Performance of a very high resolution soft x-ray beamline BL25SU with a twin-helical undulator at SPring-8

We report on the excellent performance of a newly constructed soft x-ray he lical undulator beamline BL25SU of SPring-8 for photon energies 500-1800 eV . The full beamline was designed to perform very high resolution soft x-ray spectroscopy of solids with using high brilliance, highly circularly polar ized undulator radiation. The grazing incidence monochromator employs varie d-line-spacing plane gratings which operate in convergent light from a sphe rical mirror and focuses monochromatic light onto the exit slit. A resolvin g power in excess of 15 000 was measured at 540 and 870 eV for a grating wi th a central groove density of 600 lines/mm from the photoemission spectra of Au. A resolving power of more than 20 000 is estimated near 870 eV for a grating with a central groove density of 1000 lines/mm. A photon flux of m ore than 1x10(11) photons/s/100 mA/0.02% b.w. is supplied onto the sample b etween 500 and 1800 eV with very low amount of higher-order light. The low heat load from the twin-helical undulator gives high stability to all optic al components, which is essential to obtain high resolution in a wide energ y region. Three experimental stations are installed in tandem on this beaml ine for various high resolution spectroscopy measurements. (C) 2000 America n Institute of Physics. [S0034- 6748(00)02409-6].