KMC-1: A high resolution and high flux soft x-ray beamline at BESSY

The crystal monochromator beamline KMC-1 at a BESSY II bending magnet covers the energy range from soft $(1.7keV)$ to hard x-rays $(12keV)$ employing the $(n,−n)$ double crystal arrangement with constant beam offset. The monochromator is equipped with three sets of crystals, InSb, Si (111), and Si (422) which are exchangeable in situ within a few minutes. Beamline and monochromator have been optimized for high flux and high resolution. This could be achieved by (1) a windowless setup under ultrahigh-vacuum conditions up to the experiment, (2) by the use of only three optical elements to minimize reflection losses, (3) by collecting an unusually large horizontal radiation fan $(6mrad)$ with the toroidal premirror, and (4) the optimization of the crystal optics to the soft x-ray range necessitating quasibackscattering crystal geometry $(θBragg,max=82°)$ delivering crystal limited resolution. The multipurpose beamline is in use for a variety of user facilities such as extended x-ray absorption fine structure, ((Bio-)EXAFS) near-edge x-ray absorption fine structure (NEXAFS), absorption and fluorescence spectroscopy. Due to the windowless UHV setup the $k$ edges of the technologically and biologically important elements such as Si, P, and S are accessible. In addition to these experiments this beamline is now extensively used for photoelectron spectroscopy at high kinetic energies. Photon flux in the $1011–1012photons∕s$ range and beamline resolving powers of more than $E∕ΔE≈100.000$ have been measured at selected energies employing Si $(nnn)$ high order radiation in quasibackscattering geometry, thus photoelectron spectroscopy with a total instrumental resolution of about $150meV$ is possible. This article describes the design features of the beamline and reports some experimental results in the above mentioned fields.