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ALSNews           Vol. 60           September 4, 1996

Table of Contents


    1. OPERATIONS UPDATE
    2. CIRCULAR POLARIZATION GENERATED ON A LINEAR UNDULATOR BEAMLINE
    3. LONG-TERM SCHEDULE SET FOR NOVEMBER 1996-APRIL 1997
    4. ALS USERS' MEETING POSTER REMINDER

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

Beam availability for the last two weeks was 93.2% overall and 91.7% for user shifts.

Operations Summary for September 4 - September 23

Sep 04, 00:00-24:00           Accelerator Physics
Sep 05, 00:00-08:00           User Scrubbing & Special Operations
                              (1.1-GeV/2-bunch requested)
Sep 05, 08:00- Sep 09, 07:15  1.5-GeV/400-mA/320-bunch User Operations
Sep 09, 07:30-24:00           Maintenance & Startup
Sep 10, 00:00-24:00           Accelerator Physics
Sep 11, 00:00-08:00           User Scrubbing & Special Operations
Sep 11, 08:00- Sep 16, 07:15  1.9-GeV/260-mA/320-bunch User Operations
Sep 16, 07:30-24:00           Maintenance & Startup
Sep 17, 00:00-24:00           Accelerator Physics
Sep 18, 00:00-08:00           User Scrubbing & Special Operations
Sep 18, 08:00- Sep 23, 07:15  1.9-GeV/260-mA/320-bunch User Operations

2. CIRCULAR POLARIZATION GENERATED ON A LINEAR UNDULATOR BEAMLINE
(contact: jbkortright@lbl.gov, tobin1@llnl.gov)

A research team working on undulator Beamline 7.0 recently performed magnetic x-ray circular dichroism (MXCD) experiments using the output of a novel phase retarder (approximately a quarter wave plate) that converts linearly polarized light to circularly polarized light in the VUV and soft-x-ray spectral regions. Successful use of the phase retarder to generate circularly polarized light means that the technology can be used on any linear undulator beamline, thereby extending the range of spectroscopy and microscopy experiments possible. Moreover, with an undulator source, the flux of circularly polarized photons was much larger than that obtainable from off-axis bend magnet radiation, even though absorption in the phase retarder reduced the intensity of transmitted light by slightly more than 90%.

Circularly polarized synchrotron radiation is an increasingly useful tool for probing spin-dependent phenomena, such as the behavior of magnetic materials and transition metals in proteins. Spin information comes from the difference, or dichroism, between photoemission intensity obtained with left and right circularly polarized light. Understanding magnetic behavior at such a fundamental level is critical for controlled design of ultrathin magnetic layers and other nanoscale magnetic structures and, ultimately, devices such as sensors and storage media.

The phase retarder is a relatively low-cost alternative for producing circular polarization at photon energies up to 150 eV. In use, the transmission phase retarder and a linear polarizer, both based on multilayer films, combine with a reflected intensity detector to form a tunable x-ray polarimeter (see ALS News, Vol. 31, July 11, 1995). Using Mo/Si multilayers (in both the phase retarder and polarizer) and 95 eV photons, the polarimeter was used to measure as much as 93% circular polarization produced by the retarder. A translation stage moves the linear polarizer into or out of the beam as needed, and tuning between right and left circularly polarized light is accomplished simply by rotating the retarder.

The high flux of circularly polarized light from the tunable polarimeter enabled the research team to do the MXCD photoemission experiments quickly at very high resolution (total energy resolution of 100 meV or less and angular resolution of 2 degrees or better). They observed strong dichroism signals in two different types of samples: FeNi ultrathin alloy films grown on a Cu(001) substrate with a specific orientation, and Gd (0001) grown on Y(0001). FeNi films exhibit a nanoscale version of Invar quenching (disappearance of magnetic properties when the concentration of nickel is too low), whereas Gd is a prototype for the study of magnetism in rare earths.

Besides converting linearly to circularly polarized light, another way to obtain spin information is to find special "handed" or chiral configurations of the experimental vectors (electron-emission direction, photon direction, photon linear polarization, crystallographic vectors) in which linearly polarized light is sensitive to spin. This technique is called magnetic x-ray linear dichroism (MXLD). One of the goals of the present experiments was to make a direct experimental comparison of MXCD with MXLD. A partial analysis of the results suggests that existing theories give an incomplete description of the relationship, since the MXCD signal is larger relative to the corresponding MXLD signal than predicted by theory.

Polarimeter design and commissioning were conducted by J.B. Kortright (principal investigator), K. Franck, and A. Warwick (Berkeley Lab).
The research was performed using the ultraESCA endstation on Beamline 7.0 by researchers from Lawrence Livermore National Laboratory, Pennsylvania State University, Virginia Commonwealth University, Berkeley Lab's Center for X-Ray Optics, and the ALS.
Funding: Office of Basic Energy Sciences of the U.S. Department of Energy.

3. LONG-TERM SCHEDULE SET FOR NOVEMBER 1996-APRIL 1997

The draft ALS operating schedule for November 1996 to April 1997 published in the last edition of ALSNews (Vol. 59, August 21, 1996) has been reviewed by ALS users and beamline spokespersons and adopted as is. Active and recent users will receive a copy by mail; others may request a copy by sending their complete mailing address to alsuser@lbl.gov with a "please send long-term schedule" message.

4. ALS USERS' MEETING POSTER REMINDER
(contact: alsuser@lbl.gov)

Just a reminder that abstracts for the ALS Users' Meeting poster session are due by September 30. The poster session will be held on the ALS Experiment Floor in conjunction with vendor exhibits on Tuesday, October 22, 12-3 p.m. To propose a poster, please submit a one-page abstract to ALS Administration by September 30. In the upper right-hand corner of the abstract, please include the associated ALS beamline number and the phone number, fax number, and email address of the first author. The poster board size is 1.2 m (4 feet) square. Push pins will be provided.

The number of posters accepted will be limited by the availability of space. Notification of abstracts that have been accepted or declined will be sent via email to the first author by October 11. Please submit abstracts and address any additional questions to:

     Elizabeth Saucier
     ALS Program Administrator
     Advanced Light Source
     MS 80-101
     Lawrence Berkeley National Laboratory
     Berkeley, CA 94720

     Voice -- 510-486-6166
     Fax -- 510-486-4960
     Email -- alsuser@lbl.gov