Electron energy loss spectrometry in solids

When an electron from an external medium- or low-energy incident beam comes into contact with condensed matter, it can interact with it in one of two ways, either by transferring energy to the ion lattice or to the electrons. The energy loss spectra of electrons backscattered (or possibly transmitted) by a solid target reveal characteristic structures linked to the excitation of collective or individual phenomena.

The energy of the incident electron beam (from a few eV to a few hundred eV) is chosen so as to maximize the effective cross-section of the interaction with the phenomenon you wish to highlight. It can be established that the incident electrons must have at least three to four times the energy of the characteristic losses to have a reasonable probability of exciting them. These energies correspond to relatively short electron mean free paths (of the order of a few nanometers at most), which means that electron energy loss spectrometry should be considered as a specific technique for materials surface analysis.

The methods described here are non-destructive: they enable the identification of compounds or chemical elements present on the surface of a sample, mainly by means of their vibrational signature, but also by their other characteristic electronic excitations: plasmons and interband transitions. This information is essentially qualitative, but calibration using other techniques has established that, in the case of carbon monoxide adsorbed on a metal, the detection limit is less than 0.001 monolayer (1 monolayer corresponds to a density of around 10 ¹⁴ atoms per cm ² of surface).

Unlike other electron spectrometers, high-resolution electron energy loss spectrometry is highly sensitive to light elements, particularly hydrogen. Analysis of insulating samples is possible with HREELS spectrometry, using a neutralizing electron gun. This spectrometry enables quantitative determination of the dielectric and elastic surface constants of samples, based on the observation of optical and acoustic surface phonons respectively.