Surface analysis by x-ray photoelectron spectroscopy (XPS), more commonly known as electron spectroscopy for chemical analysis (ESCA), involves irradiation of the solid in vacuo with monoenergetic soft x-rays and sorting the emitted electrons by energy. The spectrum obtained is a plot of the number of emitted electrons per energy interval versus their kinetic energy. Each element has a unique elemental spectrum, and the spectral peaks from a mixture are approximately the sum of the elemental peaks from the individual constituents. Since the mean free path of the electrons is very small, the electrons which are detected originate from only the top few atomic layers. Quantitative data can be obtained from the peak heights or areas and identification of chemical states often can be made from the exact positions and separations of the peaks, as well as from certain spectral contours. (Source: Perkin-Elmer Corporation Physical Electronics Division, Handbook of X-Ray Photoelectron Spectroscopy, 1979.)

Ultra High Vacuum (UHV)

UHV is a prerequisite for a complete control of the environment in the study of a material. At this extreme low pressure level of 10^-10 torr, comparable to the Earth's ionosphere, a gas molecule travels 300 miles on average before colliding with another gas molecule. (Usually the gas molecule hits the wall of a UHV chamber long before that). With UHV, researchers can be sure that the material under study is not affected by contamination or oxidation. Achieving UHV requires researchers to carefully consider the material used and its cleanliness. A schematic diagram of the UHV chamber is shown below.