New setup for in situ x-ray photoelectron spectroscopy from ultrahigh vacuum to 1 mbar

In an effort to extend the pressure range for electron-based spectroscopies from ultrahigh vacuum into the so-called pressure gap region, we have built a new apparatus for in situ x-ray photoelectron spectroscopy up to 1 mbar. The principle of the experimental setup is based on a modified hemispherical electron energy analyzer, a modified twin anode x-ray source, and several differential-pumping stages between sample region and electron detection. The reaction gas is provided in situ either by background dosing or, as a new feature, by beam dosing, using a directed gas beam from a small tube. The latter allows for higher local pressures. The performance of the new setup is discussed, deriving normalization procedures from the analysis of the attenuation of the substrate photoemission intensity by the increasing gas phase pressure. In addition, the change of the work function due to changes in surface composition can be evaluated in situ by analyzing the binding energy shift of the gas phase core-level peaks. As a first study, measurements for the pressure dependence of CO adsorption on Pd(111) between 5x10-8 and 1 mbar are presented