XPS technique

XPS is a surface sensitive and non destructive technique used for analysis of the elemental composition. Only the outermost atomic layers (some nanometers) are probed, but with an ion gun etch it is possible to probe deeper laying layers.

In the XPS spectrometer system the probed samples are irradiated by photons with a specific energy, and the photoelectrons that leaves the sample are detected. The energy levels of the electrons are elemental specific, and by measuring the energy of the outgoing electrons, it is possible to detect which elements that are present in a sample.

You can read further about the technique here: [1]