. ALSNEWS AND BERKELEY LAB ON HOLIDAY

Everyone at Berkeley Lab, including the ALSNews editors, will take a much-needed break for the holidays. The Lab will shut down on the evening of December 23 and reopen on the morning of January 2. ALS user operations will resume on January 7, and the next issue of ALSNews is due to be delivered that same day. We wish you all a pleasant and relaxing holiday season.

4. THE INSIDE STORY ON OXYGEN
(contact: cyng@ameslab.gov)

[Editor's note: This article gives a look ahead at the ALS Activity Report 1996/97, which is due for publication later this month. A little longer than our usual ALSNews items, the article explains more than we usually do about the motivation and methods of the research. We hope you'll enjoy the change of pace; many of those who responded to our readers' survey were interested in more explanations of research techniques.]

Oxygen molecules, which make up about 20 percent of the earth's atmosphere, serve as a shield (along with nitrogen, ozone, and other molecules) against ultraviolet radiation from the sun by absorbing the radiation before it reaches the earth's surface. Therefore, understanding the absorption process and the lengthy cycle of subsequent chemical reactions that may lead to the regeneration of oxygen is of deep interest to atmospheric chemists. A key absorption process is photoionization. Upon absorption of a short-wavelength ultraviolet ray (vacuum ultraviolet or VUV) that cannot penetrate through air, an oxygen molecule ejects an electron, thereby becoming a positively charged ion (cation). The ultimate fate of the oxygen depends on the distribution of energy in the cation as the result of photoionization (the internal state of the cation), because this state determines the result when the cation collides with an electron or with another atmospheric molecule.

Understanding the internal state of the oxygen cation (O2 with a charge of +1) comes down to unraveling its electronic structure. In an atom, the outer (valence) electrons contribute to the chemical bonding that turns a group of atoms into a molecule. The bonding electrons form orbitals associated with the molecule as a whole rather than the individual constituent atoms. In addition, the atoms in the molecule may vibrate and rotate. Overall, the molecule has a spectrum of energies that depends on the orbitals, with small contributions from the vibrations and smaller ones still from the rotations. This is the electronic structure.

Chemists use VUV spectroscopy to study the electronic structure of molecules. Measuring the wavelengths at which electrons in the molecular orbitals receive just enough energy to escape but no more (threshold photoelectron spectroscopy) is a sensitive method of probing the molecular electronic structure. Sensitivity is essential, since absorption peaks for molecules that differ only in their rotational motions are very closely spaced and tend to overlap. One big problem is finding a VUV source that provides an intense beam in a narrow wavelength band that can be tuned to the absorption peak of interest. Another is limiting the detection to only the threshold electrons with just enough energy to escape, since electrons in other orbitals that need less energy to escape are also ejected.

At the ALS, researchers have successfully attacked these issues by adapting a laser technique to synchrotron radiation. The basic idea is to use VUV light from the ALS to give target electrons almost enough energy to escape, but not quite (exciting them to high-n Rydberg states). A voltage pulse applied a few nanoseconds later then adds the extra energy needed to escape from the Rydberg states, but in the meantime, all the higher-energy electrons have moved out of the area. The researchers call this technique pulsed-field ionization photoelectron spectroscopy.

After an electron escapes from the oxygen molecule, the resulting cation may be left in a variety of internal states with different energies specified by the molecular orbitals and the vibrations and rotations. The ALS provides bright beams of VUV light over a wide range of wavelengths, which scientists can use to probe molecules with electrons in high-energy orbitals (excited electronic states) that are inaccessible to lasers because of the limited VUV-wavelength range they cover. However, switching light sources creates a new challenge. In the laser version, the time delay between light and voltage pulses is a few microseconds, but light pulses at the ALS come much faster. To meet this challenge, the researchers designed a special photoelectron spectrometer that can reduce the delay to nanosecond intervals.

Researchers have now demonstrated this high-speed, high-resolution capability at the ALS with the molecular oxygen cation. Their results exceed the highest spectral resolving power ever achieved for molecular photoelectron spectroscopy using synchrotron radiation. And the researchers were able, for the first time, to obtain spectra with clearly resolved peaks for the different rotational states of the cation in excited states at energies above those accessible with lasers.

Research conducted by C.Y. Ng (principal investigator), M. Evans, and S. Stimson (Ames Laboratory and Iowa State University); C.-W. Hsu (Berkeley Lab); and P. Heimann (ALS), using the photoionization endstation at Beamline 9.0.2.2. Funding: Office of Basic Energy Sciences of the U. S. Department of Energy.

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

Operations Summary for December 10 - January 11

Dec 10, 00:00-08:00           User scrubbing & special operations
Dec 10, 08:00- Dec 15, 07:15  1.9-GeV/400-mA/287-bunch user operations
Dec 15, 07:30- Dec 16, 24:00  Maintenance & startup
Dec 17, 00:00-08:00           User scrubbing & special operations
Dec 17, 08:00- Dec 23, 23:15  1.9-GeV/400-mA/287-bunch user operations
Dec 24, 00:00- Jan 01, 24:00  Winter holidays
Jan 02, 00:00-24:00           Maintenance
Jan 03, 00:00- Jan 05, 07:30  No operations
Jan 05, 07:30-24:00           Maintenance & startup
Jan 06, 00:00-24:00           Accelerator physics & machine setup
Jan 07, 00:00-08:00           User scrubbing & special operations
Jan 07, 08:00- Jan 11, 16:00  1.9-GeV/400-mA/287-bunch user operations