Electron Spectroscopy for Chemical Analysis (ESCA)
aka X-Ray Photoelectron Spectroscopy (XPS)
Electron Spectroscopy for Chemical Analysis (ESCA) is also known as X-ray Photoelectron Spectroscopy (XPS). The two names are interchangeable and do not denote any differences in technique. In this analysis method, the sample is flooded with monochromatic X-rays and electrons given off by the sample in response are collected. The x-ray penetration depth is quite significant, several µm's at least. However, the electrons leave with very low energy and so they can only escape from the top few nm's making this a true surface analytical method. When combined with sputtering, compositional changes can be monitored with depth resolution of 1 or 2 nm's.
Electrons leaving the sample do so with energies that are characteristic of the element from which they came. Thus, by determining the energy, the origin of the electron can be determined and the composition of the specimen is determined. Since most of the detected electrons come from the outermost valence orbit and since the energy of electrons in this orbit is a function of bonding, much chemical information about the atoms can also be determined. Different compounds of the same element result in quite different bonding energies and these differences can be detected by the ESCA spectrometer. It is for this reason that separate concentrations can be determined for elemental and oxidized metals or various families of organic bonds by this method.
Concentrations are determined by calculating the area under the peak The area is relatively insensitive to variations due to chemical state and so ESCA is generally quite accurate in determining compositions.