Causes and prevention of photochemical reduction of hexavalent chromium during x-ray photoelectron spectroscopy

We report on the photoreduction of hexavalent chromium during exposure to soft x-rays used in x-ray photoelectron spectroscopy (XPS). Photoreduction of hexavalent chromium to the tetravalent state in both CrO₃ and K₂CrO₄ was found to involve hydrocarbons adsorbed onto the sample surface during rough pumping with a rotary pump during sample entry. When rough pumping was performed by a Venturi pump and a sorption pump, no photoreduction was observed over a period of 6 h of exposure to soft x-rays. We also observed that Cr(IV) was stable under soft x-ray radiation. In the presence of water vapor in the vacuum chamber, Cr(IV) was chemically reduced to Cr(III). The findings of this study bring into question the applicability of current models explaining the photoreduction of high-valent chromium oxides during XPS, and are also of general interest.