An X-Ray Photoelectron Spectrometer Designed for Surface Research

An x-ray photoelectron spectroscopy (ESCA) apparatus designed specifically for surface research is described. The features, included in one vacuum envelope, are ultrahigh vacuum, a high intensity x-ray source, a 10 cm mean radius hemispherical electron energy analyzer, sample cleaning by argon ion bombardment, sample heating by electron bombardment, and the facility for the collection of electron spectra as a function of angle relative to the surface plane of the sample. It is shown that (1) sample cleaning increases net signal strengths by factors as great as 14; (2) less than 2% of a monolayer of cesium on a clean molybdenum substrate can be readily detected; and (3) that ultrahigh vacuum is essential for investigation of well-defined surfaces. Surface sensitivity is increased by an order of magnitude when spectra are collected at low electron take-off angles relative to the surface plane, and the importance of measuring angular dependence of spectra for interpretation of data from surfaces is demonstrated.