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  • Aladdin Highlights the HERMON Beamline


    This Aladdin contains the second in a series of articles to highlight various members and points of interest of the SRC community. The HERMON beamline is highlighted in this issue.

    Since being commissioned in the summer or 1993 the High Energy Resolution MONochromator (HERMON) has maintained its status as a very unique source of synchrotron radiation. The beamline monochromator uses a combined rotation and translation of a spherical variable line spaced grating to maintain focus and minimize aberrations across a broad energy range. Using sophisticated real-time feedback computer control, the beamline achieves resolving power in excess of 10,000 in the soft X-ray range.

    The monochromator currently installed on the beamline has one grating that covers the 500 to 1100 eV range. “The monochromator mechanical design is unique in the world and provides the highest resolution in the United States in the soft X-ray range”, says beamline manager Mark Bissen. Bissen maintains the beamline and its complex control system for users. He is also responsible for beamline upgrades like an additional grating that will allow the monochromator to cover the 250-550 eV range. Other future improvements for the beamline include a beam position monitor system supplying feed back to the entrance mirror control system, modifying the monochromator control system, and improving the quality of the optics.

    A picture of HERMON users.

    Alexander Menzel and Sacia Benzaid hard at work at the end of the HERMON beamline.

    This soft X-ray user beamline is based on an In Focus Monochromator (IFM) invented by M. C. Hettrick of Hettrick Scientific Inc.. HERMON was jointly designed and built by SRC and PSL under a licensing agreement between the UW-Madison and Hettrick Scientific Inc..

    The National Science Foundation (NSF) funded beamline typically attracts experiments for molecular vibrational modes, gas phase characterization, and high temperature superconductor research. Recent research includes a study of K-shell excitations of atomic oxygen with high resolution by A. Menzel, S. Benzaid, C.D. Caldwell, University of Central Florida, and M.O. Krause, Oak Ridge, Tennessee.

    In this experiment, ion yield measurements have been performed in order to study the K-shell edge and the associated Rydberg states of atomic oxygen and small molecules (O2, CO, NO, F2, and HF) in the gas phase.

    The high resolving power (104) of the variable line density grating monochromator beamline HERMON, at the Synchrotron Radiation Center (SRC), enabled the U. Central Fla./Oak Ridge group to resolve higher Rydberg members of the oxygen 1s®np series up to n=5.

    Atomic oxygen is created in a 2460 GHz microwave discharge and directed toward a gas cell separated from the monochromator by a 120 nm carbon window. The operating pressure maintained in the cell is about 1.5x10-4 Torr. A dissociation rate of 55% is reached when a small amount of nitrogen is added to the oxygen in the discharge.

    Spectrum from atomic oxygen

    The atomic spectrum of oxygen recorded on the HERMON beamline. Shell transitions are clearly seen. The higher resolution spectrum shown in the inset reveals higher members of the atomic series superimposed on the molecular resonances.

    The ions resulting from the interaction with the photons are accelerated and detected by a channeltron. The number of counts is recorded as a function of the photon energy, and the photon flux is simultaneously measured for normalization purposes.

    For further information on the HERMON beamline, contact the beamline manager at (608) 877- 2146.


    Aladdin -- Editor Brian Tonner tonner@src.wisc.edu