The high energy spherical grating monochromator−A new source of soft x rays at Daresbury (abstract)

Abstract

In this article we report the characteristics of the new High Energy Spherical Grating Monochromator beam line on the SRS. The instrument, which has no entrance slit, was designed to provide high photon flux with small spot size, in the energy range covering the 1s binding energies of carbon, nitrogen, oxygen, and fluorine. Radiation from a bending magnet is horizontally focused onto the exit slit by a long, Pt-coated meridian cylinder (R=299 m, 2 mrad horiz. aperture, 2° glancing angle). The light is vertically diffracted and focused by one of three interchangeable spherical gratings (1050, 1500, and 1800 lines mm−1) operating in negative order. Finally, the light is refocused by an ellipsoidal mirror. The photon flux, determined with copper and carbon photocathodes, is presented for the three gratings. Useful flux is obtained in the range 250–1200 eV, with intensity maxima for each grating at 600, 700, and 800 eV of 11, 9, and 5×1010 photons s−1 per 100-mA stored beam into a band pass of 0.05%. The influence of contaminants which are present on the optical elements is discussed, together with details of beam line operating conditions which minimize the build up of such contaminants. Photoabsorption and photoemission measurements indicate a high (up to 30%) second order and some third order light content. Resolution determinations obtained from photoabsorption measurements are presented. Although features as narrow as 250 meV have been resolved, the resolving power of the instrument is found to depend strongly on stored beam current. We suggest this may be due to electron beam (i.e., source) blow-up. We critically discuss the suitability of the new facility for surface EXAFS of low Z adsorbates, in particular above the carbon, nitrogen, and oxygen 1s edges, using examples from recent studies which have been undertaken on the beamline.