Soft x-ray fluorescence (UHV compatible) proportional counters for NEXAFS and SEXAFS above the C,N,O, and S K edges

The authors describe the first ultra-high-vacuum-compatible soft x-ray detector for fluorescence yield (FY) detection of the near edge (NEXAFS) and extended (EXAFS) x-ray absorption fine structures above the C,N,O and S K edges. This soft x-ray proportional counter is shown to offer more than a factor of twenty increase in surface sensitivity for chemisorbed submonolayer (ML) coverage with comparable signal-to-noise as compared with any conventional electron yield detector. The FY detector allows one to record SEXAFS spectra for ≅5/100 ML for both sulfur and carbon. The high sensitivity of the FY technique is due to the small amount of elastically and inelastically scattered background from the sample. In addition, the characteristic fluorescence radiation from the bulk or other contaminants can easily be eliminated using standard pulse height analysis techniques. Recently, experiments were performed at the NSLS VUV storage ring demonstrating that carbon edge FY NEXAFS measurements can be performed under 1 Torr of hydrogen gas. These prototype experiments coupled a soft x-ray proportional counter, reaction cell, and suitable soft x-ray entrance window to form a system allowing the experimenter to bridge the gap between UHV surface science and real reaction conditions encountered in catalysis