PHOTOEMISSION OF ADSORBED XENON, X-RAY PHOTOELECTRON-SPECTROSCOPY, AND TEMPERATURE-PROGRAMMED DESORPTION STUDIES OF H2O ON FES2(100)

The reaction of H2O with the (100) crystallographic plane of pyrite, F eS2, has been investigated in the vacuum environment with photoemissio n of adsorbed xenon (PAX), temperature-programmed desorption (TPD), an d X-ray photoelectron spectroscopy (XPS). TPD data indicate that H2O d esorbs from FeS2(100) in a broad range of temperatures (150-300 K). XP S data suggests that the vast majority of H2O that initially adsorbs o n pyrite at 79 K desorbs from pyrite during thermal annealing to 300 K . PAX, a technique that is sensitive to the short range order of a sur face, has been used to elucidate the types of sites that are available on FeS2(100) for the binding of adsorbate. Within the resolution of o ur PAX data, the surface of FeS2(100) consists of at least two types o f sites. It is proposed that these two types of sites are associated w ith the stoichiometric surface and defect (i.e., sulfur-deficient or a nion vacancy) sites. PAX further suggests that at low adsorbate covera ge, H2O predominately resides on defect sites. As the coverage of H2O is increased, defect sites become saturated and additional adsorption occurs on the less reactive stoichiometric surface.