Soft x-ray beam line for surface EXAFS studies in the energy range 60≤hν≤11 100 eV at the Daresbury SRS

Abstract

The characteristics of the beam line used for surface EXAFS (extended x-ray absorption fine structure) studies at the Daresbury Synchrotron Radiation Source (SRS) are described. Monochromatic photons are available in the energy range 60≤hν≤11 100 eV. This has been achieved using a monochromator with a combination of crystal and grating optics. The radiation is focused through the monochromator by a toroidal premirror at 0.5° grazing angle. Three pairs of crystals and a plane grating with focusing mirror are available which can be interchanged under ultrahigh vacuum (UHV). The plane grating/mirror monochromator has two ranges of zero-order angles, 6.4° and 2°, giving photon energy ranges of 60–550 eV and 600–2000 eV, respectively, with a 1200 l mm−1 grating. The crystals in use are InSb(111), Ge(111), and Ge(220) having photon energy ranges of 1745–7360 eV, 2000–8430 eV, and 3260–11 100 eV. The performance of the gratings and crystals in respect to intensity, resolution, scattered light, and higher orders is discussed, and some representative results from the beam line are given. Detailed consideration has been given to thermal effects on the crystal and grating optics. The grating can be exposed to a maximum power density of 9 W cm−2 and has so far withstood the intense radiation. The Ge crystals, which can see power densities of up to 150 W cm−2, have survived, but exit beam movement and energy scale shifts are observed. Efforts to reduce these beam movement effects have been successful inasmuch as the instrument is usable. InSb crystals suffered some radiation damage, and eventually surface melted. Beryl and quartz crystals have been tried but did not survive. The crystal part of the monochromator is in routine use, but the grating part has yet to be proved successful for surface EXAFS experiments.