Theory of Angle‐Resolved Photoemission. The Role of Electron Diffraction in the XPS Spectra

The external photoelectron current can be expressed by the one‐electron Green's function for the crystal with surface. By calculating this GF with the matching formalism a general expression for the bulk contributions to the photoelectron spectra is obtained. As a consequence of the electron diffraction the transmission matrix of the surface enters this expression. The result is analyzed for the conditions of the angle‐resolved XPS measurements. Thereby it is shown what types of approximations or errors are contained in the two models which are used till now frequently for the calculation of angle‐resolved XPS spectra. Further a practicable approximate formula is given for these spectra which takes into account the electron diffraction in a simple manner.