Main Features of Photoelectron Spectra

In a photoelectron (PE) spectrometer, an intense beam of monochromatic light ionises molecules or atoms in an Ultra-High Vacuum (UHV) chamber which is a necessity for the study of the solid surface. The accompanying figure exhibits the ingredients of a modern PE experiment. The light source is either a gas discharge lamp, an X-Ray tube, or a synchrotron radiation source. The light impinges on the sample, which is a gas or the surface of a solid, and the electrons excited by the photoelectric effect are then analysed for their kinetic energy E_kin> and their momentum p (wave vector p/h) in a electrostatic analyser. The polarisation of the light is a useful property in an angle-resolved PE experiment. The important parameters to be measured are then the kinetic energy E_kin of the photoemission electron, and its angle with respect to the impinging light (Y + Q) and the surface (Q) as shown in the figure. Knowing the energy of the light and the work function f, one can determine the binding energy E_B of the electrons in the sample from the following equation:

The momentum p of the outgoing electron is determined from the kinetic energy by

The direction of p/h is obtained from q and g which are the polar and azimuthal angles under which the electrons leave the surface.

The figure below shows schematically how the energy-level diagram and the energy distribution of photoejected electrons relate to each other. The solid sample has core level and valence band electrons. If photon adsorption takes place in a core level or valence band with binding energy E_B the photoelectrons can be detected with kinetic energy by equations used above. The ejected photoelectrons are separated according to their kinetic energies in an electron analyser, detected and recorded. The PE spectrum is a record of the number of electrons detected at each energy, and consists of a peak at each E_kin corresponding to the binding energy, E_B, of an electron in the atom, as illustrated schematically below. The PE spectrum is a simple reflection of the molecular orbital diagram. This is an attractive feature of PE spectroscopy; it is able to provide information on the electron energy distribution in a material.