Synchrotron Radiation Center

HERMON Makes High Resolution Debut

Proof-of-concept experiments on the newest beamline at SRC have demonstrated that the limits to very high resolution in soft X-ray spectroscopy have been pushed a step further. According to Mark Bissen, optics specialist at SRC, who together with Hartmut Hochst (Research and Safety-SRC) is taking the new varied- line-space monochromator through its test phase, researchers interested in gas-phase studies will find the monochromator especially useful due to the very high resolving powers. First results show a performance approaching the theoretical resolving power of 10,000, a new record at SRC for the soft X-ray region.


SRC's motivation to develop a new high energy monochromator utilizing a varied line space grating was to overcome intrinsic limitations in the classical SGM configuration, such as the need for a moving exit slit. The new "HERMON" (High Energy Resolution Monochromator), the first of its kind used in vacuum ultraviolet study, is a step forward for SRC monochromators due to its success in eliminating higher order aberrations intrinsic in conventional designs. The HERMON project is the result of joint efforts of many staff in UW-PSL and SRC. The original optical design of the HERMON was laid out in a paper by Hochst, Bissen, Mike Engelhardt, and Dennis Crossley of SRC (NIM A319, (1992) 121). The design includes a unique combination of grating rotation and translation, which varies the grating's effective ruling density, thereby maintaining a focus at a fixed exit slit. The grating motion mechanism and chamber were designed by Fred Middleton of UW-PSL, with contributions to design improvements and additional beamline components from Mike Fisher and Greg Rogers of SRC.

Researchers in need of high resolution in the soft X-ray region will find the exceptional performance of HERMON especially useful. A scan range from about 60 eV to 1100 eV will ultimately be covered, using one of the four gratings that can be accommodated by the grating housing. Presently, the monochromator is equipped with a partially ruled test grating, which covers an energy range of 480 to 1120 eV. This range is not currently available with a high resolution monochromator at SRC. Initial experiments are taking place using an ionization chamber built by Dan Wallace of the Optics group, and control and instrumentation software and hardware assembled by Dave Eisert, Kevin Kleman, and John McMurry of SRC. The HERMON design produces calculated resolving powers of 10,000 to 5,000 over the energy range from 500 to 1150 eV. Early tests show resolving powers in excess of 5,000 with the test grating.

An improved grating is now being manufactured by Hyperfine, Inc., of Boulder, Colorado. Several upgrades are planned before the monochromator will be fully operational, including: replacing the test grating with the final ruled grating, modifying the focus drive mechanism, and implementing a beam position monitoring station.