1. DYNAMIC SCATTERING OF COHERENT SOFT X RAYS
by Lori Tamura
Contact: kevan@oregon.uoregon.edu

The coherent light provided by lasers revolutionized experimental science in many ways. For example, light-scattering experiments using coherent visible-wavelength light allow the detailed measurement of dynamic microscopic phenomena such as the Brownian motion of small particles. An analogous technique for studying motion on the molecular scale, however, requires shorter-wavelength coherent light in the soft x-ray range (10 to 100 angstroms). To study thermally driven fluctuations inside molecules, a group of scientists at the ALS has applied coherent soft x rays from Beamline 7.0.1 to dynamic scattering experiments on liquid-crystal films.

Hard x rays (at wavelengths near 1 angstrom) have recently been used to probe relatively slow (on the order of tenths of a second to hours) atomic motion. The measurement of fast (microsecond) molecular motion, however, requires more coherent photons than can be obtained in the hard x-ray wavelength range. Fortunately, because the available coherent flux of an x-ray source (hard or soft) is proportional to the wavelength of the light squared, one can get 2,000 times more coherent photons using 44-angstrom soft x rays than from using 1-angstrom hard x rays. At the molecular scale, a longer wavelength is not detrimental to resolution. At Beamline 7.0.1, a double-pinhole coherence filter converted the undulator beam into a high-coherence incident beam. With this setup, the researchers were able to achieve the same time resolution as with conventional laser-light scattering (about 1 microsecond) and 100 times better spatial resolution (44 vs. 6,360 angstroms).

The goal is to develop this technique so that it can be used to probe fluctuations during phase transitions and to study the internal motions of biological molecules that are thought to be crucial to chemical reaction rates, catalysis, and biological function. Before applying the technique to the difficult problems of phase transitions and biology, however, the researchers decided to apply it to a simpler problem: measuring the layer fluctuations of freely suspended liquid-crystal films.

The liquid-crystal films used in this experiment were heated to the smectic-A transition point. In the smectic-A phase, the layers of the crystal can "slide" back and forth and the molecules are free to move within the layers. When photons are reflected from this fluctuating system, the intensity of the reflected beam will fluctuate because of the thermally driven motions within the sample. If the incident beam is spatially coherent, the reflected-beam fluctuations are an average over all of the illuminated molecules. By fitting the normalized reflected-beam intensity as a function of time to an exponential curve, the researchers determined the characteristic decay time of the layer fluctuations for a given film thickness. A plot of decay time vs. film thickness for the five crystal types studied shows obvious linearity, in excellent agreement with the predictions of theoretical models.

With future access to the raw undulator beam on Beamline 9.0.1 (unfiltered by a monochromator and not subject to losses from optical components), the available coherent flux will increase by 1,000. Once suitable technique and source improvements are made, researchers hope to be able to go beyond the usual time-averaged x-ray snapshots to produce x-ray movies of molecular motion.

Research conducted by A.C. Price and L.B. Sorenson (University of Washington, Seattle); S.D. Kevan and J. Toner (University of Oregon); and A. Poniewierski and R. Holyst (Polish Academy of Sciences), using Beamline 7.0.1.

Funding: U. S. Department of Energy, Office of Basic Energy Sciences; American Chemical Society Petroleum Research Fund; University of Washington Molecular Biophysics Training Grant; KBN Grants; and Maria Sklodowska Curie Joint Fund II.

Publication about this experiment: A.C. Price et al., Phys. Rev. Lett. 82, 755 (1999).

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

To highlight the richness of our user community and help introduce recent arrivals, we offer this listing of some of the experimenters who will be collecting data during the next two weeks at the ALS.

Beamline 1.4.3: Giuseppina Conti (Applied Materials) will be testing the infrared microscope's capabilities to measure small contaminants on wafer surfaces.

Beamline 7.3.3: Phil Heimann (Berkeley Lab) and Roger Falcone (Univ. of California, Berkeley) will conduct femtosecond x-ray experiments, looking at phase transitions in silicon induced by a laser pulse. Wenbing Yun (Berkeley Lab) will be developing a system for x-ray tomography.

Beamline 9.0.2.1: W. Sun and D. Neumark (Univ. of California, Berkeley) will be investigating the dissociation dynamics of butadiene.

Beamline 9.3.2: Ed Moler (Berkeley Lab) will be commissioning the Fourier transform soft x-ray spectrometer.

Beamline 10.0.1: Orhan Yenen and Duane Jaecks (Univ. of Nebraska-Lincoln) will study alignment of substates in argon by using polarization-resolved fluorescence with linearly and circularly polarized incident radiation. Z.X. Shen and his group (Stanford Univ.) will conduct electron spectroscopy with high momentum resolution to study the electronic structure of high-Tc materials.

Beamline 10.3.1: Winfred Seifert (Technische Univ., Freiberg) will be studying impurities in solar cells. Tom Cahill and Ken Verosub (Univ. of California, Davis) will probe man-made contamination of soil and conduct analyses of airborne contaminants.

Beamline 10.3.2: Bill Glassley (Lawrence Livermore National Laboratory) and Ardyth Simmons (Berkeley Lab) will study the chemical heterogeneity, pore structure, and chemistry in the region of rock fractures. Daniel Strawn and Harvey Doner (Univ. of California, Berkeley) will investigate the speciation of metal contaminants in soils. Katrina Peariso and Jim Penner-Hahn (Univ. of Michigan) will be studying the chemical and temporal changes in the zinc environments of zebrafish embryos during their development.

3. OPERATIONS UPDATE
Contact: RMMiller@lbl.gov

Beam reliability for user shifts during the last two weeks (March 29-April 11) was essentially perfect. Users had more beamtime than scheduled, and there were no significant outages.

Long-term and weekly operations schedules are available on the Web (http://www-als.lbl.gov/als/accelinfo.html). Weekly operations scheduling meetings are held on Fridays at 3:30 p.m. in the Building 6 conference room. 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.