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. 58           August 7, 1996

Table of Contents


    1. OPERATIONS UPDATE
    2. A NEW VIEW OF DISSOCIATION
    3. USERS' TOWN MEETING AUGUST 14
    4. ALS ADMINISTRATION AND ACCELERATOR GROUPS MOVE BACK TO BUILDING 80

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

Beam availability for the last two weeks was 88.7% overall and 89.7% for user shifts. Interruptions to operations were caused by problems with the longitudinal feedback timing system, a motorized water temperature control valve for storage ring RF cavity number 2, low-conductivity water flow interlock trips on two storage ring girders, and a failed power supply in the ALS control system. All problems were repaired and were of short duration.

Operations Summary for August 7 - August 26

Aug 07, 00:00-08:00           User Scrubbing & Special Operations
                              (1.1-GeV/400-mA/320-bunch user operations)
Aug 07, 08:00- Aug 12, 07:15  1.1-GeV/400-mA/320-bunch user operations
Aug 12, 07:30-24:00           Maintenance & Startup
Aug 13, 00:00-24:00           Accelerator Physics
Aug 14, 00:00-08:00           User Scrubbing & Special Operations
Aug 14, 08:00- Aug 19, 07:15  1.9-GeV/260-mA/2-bunch User Operations
Aug 19, 07:30- Aug 20, 24:00  Maintenance & Startup
Aug 21, 00:00-08:00           User Scrubbing & Special Operations
Aug 21, 08:00- Aug 26, 07:15  1.9-GeV/260-mA/2-bunch User Operations
Aug 26, 07:30-24:00           Maintenance & Startup

2. A NEW VIEW OF DISSOCIATION
(contact: musa@me13.lbl.gov)

Initial images from the Universal Ion Imaging chemical dynamics endstation are treating chemists to new views of the dynamics behind dissociation. Investigators from Berkeley Lab's Chemical Sciences Division have recently studied the dissociative ionization of sulfur hexafluoride (SF6), selectively looking at velocity distributions for SF4+ and SF5+ product ions. The SF6 molecule serves as a model for studying the chemical process of dissociation.

The two-dimensionalimage captured for each product ion contains the information necessary to reconstruct a three-dimensional distribution, representing the velocities (speeds and directions) of the many ions of that type produced in the dissociation. Higher intensities correspond to greater numbers of product ions with the corresponding speed and direction. The velocity information can be used to calculate translational energies, which provide insight into what forces are acting during the dissociation. Understanding the forces helps chemists to figure out the chain of events by which a molecule breaks apart. The distribution for SF5+ revealed an intriguing anisotropy (i.e., the distribution's cross-section is different when measured along different axes), which may provide clues about the forces at work when SF6 dissociates.

The imaging process begins with pulses of SF6 in a molecular beam intersecting with pulses of undulator light (15.8 eV, deltaE = 0.3 eV) from the ALS. The light is pulsed by a chopping mechanism on the beamline. Where the two pulses meet, SF5+ and SF4+ ions are formed by dissociative ionization. The ions are accelerated upward in a time-of-flight tube, through which different ions with the same charge pass at different rates because they have unequal masses. They then reach a position-sensitive detector, which is gated so that it detects particles only during the time at which the ions of interest (e.g., SF5+) reach it. A CCD camera above the detector records the intensity (the number of particles detected) in each position as a two-dimensional image. This single image represents the entire distribution of the particles' velocities in a flattened form (something like a pancake). It can be converted into a three-dimensional velocity distribution through a tomographic reconstruction (a mathematical transformation made possible by assuming cylindrical symmetry about the polarization direction of the synchrotron light, mathematically turning the pancake into a souffle).

The data can be collected in a matter of minutes, rather than days as with other setups, in part because the detector is position sensitive, collecting multiple-point images rather than single data points. This eliminates the need for repetitive detector positioning to measure intensity at multiple angles. In addition, the pulse-after-pulse repetition of the dissociation and detection quickly accumulates more and more distribution data as the experiment is essentially run again and again.

Working on an undulator beamline gives researchers the brightness and tunability needed to image virtually any ion. In future studies, the researchers hope to examine reactions between two molecules or a molecule and an atom by using reactive scattering. This technique adds a second molecular beam to the current ion imaging setup on Beamline 9.0.2, making it possible to image two-reactant processes.

3. USERS' TOWN MEETING AUGUST 14

The next town meeting of ALS management and users will occur on August 14 from 3 to 5 p.m. in the Building 4 conference room. The primary topic will be the long-term operations schedule for the period from November 6, 1996 to April 27, 1997. Users are invited to come with comments or requests regarding energy and other aspects of operations. There will also be a short presentation by the accelerator physics group on operating conditions, brightness, beam lifetime, etc. Please send any issues you may wish to raise at the meeting to ALS Users' Executive Committee Chair Jeff Bokor (jbokor@eecs.berkeley.edu) or to Fred Schlachter (fred_schlachter@lbl.gov) in advance of the meeting.

4. ALS ADMINISTRATION AND ACCELERATOR GROUPS MOVE BACK TO BUILDING 80

During the week of August 5-9, most of ALS management, administration, and accelerator group staff who temporarily moved to Building 4 to make way for construction of the ALS Structural Biology Support Facilities will move back to the second floor of refurbished Building 80. Note that the ALS User Office, Neville Smith (ALS Scientific Program Head), Fred Schlachter (ALS Scientific Outreach and Communications Coordinator) and Art Robinson (ALS Scientific Outreach and Communications) will remain in Building 4. Telephone numbers, fax numbers, and mailing addresses will not be affected.

The visitor's log and badges for casual, escorted visitors (those not on tours organized through Berkeley Lab's Reception Center) will continue to be located in Building 2, Room 400 (Joan Minton's office).