NESAC/BIO Tutorials - ESCA Technique

Description of the ESCA Technique

Electron spectroscopy for chemical analysis (ESCA, also known as X-ray photoelectron spectroscopy or XPS) exploits the photoelectric effect to obtain information about the chemical composition and structure of a surface. When a photon source (e.g., X-rays) is directed at a sample, the photons interact with the electrons present in the sample material. If the photon has sufficient energy, it causes an electron to be emitted from its orbital. The simple theoretical relationship that describes this process is

KE = hv - BE

where KE is the kinetic energy of the emitted photoelectron, hv is the energy of the photon, and BE is the binding energy for the emitted photoelectron. KE is measured in the ESCA experiment, hv is known, and BE can be calculated, yielding the energy with which the electron was held in its atomic or molecular environment.

For photoemission from solids, the work function term must be added to this equation. A unique work function is established for each ESCA instrument. This term expresses the additional energy required after the ionization process to get the emitted electron away from the surface and into the surrounding gas or vacuum space. Thus, the measured kinetic energy of the electron will be indicative of the element from which it came and the chemical environment of that element. A pdf file listing elemental BEs and photoionization cross-sections can be downloaded by clicking here.

Main Chamber of S-Probe

For ESCA analysis, a sample is placed in an ultrahigh vacuum environment, typically less than 10^-8 Torr. The sample is then exposed to a low-energy, monochromatic X-ray source which causes the emission of photoelectrons from atomic shells of the elements present on the surface. These electrons possess an energy characteristic of the element and molecular orbital from which they are emitted. The electrons are detected and counted according to the energy they possess. By counting the number of electrons detected at each energy value (KE < hv), a spectrum of peaks corresponding to the elements on the surface is generated. the area under these peaks is a measure of the relative amounts of each element present, while the shape and position of the peaks reflect the chemical environment of each element.

ESCA is a surface sensitive technique because only those electrons that leave the surface without energy loss will contribute to the peak signifying that element. Those electrons originating from far below the surface (>100 Å) suffer energy loss through collisions and are unable to make it out of the surface, or they escape the surface with considerable energy loss. The information-rich quality of the ESCA method, combined with its ability to generate data that correlate well with observed biological interactions, makes ESCA one of the most valuable surface analysis methods available for the study of biomaterials.