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.

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ALSNews           Vol. 37           October 3, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. ALS WELCOMES VISITORS TO OPEN HOUSE
    3. UNDULATORS ON THE MOVE 
    4. FIRST PHOTOELECTRON DIFFRACTION MEASUREMENTS USING CIRCULARLY POLARIZED LIGHT ON BEAMLINE 9.3.2

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

The ALS is in a scheduled shutdown for major installations and maintenance (a list of major shutdown activities appears in ALSNews Vol. 35, September 5, 1995). User operations are scheduled to resume on November 1. The installation of a new undulator, completed September 19, is described in item #3 below.

2. ALS WELCOMES VISITORS TO OPEN HOUSE
(contact: fred_schlachter@lbl.gov)

ALS staff are preparing an exciting set of activities and presentations for a lab-wide open house to be held Saturday, October 28, from 10 a.m. to 3 p.m. All over Lawrence Berkeley National Laboratory, tours, displays, and hands-on activities will give the public a chance to share scientists' excitement about their work. Everyone is welcome and encouraged to attend -- the open house is designed to appeal to adults, young people, scientists, science students, and non-scientists alike.

At the ALS, outdoor activities especially for young people will include "The Electron Adventure," a game in which participants follow the course of electrons in the ALS and see examples of how its light is used, and "Soapy Science," where the swirling colors in giant soap bubbles will reveal some of the properties of light such as interference and wavelength.

Inside the ALS, visitors of all ages can explore the nature of light, the process of aligning a beamline, and how the electron beam of the ALS is made to bend into a circular path, all through hands-on activities. There will be interactive computer graphic displays from ALS engineering groups and exhibits on the history of x rays and the transition from cyclotrons to synchrotrons. Visitors will receive a large, colorful poster explaining the workings of the ALS as a reminder and souvenir of this special day. ALS users are invited to assist ALS staff in explaining their research to visitors; interested users should contact Jane Toby at (510) 486-6793.

For more information on programs and transportation for the open house, please call (510)486-6999.

3. UNDULATORS ON THE MOVE

"Moving Day" took on a weighty meaning at the ALS on September 18 and 19 as the U8 (8-cm-period) undulator in sector 9 was successfully moved to its new location in sector 12 and the recently completed U10 (10-cm-period) undulator was installed in its place. This brings the "in-the-ring" total for ALS insertion devices to four.

The U10 undulator with its support structure, a combined weight of nearly 25 tons, had to be lifted above the storage ring from the assembly area on the experiment floor so that it could be lowered into place through the top of the shielding. The U10 will generate light for Beamline 9.0.1, primarily used for atomic and molecular science, and the Chemical Dynamics Beamline 9.0.2. The U10, which has a higher peak field level (0.98 T) than the U8 (0.81 T), will produce light in the range of 5 to 950 eV at a storage ring operating energy of 1.5 GeV. The low energy photons (5-30 eV) are extremely useful for chemistry research because they cover the valence energies of many of the atoms and molecules that play a vital role in combustion and atmospheric chemistry. They allow selective ionization without causing unwanted dissociative ionization, thus achieving a near-zero background detection capability. The flux from the U10 will be approximately 10e16 photons/sec.

The relocated U8 will serve Beamline 12, now under construction with commissioning scheduled for December 1995. This will be the first beamline optimized to take full advantage of the spatially coherent undulator radiation available at third-generation synchrotron sources. This optimization is expected to yield a coherent photon flux several hundred times greater than available in previous experiments. The primary user will be the Center for X-Ray Optics' endstation for EUV interferometry, which has been operating on Beamline 9.0.1. The high coherent flux will provide excellent fidelity for reference waves in interferometry. Another planned Beamline 12 branchline will serve the MAXIMUM photoemission microscope from the University of Wisconsin. This endstation uses reflective optics to focus radiation to the smallest possible spot size, a task best achieved with spatially coherent light. The endstation has already seen action on bend-magnet Beamline 6.3.2, where it has generated images comparable to those obtained using undulator light at Wisconsin. The optimized coherent photon flux of Beamline 12 would give MAXIMUM a distinct advantage in spatial resolution (the target spot size is 500 angstroms) and shorten exposure times from hours to minutes.

4. FIRST PHOTOELECTRON DIFFRACTION MEASUREMENTS USING CIRCULARLY POLARIZED LIGHT ON BEAMLINE 9.3.2
(contact: csfadley@lbl.gov)

Researchers on Beamline 9.3.2 have taken their first measurements of circular dichroism on an oxidized tungsten surface using circularly polarized synchrotron radiation. Synchrotron radiation emitted from bend magnets in the plane of the electron orbit is linearly polarized, but an observer moving upward or downward from this plane would measure two changes. First, there is an increasing perpendicular component to the polarization (resulting in right circularly polarized light above the plane and left circularly polarized light below it), and second, the radiation flux decreases. A vertically scanning slit assembly has been installed on Beamline 9.3.2 to give access to this circularly polarized light: at the beamline's typical operating degree of circular polarization (0.8), 30% of the total available flux is transmitted through the slit.

As a first study with this new capability, researchers used the advanced photoelectron spectrometer/diffractometer endstation on this beamline to study circular dichroism in core level photoelectrons from an oxidized tungsten surface using photoelectron diffraction (PD). In PD, radiation absorbed at a specific site on a sample causes the emission of an electron from the absorbing atom. The electron behaves as a spherical wave, scattering off neighboring atoms to create secondary, scattered waves which differ in phase from the primary wave. In certain directions, the scattered waves interfere constructively with the primary wave, increasing the intensity of the emission recorded at the analyzer. In other directions, the waves interfere destructively and the recorded intensity is reduced. By rotating the sample and the Scienta ES200 electron analyzer in their endstation while keeping the sample and incident photon beam in a fixed relative orientation, the Beamline 9.3.2 researchers can map the intensity of photoelectron emission over a section of a hemisphere above their sample surface. Analysis of this intensity, called a photoelectron diffraction pattern because it results from the wave behavior (diffraction) of photoelectrons, provides a model of the ordered atomic structure at the sample surface.

An intriguing effect known as peak rotation occurs when PD is performed using circularly polarized light: the hemispherical photoelectron diffraction pattern is rotated relative to the pattern measured from the same sample using linearly polarized light. This is because circularly polarized light imparts an angular momentum to the photoelectrons it liberates. In the current study, peak rotations of +/- 9 degrees were observed for both the tungsten and tungsten oxide 4f peaks when the incident radiation was changed from right to left circular polarization. Similar effects have previously been observed only in PD measurements of 2s and 2p photoelectrons from silicon (001), taken by Daimon et al. (Osaka University, and also a collaborator on this research project); thus the present work at the ALS supports the generality of this phenomenon in the excitation of any system (magnetic or non-magnetic) with circularly polarized radiation.

Peak rotations and other distortions of photoelectron angular distributions away from mirror-plane symmetry will be important considerations in the quantitative understanding of circular dichroism in photoelectron measurements. In particular, the existence of these effects and how to calculate them are important in the interpretation of magnetic circular dichroism, a technique used to study the magnetic order in materials for the computer and data storage industries.

Researchers involved in this study include groups led by Charles Fadley (U. of California-Davis and LBNL Materials Sciences Division) Zahid Hussain (ALS), and David Shirley (Pennsylvania State U.). Future plans include studies of magnetic order near surfaces and interfaces via spin polarized photoelectron angular distribution measurements using circularly polarized light. Polarization measurements of the Beamline 9.3.2 radiation were made using the Kortright multilayer polarimeter (see ALSNews Vol. 31, July 11, 1995).