The sample is illuminated by X-rays of sufficient energy to excite the core atomic levels of atoms within the sample. The intensity of the photoelectrons (see XPS) for a certain peak is determined as a function of azimuthal or polar angle. The intensity of the photoelectron peak can also be studied as a function of electron kinetic energy by changing the incident photon energy in synchrotron radiation experiments. The photoelectrons can be diffracted by the neighbouring atoms. Hence this can provide information on the surface crystallographic structure and is therefore complementary to LEED and STM. The diffraction pattern determined will be specific to a single element corresponding to the element which emitted the photoelectron. Therefore the nature of the surroundings of each element in that sample can be investigated in turn. XPD is also the basis of the technique known as photoelectron holography.