Photoemission spectroscopy

X-Ray Photoemission Spectroscopy (XPS, formerly known as ESCA - Electron Spectroscopy for chemical analysis) was developed at Uppsala University, Sweden in the 60's by a group headed by Kai Siegbahn, who in 1981 won the Nobel Prize for Physics for his work in developing the tecnique.

Physical principle

The physics behind the XPS technique is really an application of Einstein's result on the photoelectric effect. The material to be analyzed is exposed to a monochromatic beam of X-rays, inducing photoelectric ionization of sample atoms. In XPS, these photoelectrons are collected and their kinetic energy is determined resulting in a recorded spectrum of electron intensity as a function of the measured energy. Using Einsteins relation $E_{k}=h\nu +E_{B}$ it is now possible to calculate the binding energy for a certain intensity peak since the incoming X-rays have a known frequency. This binding energy is material dependent, and moreover, it is also affected by the precise chemical structure and bonding of the material. By comparing the spectrum with tables of known elements and bindings, the chemical contents on the surface is determined.

Due to the short range in the material (due to absorption for photoelectrons excited in this process, XPS tends to be highly surface sensitive. The information depth is typically of the order of 30 Å. The lateral resolution is on the other hand negatively affected by the difficulty in focusing X-ray beams effectively. The XPS technique is therefore not commonly applied for surface imaging, as opposed to e.g. SEM or Auger spectroscopy.

Instrument details

Commercial XPS instruments include an x-ray source, e.g. Aluminium $K_{\alpha }$ radiation is common. The beam is monochromatized using Bragg reflection on a crystal and is then directed towards the sample. This whole part of the instrument is kept in an UHV (Ultra High Vacuum) environment to avoid sample and instrument contamination due to adsorption of molecules. Most instruments are also equipt with a sputtering gun to remove unwanted molecules from the surface prior to measurement, and for extracting depth profiles.

The photoelectrons are collected using a magnetic lens system and an electron spectrometer is used to determine their kinetic energy. A commercial