1. FEMTOSECOND STRUCTURAL DYNAMICS OBSERVED IN SEMICONDUCTOR CRYSTALS
by Art Robinson
(Contact: RWSchoenlein@lbl.gov)

A group from Berkeley Lab; the University of California, Berkeley; and the University of Aarhus in Denmark has measured ultrafast structural changes in indium antimonide crystals by means of time-resolved x-ray diffraction with a novel femtosecond x-ray source at the ALS. The experiments quantitatively resolved lattice expansion occurring over tens of picoseconds after excitation with a high-power infrared laser. They also demonstrated the subpicosecond creation of a disordered region near the surface.

Femtosecond x-ray experiments are an emerging area of research in chemistry, solid-state physics, and biology. Dynamic processes in condensed matter occur on a time scale dictated by the period of a lattice vibration, typically about 100 femtoseconds. However, the time resolution of established techniques for determining atomic structure, such as x-ray diffraction and x-ray absorption fine structure, is limited by the pulse length of the x-ray source. For example, the ALS emits pulses 30 picoseconds in length, which is 300 times too long to resolve events lasting 100 femtoseconds. A number of ways to circumvent this limitation are under investigation at the ALS.

The Beam Test Facility (BTF) makes use of the electron beam in the ALS linear accelerator rather than in the storage ring. In brief, firing a high-power infrared laser across the tightly focused linac beam at 90 degrees generates femtosecond x-ray pulses by the process of Thomson scattering. The x-ray photon energy depends on the electron-beam energy. In the present experiments with a 100-femtosecond titanium-sapphire laser, 50-MeV electrons in the linac resulted in 300-femtosecond pulses of x rays with a photon energy of 30 keV, and 25-MeV electrons resulted in 500-femtosecond x rays at 7.3 keV. The Berkeley group used the BTF source to measure x-ray diffraction from an indium antimonide crystal every few picoseconds both before and after laser irradiation at intensities below the threshold for damage. (A highlight with images from this experiment appears on the Web at http://www-als.lbl.gov/als/science/sci_archive/InSb.html.)

The data at 30 keV demonstrate that expansion of the lattice, which shifts the diffraction peak to lower photon energies, begins only after a delay of about 10 picoseconds after laser irradiation. This is the first direct evidence of such a delay, which the researchers quantitatively modeled as a combination of energy-relaxation processes by which energy is transferred from the electrons excited by absorption of laser light to lattice vibrations and subsequent propagation of the resulting lattice expansion inward from the surface. A decrease in diffraction intensity from the surface was observed with 7.3-keV photons, which do not penetrate the sample as deeply as 30-keV photons. The decrease is due to the high concentration of photoexcited electrons near the surface, which causes a nonthermal disordering of the lattice in less than a picosecond, a time too short to invoke transfer of energy to lattice vibrations and subsequent bond breaking.

In collaboration with researchers from the Berkeley Lab Center for Beam Physics and the ALS, the experimenters are building a new bend-magnet beamline (Beamline 5.3.1) that will provide 100-femtosecond x-ray pulses via another laser-based technique, recently demonstrated at the ALS, called time slicing. In addition to the improved time resolution, this beamline will be about10,000 times brighter than the BTF source and will cover the entire spectral range of the bend magnet, thereby opening up a wide range of scientific applications for time-resolved x-ray measurements.

Research conducted by R. W. Schoenlein, T. E. Glover, and W. P. Leemans (Berkeley Lab); A. H Chin and C. V. Shank (University of California, Berkeley, and Berkeley Lab); and P. Balling (University of Aarhus, Denmark), using the Beam Test Facility.

Funding: U. S. Department of Energy, Office of Basic Energy Sciences; National Science Foundation.

Publication about this experiment: A. H. Chin et al., Phys. Rev. Lett. 83, 336 (1999).

2. USER MEETING REGISTRATION EXTENDED TO OCTOBER 8
(Contact: alsuser@lbl.gov)

The deadline to register for the 1999 ALS Users' Meeting has been extended to October 8. Registration materials are available from the ALS User Services Office (phone, 510-486-7745; fax, 510-486-4773; email, alsuser@lbl.gov) and on the Web at http://www-als.lbl.gov/als/usermtg/registration.html. The meeting will be held October 18-20, with a full scientific program on the 18th and 19th and workshops on the 20th. Further information about the meeting is also available on the ALS Users' Meeting Web site (http://www-als.lbl.gov/als/usermtg/).

3. INTERNATIONAL WORKSHOP ON SYNCHROTRON RADIATION
(Contact: Fadley@lbl.gov)

The Second International SLS (Swiss Light Source) Workshop on Synchrotron Radiation will be held at the Paul Scherrer Institut in Brunnen, Switzerland on October 26-30, 1999. The final program for this meeting is now available, and it will involve 40 invited talks on various aspects of third-generation synchrotron radiation and its applications, including five talks by ALS users or staff members. For more information on this year's meeting, see the SLS Workshops and User Meetings Web pages at http://www.psi.ch/sls/.

4. KEY ACCESS TO EXPERIMENT FLOOR DISCONTINUED
(Contact: RMMiller@lbl.gov)

The locks on the entry doors to the ALS experiment floor have been changed. Access to the floor is now by key card only. Anyone needing a key card should contact Ruth Pepe at ext. 5268. All 6-37 keys that are no longer needed can be dropped off at the User Services Office on the ALS mezzanine so they can be returned to the Lock Shop.

5. UEC CORNER: NOTES FROM THE USERS' EXECUTIVE COMMITTEE
by Stephen Kevan
(Contact: kevan@oregon.uoregon.edu)

Two important meetings have been held in the past week. I will briefly describe each of these in this communication.

A few UEC members were able to limp to the ALS Scientific Advisory Committee (SAC) meeting on Saturday, September 18. (Editor's note: Kevan is recovering from an achilles tendon tear.) My overall impression was quite positive. This is a very thoughtful and strong committee. Their opinions are sought and seriously considered by the ALS. The following things were discussed:

(1) Results of a review of the ALS microscopy program, chaired by Janos Kirz, were presented. The committee worked very hard and produced a very useful document that will be available shortly. Most programs reviewed quite well, notably PEEM, STXM, XRM, and the microfocus x-ray lines.

(2) Results of a review of Beamlines 10.3.1 and 10.3.2, the microdiffraction and microabsorption beamlines, were also presented. The discussion focused quite a bit on developing a strong Participating Research Team (PRT) or PRTs to help manage and maintain these valuable facilities. There will be (another) workshop/meeting of the principals during the Users' Meeting to try to thrash this out. This issue of PRT development is one that is coming into focus around the ring: the ALS cannot own and operate many beamlines, so developing strong PRTs is crucial to the long-term viability of the facility.

(3) We heard about the crystallography program from Graham Fleming. He is chairing a committee to look into prioritizing superbend allocation. Paul Alivisatos will represent the UEC on this committee.

(4) The need for "generic" (e.g., EXAFS, powder diffraction, etc.) beamlines was discussed, with many agreeing that these would be valuable. Generic beamlines often need to be facility operated and owned, which counters the "strong PRT model" discussed above.

On Wednesday, September 22, an off-site management retreat was held at the Clark Kerr campus, with several users in attendance. The focus of these meetings is on near-term budgeting and long-term planning. In other words, this is where advice, primarily from the SAC but also some from the UEC, is folded into ALS policy. These meetings have proven quite useful in the past. This meeting served three purposes:

(1) We discussed and validated past decisions, in terms of long-range planning for the facility. A planning document is in preparation and should be available soon.

(2) We were presented with the near-term budgetary needs of the ALS group leaders. We will be providing input in the coming weeks and would be delighted to have your comments.

(3) Jim Krupnick provided a very useful description of the ALS budget, and we were able to see very clearly the financial stress under which the ALS is operating. The statistic that stuck in my mind is that, in indexed dollars, the ALS operating budget is smaller now than in 1993 when there were no users. Serious and heroic efforts are being made to improve efficiency, but "fat" in the ALS budget was not readily apparent. This will, of course, be discussed further at the upcoming ALS Users' Meeting.

We look forward to seeing you at the Users' Meeting in a couple of weeks. The agenda is nearly complete, and we look forward to an energetic and informative series of talks.

6. RECENT PUBLICATIONS: RIBOSOMES TAKE CENTER STAGE

The September 24 issue of Science Magazine features a suite of articles about groundbreaking ribosome research. The centerpiece is a research article reporting a study conducted at the Macromolecular Crystallography Facility (MCF) at the ALS. A team from the University of California, Santa Cruz, led by Harry F. Noller, in collaboration with MCF/ALS group leader Thomas Earnest, has solved the structures of 70S ribosome functional complexes containing messenger RNA and transfer RNA. Their structures give a wide-reaching glimpse into the making of proteins, at 7.8-angstrom resolution. This work is also featured on the cover of the issue. In a second study reported in the same issue, the group extends the crystallography work by using directed hydroxyl radical probing to study the interface between ribosomal subunits. The publications are accompanied by an Anders Liljas perspective and a pair of "News Focus" pieces by Elizabeth Pennisi.

Cate, J.H., M.M. Yusupov, G.Zh. Yusupova, T.N. Earnest, and H.F. Noller, "X-ray Crystal Structures of 70S Ribosome Functional Complexes," Science 285(5436), 2095 (September 24, 1999).

Culver, G.M., J.H. Cate, G.Zh. Yusupova, M.M. Yusupov, and H.F. Noller, "Identification of an RNA-Protein Bridge Spanning the Ribosomal Subunit Interface," Science 285(5436), 2133 (September 24, 1999).

Liljas, A. "Function is Structure," Science 285(5436), 2077 (September 24, 1999).

Pennisi, E., "Challenge from Electron Microscopy," Science 285(5436), 2050 (September 24, 1999).

Pennisi, E., "Race to the Ribosome Structure," Science 285(5436), 2048 (September 24, 1999).

7. WHO'S IN TOWN: A SAMPLING OF ALS USERS

Beamline 1.4.3: Hoi-Ying Holman and collaborators (Berkeley Lab) will be studying living cells with IR spectromicroscopy. Mike Chin, Mike Martin, and Wayne McKinney (Berkeley Lab) will be comissioning phase two of the active feedback optical system. Sherry Zhang and Phil Ross (Berkeley Lab) will study buildups on battery anodes. Felicia Hendrickson and Bob Glaeser (Univ. of California, Berkeley) will continue investigations of the photocycle of bacteriorhodopsin microcrystals.

Beamline 7.0.1: Z.Q. Qiu (Univ. of California, Berkeley) will study quantum states in metallic double-quantum-well systems. Susan Kauzlarich (Univ. of California, Davis) will conduct a spin-polarized photoemission study of rare-earth transition-metal zintl compounds.

Beamline 7.3.1.1: Boris Sinkovic and Laurent Seve (Univ. of Connecticut) will study ferromagnetic and antiferromagnetic nanostructures and surfaces with PEEM2.

Beamline 8.0.1: Dennis Lindle (Univ. of Nevada, Las Vegas) will study the limits of fundamental approximations in x-ray photoelectron spectroscopy. Michael Lubell (American Physical Society) will conduct photoemission studies of global change molecules.

Beamline 9.3.1: Heinz Frei, N. Ulagappan, and Young-Hoon Yeom (Berkeley Lab) are making x-ray absorption measurements (EXAFS) of transition-metal molecular sieves. These molecular sieves are used as photoreactors for chemical synthesis.

Beamline 10.3.2: Richard Reeder (State Univ. of New York at Stony Brook) will be studying trace metal structural configurations in various minerals and contaminated soils.

8. OPERATIONS UPDATE
(Contact: RMMiller@lbl.gov)

Beam reliability for the last two weeks (September 13 to 26) was 96.4% for user shifts. There was one outage of 6-1/2 hours to repair a water leak and dry a storage-ring magnet power supply. All other outages were of short duration.

Long-term and weekly operations schedules are available on the Web (http://www-als.lbl.gov/als/accelinfo.html). Requests for special operations use of the "scrubbing" shift should be sent to Bob Miller (RMMiller@lbl.gov, x4738) by 1:00 p.m. Friday. The Accelerator Status Hotline at (510) 486-6766 (ext. 6766 from Lab phones) features a recorded message giving up-to-date information on the operational status of the accelerator.