Essay On Photoelectron Spectroscopy

Surface analysis by X-ray photoelectron spectroscopy (XPS), initially designated as electron spectroscopy for chemical analysis (ESCA), is accomplished by irradiation of a sample under vacuum with monoenergetic soft X-rays and energy-analysing the emitted electrons. The spectrum obtained represents the number of emitted electrons in a fixed small energy interval versus their kinetic energy. The X-rays that are usually used for XPS are derived either from an aluminium or magnesium anode which have photon energies of 1486.6 eV or 1253.6 eV, respectively. These photons interact with atoms on the surface region of the sample by the photoelectric effect. An electron excited by the incident radiation may escape from the sample, provided it has sufficient energy

Photoelectrons emitted within the sample may reach the surface of the sample without suffering any collisions or being elastically scattered. These photoelectrons form the well-defined core-peaks in the spectra and are most useful. Other photoelectrons suffer inelastic collissions and loss of energy may occur by the creation of electron-hole pairs or by the generation of collective electrons or plasmon oscillations. Inelastically scattered electrons form the raised background (on which the photoelectron peaks ride) at binding energies higher than the peaks .The background is continuous because the energy loss processes are random and multiple. An ionised atom can relax back to its equilibrium state by either X-ray fluorescence or Auger emission X-ray fluorescence results in the emission of a characteristic photon from the atom as an electron from a higher level fills the hole left by the photoelectron. This process is a minor one in this energy range (below 2keV), occurring less than one percent of the time. Auger emission occurs when an outer electron is emitted, carrying off the excess