This method refers to the measurement of specific energy losses suffered by a Primary Electron (PE) traveling through a surface or inside a solid target. All interactions belong to elementary excitation in solids that theoretically describe quantified interactions of the PE with the solid bulk and surface. Each kind of interaction has its own energy, meaning that the PE will lose a characteristic amount of energy for each excitation, which is the basic feature of EELS.
The most common interactions are:
- ionization of a core atomic level (followed by X-ray or Auger emission)
- the excitation or ionization of outer energy levels leading to intra- and inter-band transitions
- the plasmon excitations of the electron gas of the valence / conduction band. Experimentally, plasmon excitation are normally dominant.
There are two different ways to measure energy losses, and their results are very different:
- In Transmission, where the PE crosses a thin sample as found in Transmission Electron Microscopy (TEM). This technique is referred to as transmission Electron Energy Loss Spectroscopy (commonly understood as EELS).
- In Reflection, where the PE impacts a thick sample and the energy loss spectra is measured from electrons back scattered from the surface. This technique is referred to as Reflection Electron Energy Loss Spectroscopy (REELS).
The following figure shows two possible configurations, for transmission and for reflection. The incidence angle is normal to the surface in transmission, but it can have any orientation in reflection. An interesting geometry is using a high energy PE beam at grazing angle to the surface (RHEED geometry) and a RHEA analyzer.
Although both techniques generate similar excitations, they differ by the fact that in transmission by choosing proper sample thicknesses the background of electrons scattered inside the target is kept very low as compared to energy loss intensities. In contrast, all electrons in reflection mode must be scattered by a large angle. Therefore a larger background of scattered electrons is added to the energy loss signals. Fortunately, this background is negligible for energies near the PE energy. Thus, Characteristic Energy Losses (CEL) like plasmon and band transition can be well measured in reflection mode whereas the larger loss energies corresponding to deeper core level excitation will be superposed to a large background of scattered electrons.
The dominant CEL structures measured in reflection are Plasmons, low energy ionization levels and band transitions. The corresponding loss energies do not depend on the atomic number Z but are governed by the outer shell structure of the solid, thus making CEL very sensitive to the chemical environment of surface layers.
For REELS, energy analyzers with very good energy resolution are required. The analyzers should reach this resolution especially at higher PE energies in the keV range, and, additionally work in constant energy resolution mode (providing constant resolution at all energies). Standard CMA are therefore not well suited for this kind of operation.
In contrast, all STAIB Instrument energy analyzers are powerful REELS analyzers.