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. 43           January 10, 1996

Table of Contents


    1. OPERATIONS UPDATE
    2. ALS-RELATED RESOURCES ON THE WORLD WIDE WEB
    3. EXPERIMENTS ON SYMMETRIC MOLECULES TEST KEY PRINCIPLE OF QUANTUM MECHANICS
    4. ALS OFFERS FREE TEACHER WORKSHOP
    5. SCHEDULING MEETING WILL BE JANUARY 19

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

Beam availability for the last two weeks was 92.9% overall and 92.6% during user shifts.

Operations Summary for January 10 - January 29

Jan 10, 00:00-08:00           Scrubbing & Tests
Jan 10, 08:00- Jan 14, 23:15  1.5-GeV/400-mA/320-bunch user operations
Jan 15, 00:00- Jan 16, 08:00  Holiday 
Jan 16, 08:00-24:00           Maintenance & Startup
Jan 17, 00:00-16:00           Accelerator Physics
Jan 17, 16:00- Jan 22, 08:00  1.9-GeV/260-mA/2-bunch user operations
Jan 22, 08:00-24:00           Maintenance & Startup
Jan 23, 00:00-24:00           Accelerator Physics
Jan 24, 00:00-08:00           Scrubbing & Tests
Jan 24, 08:00- Jan 29, 08:00  1.5-GeV/400-mA/320-bunch user operations

2. ALS-RELATED RESOURCES ON THE WORLD WIDE WEB

If you have access to the World Wide Web, you probably have already visited the ALS home page (http://www-als.lbl.gov/als/), which provides links to a variety of information about ALS operations, science, workshops, ALSNews archives, etc. If you haven't visited recently, you may want to check out two new links from the page: Recent ALS Science Highlights and Microworlds. The Center for X-Ray Optics has also expanded its Web offerings recently, providing online technical information for x-ray researchers. Read on to learn more about each of these resources.

** RECENT ALS SCIENCE HIGHLIGHTS **
(contact: jccross@lbl.gov)

Sometimes a picture is worth a kilobyte of text-only information! Recognizing this, the ALS has inaugurated links from its home page (http://www-als.lbl.gov/als/) to figures showing results from ALS research. These new files are accompanied by links to related stories in ALSNews, so curious Web-surfers can always get the text as well.

** MICROWORLDS **
(contact: jccross@lbl.gov)

Microworlds, an electronic science magazine developed by Lawrence Berkeley National Laboratory, features educational articles and hands-on activities related to how the ALS works and its use as a research tool. Over 1000 teachers and students from 55 countries have registered to receive updates on Microworlds since January 1995 when it first went online.

The Eisenhower National Clearinghouse for Mathematics and Science Education (ENC) selected Microworlds as one of their Digital Dozen of highlighted Web sites for December. ENC, funded through a contract with the U. S. Department of Education, provides K-12 teachers with a central source of information on mathematics and science curriculum materials. The Digital Dozen are selected based on their content, usefulness in the classroom, and overall ease of use. Microworlds is at http://www.lbl.gov/MicroWorlds/ and ENC is at http://www.enc.org/

** X-RAY OPTICAL CONSTANTS AND CALCULATIONS **
(contact: emgullikson@lbl.gov)

Beamline designers and scientists conducting synchrotron-based science can now access a number of useful tables and calculation programs on the Web, thanks to web pages assembled by the Center for X-Ray Optics. The pages' forms-based interface (use later versions of Netscape for best results) provides atomic scattering factors and other information for the first 92 elements. The pages also link to programs which calculate practical quantities such as the index of refraction of an arbitrary solid, the reflectivity of a layered mirror, or the x-ray spectrum from a bend magnet, all under user-specified conditions. Set your browser to http://www-cxro.lbl.gov/optical_constants/ to try out this resource.

3. EXPERIMENTS ON SYMMETRIC MOLECULES TEST KEY PRINCIPLE OF QUANTUM MECHANICS
(contact: joseph@fysik.uu.se)

A group led by Joseph Nordgren of Uppsala University has used resonant soft x-ray emission spectroscopy (resonant SXES) to study symmetric molecules, providing strong experimental support for one of the fundamental ideas of quantum mechanics: the idea that a molecule can exist in a combination of two or more distinct states.

The group's first experiment of this type involved exciting a gas of oxygen molecules (O2) with soft x rays from undulator Beamline 7.0.1 at precisely tuned photon energies. These energies were chosen to match the energy gaps between the molecules' core (inner electron) shells and higher-energy molecular orbitals ("resonant" refers to this matching of energies). As an x ray struck an oxygen molecule, it excited an electron from the core shell of one of the molecule's two atoms to a higher state. Then, in some molecules, either the excited electron or another electron from a valence orbital dropped to fill the core hole, emitting an x-ray photon in the process. The experimenters measured a spectrum of these emitted photons for each of several incoming photon energies. Later, the group performed a similar experiment with nitrogen (N2), another diatomic, homonuclear gas.

The explanation above is classical (not quantum-mechanical) in that it describes a core hole which is localized at one atom or the other. This would break the symmetry of the original molecule, whereas quantum mechanics allows for molecular symmetry to be maintained during core electron excitation. Nordgren's group studied the issue of core hole localization and symmetry breaking by exploiting rules governing electron transitions, as follows: the orbitals of a symmetric molecule can have one of two kinds of parity (symmetry), designated by the German words gerade ("even") and ungerade ("odd"). When an electron in a symmetric molecule absorbs or emits a photon, parity-selection rules constrain it to move only from an orbital of one parity to an orbital of the opposite parity, not between orbitals of the same parity. Using resonant SXES, one can detect whether the transitions taking place are allowed by the parity-selection rules. If non-parity-allowed transitions appeared in ! the resonant SXES spectrum, this w ould indicate that the molecule's symmetry had been broken. However, the group's experiments showed only parity-allowed transitions, indicating that molecular symmetry was preserved.

Furthermore, the broad peaks in the resonant SXES spectra for nitrogen indicate that the core-hole state is relaxed and well-defined, with a bond length distinct from those of the ground state and the final state. This supports the localized-core-hole view. In order to reconcile a localized core hole with the preservation of molecular symmetry, one must consider the excited molecule as existing in a combination of two states, one for each of the possible locations of the core hole.

Diatomic, homonuclear molecules as samples, resonant SXES as a technique, and the ALS as a source were all carefully chosen to suit this experiment to the question at hand. These molecules are ideal systems for a study using orbital symmetries because of their high degree of symmetry. Larger molecules often vibrate in asymmetric modes which can break their high symmetry regardless of whether core-hole localization occurs, but O2 and N2 each have only one, completely symmetric vibrational mode. The researchers chose the resonant SXES technique because other techniques, such as photoelectron spectroscopy or SXES with non-resonant energies, would not provide information on whether molecular symmetry were broken. Since SXES has a very low yield (about one emitted photon for every 1,000 incoming photons), the high intensity of the ALS beam was essential to observe enough emitted photons within a reasonable amount of time.

4. ALS OFFERS FREE TEACHER WORKSHOP
(contact: jccross@lbl.gov)

As part of its ongoing scientific outreach activities, the ALS is organizing a free workshop for teachers designed to introduce science educators to the ALS. The workshop is designed to build ties between the ALS, its staff, and local science educators, in order to facilitate the use of the ALS as a teaching resource. It will include a close-up look at the ALS and the opportunity to interact with ALS scientists, hands-on activities to take back to the classroom, and free hand-outs and activity materials.

The hands-on activities are designed to be readily adaptable to grades 4-12, and teachers without a science background are encouraged to participate. There is no cost for the workshop, meals, or materials.

The workshop will be held on:
Monday, March 4, 4:30-7:30 p.m. (including dinner) and 
Saturday, March 16, 9 a.m.-1 p.m. (including lunch).
Participants are expected to attend both sessions.

5. SCHEDULING MEETING WILL BE JANUARY 19
(contact: fred_schlachter@lbl.gov)

The users' scheduling meeting to discuss operations following the April, 1996 shutdown has been rescheduled for January 19 at 3:00 p.m. to coincide with the ALS Users' Executive Committee meeting. The regular weekly scheduling meeting will occupy the first few minutes of the meeting; likely agenda items for the remainder of the meeting include operating energies, use of the longitudinal feedback system, and an update on accelerator physics. The meeting location will be held in the Building 4 conference room, room 102.