Electron Spectroscopy

Electron Spectroscopy Introduction Electron spectroscopies analyze the electrons that are ejected from a material for qualitative or semi-quantitative analysis. In general an excitation source such as x-rays or electrons will eject an electron from an inner-shell orbital of an atom. Detecting photoelectrons that are ejected by x-rays is call x-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA). Detecting electrons that are ejected from higher orbitals to conserve energy during electron transitions is called Auger electron spectroscopy (AES). These electron processes are described below. Ejected electrons can escape only from a depth of approximately 3 nm or less, making electron spectroscopy most useful to study surfaces of solid materials. Depth profiling is accomplished by combining an electron spectroscopy with a sputtering source that removes surface layers.

Auger Electron Spectroscopy (AES) Introduction Auger (pronounced ~o-jay) electron spectroscopy is an electron spectroscopic method that uses a beam of electrons to knock electrons out of inner-shell orbitals. Auger electrons are ejected to conserve energy when electrons in higher shells fill the vacancy in the inner shell. These Auger electrons have energies characteristic of the emitting atom due to the characteristic energy-level structure of that element.

Instrumentation

Picture of an Auger electron spectrometer

X-ray Photoelectron Spectroscopy (XPS, ESCA) Introduction X-ray photoelectron spectroscopy (XPS, also called electron spectroscopy for chemical analysis, ESCA) is a electron spectroscopic method that uses x-rays to knock electrons out of inner-shell orbitals. The kinetic energy (Ek) of these photoelectrons is determined by the energy of the x-ray radiation h(nu) and the electron binding energy (Eb) as given by: EK = h(nu) - Eb The electron binding energies are dependent on the chemical environment of the atom. XPS is therefore useful to identify the oxidation state and ligands of an atom.

Instrumentation The detection of photoelectrons requires that the sample be placed in a high vacuum chamber. Since the photoelectron energy depends on x-ray energy, the excitation source must be monochromatic. The energy of the photoelectrons is analyzed by an electrostatic analyzer and the photoelectrons are detected by an electron multiplier tube or a multichannel detector such as a microchannel plate.