Electron spectroscopy

Electron spectroscopy is an analytical technique to study the electronic structure and its dynamics in atoms and molecules. In general an excitation source such as x-rays, electrons or synchrotron radiation will eject an electron from an inner-shell orbital of an atom. Detecting photoelectrons that are ejected by x-rays is called 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).

Experimental applications include high-resolution measurements on the intensity and angular distributions of emitted electrons as well as on the total and partial ion yields. Ejected electrons can escape only from a depth of approximately 3 nanometers 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.

Synchrotron radiation research work has been carried out at the MAX Laboratory in Lund, Sweden, Elettra Storage Ring in Trieste, Italy, and at ALS in Berkeley, CA.

In typical laboratory use, a sample is placed in the specimen chamber of a scanning electron microscope (SEM). Because the SEM uses a beam of electrons to illuminate the sample and produce the three dimensional images often seen in lay news reports and publications as well as professional journals, the sample must be able to conduct electricity. If the sample is metallic, electron conduction is already present because of the natural electrical conductivity of metal. It is non-metallic, the sample is usually coated with a layer of gold and platinum approximately twenty nanometers deep. This is sufficient for the sample to be conductive without being so thick as to "hide" elements present within the sample. When the electrons emitted by the SEM collide with electrons in the atoms of the sample, the physics of typical electron-electron collisions apply, such that sample electrons having less energy in their orbital shells than the energy setting of the SEM (kev setting), will be displaced from their orbits by the higher energy SEM electrons. This leaves the atom in an unstable state, which is remediated by higher energy electrons in the sample "dropping" down to occupy the electron shells previously ejected by the higher energy SEM electrons. In order to accomplish this, the higher energy electrons within the atom must relinquish the excess energy that exists between the ejected electrons and the higher energy electrons. One way of doing this is by the creation of photons of xray energy. This energy can be detected by the electron spectrometer with a detector placed within the specimen chamber of the SEM, and the electrons can be identified as to the elements from which they were emitted. This gives a qualitative picture of which elements are present within the sample, because the energy dispersive spectrometer can scan the entire group of elements present up to the kev setting of the SEM, which is user controlled by simply switching a knob or pressing a button to set the upper limit of the kev power of the SEM's electron beam. It should be noted that by coating non-conductive samples with gold and platinum as described above, significant peaks will appear on the graph at the kev points of gold and platinum. These peaks can usually be easily removed from the graph by the software of the spectroscopy system should they overshadow areas where naturally occurring elements are present. If these elements include gold or platinum, the problem can be sidestepped in the beginning by using carbon as the coating agent. Carbon is electrically conductive but sufficiently "light" such that the typical energy dispersive spectrometer will not be able to detect it within its usual range of detection, which often goes down to sodium but has difficulty detecting lighter elements without the use of a separate wavelength detector.

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