The National Institute of Standards and Technology (NIST) operates the newly upgraded Synchrotron Ultraviolet Radiation Facility (SURF III) mainly as a light source for radiometry. SURF III provides continuum radiation from the far-infrared to the soft X-ray spectral range and has its peak output in the extreme ultraviolet. SURF III is a circular-orbit, weak-focusing (single dipole magnet) storage ring, a feature which is advantageous if the synchrotron radiation output is calculated. We report the improvements achieved during a recent upgrade from SURF II to SURF III and our strategy to accurately determine the magnetic¯ux density, radio frequency (RF), beam current, and beam size, which are the parameters necessary to characterize the source completely.

This paper reports novel measurements of x-ray optical radiation on an absolute scale from the intense and ultra-short radiation generated in the soft x-ray regime of a free electron laser. We give a brief description of the detection principle for radiation measurements which was specifically adapted for this photon energy range. We present data characterizing the soft x-ray instrument at the Linac Coherent Light Source (LCLS) with respect to the radiant power output and transmission by using an absolute detector temporarily placed at the downstream end of the instrument. This provides an estimation of the reflectivity of all x-ray optical elements in the beamline and provides the absolute photon number per bandwidth per pulse. This parameter is important for many experiments that need to understand the trade-offs between high energy resolution and high flux, such as experiments focused on studying materials via resonant processes. Furthermore, the results are compared with the LCLS diagnostic gas detectors to test the limits of linearity, and observations are reported on radiation contamination from spontaneous undulator radiation and higher harmonic content.

Total photoion yield spectra in the vicinity of the 3d thresholds of Xe, Cs and Ba are presented. These spectra were recorded with higher photon energy resolution than previously reported. For xenon the new experimental data reveal additional structures. Employing Hartree-Fock calculations, the origin of the prominent resonances is explained: for Xe and Cs the main absorption features are caused by shape resonances above threshold, whereas for Ba it is revealed that the 4f orbital has finally collapsed and the sharp resonances are caused by discrete excitations into bound states.

The 1s X-ray absorption spectra of atomic calcium, chromium, manganese and copper were determined experimentally and calculated theoretically. The spectra are dominated by a prominent line at threshold which precedes a smooth continuum extending to higher energies. Hartree-Fock calculations taking relativistic effects, relaxation and term dependence into account describe the discrete atomic resonance at threshold very well. The resonances at threshold have no counterpart in the spectra of the metals. The experimental atom to metal core level binding energy shifts agree well with the predictions of the thermodynamical model by Johansson and Martensson (1980).

The combination of a cryogenic radiometer and synchrotron radiation enables detector scale realization in spectral regions that are otherwise difficult to access. Cryogenic radiometry is the most accurate primary detector-based standard available to date, and synchrotron radiation gives a unique broadband and continuous spectrum that extends from x ray to far IR. We describe a new cryogenic radiometer-based UV radiometry facility at the Synchrotron Ultraviolet Radiation Facility II at the National Institute of Standards and Technology. The facility is designed to perform a variety of detector and optical materials characterizations. The facility combines a high-throughput, normal incidence monochromator with an absolute cryogenic radiometer optimized for UV measurements to provide absolute radiometric measurements in the spectral range from 125 nm to approximately 320 nm. We discuss results on photodetector characterizations, including absolute spectroradiometric calibration, spatial responsivity mapping, spectroreflectance, and internal quantum efficiency. In addition, such characterizations are used to study UV radiation damage in photodetectors that can shed light on the mechanism of the damage process. Examples are also given for UV optical materials characterization.