What is EELS?
Electron energy loss spectroscopy(EELS) is the study of the vibrational motion of atoms and molecules on and near the surface by the analysis of the energy spectrum of low-energy electrons backscattered from it. An electron passing through material can interact with electron clouds of the atoms present and transfer some of its kinetic energy to them. There are three kinds of EELS, they are:
1. High Resolution Electron Energy Loss Spectroscopy(HREELS)
2. Electron Energy Loss Spectroscopy(EELS)
3. Core Electron Energy Loss Spectroscopy(CEELS)
EELS - uses electrons from 0.1 to 10 keV and passes them through a thin foil of the material of interest. At high energies, the transmitted beam contains inelastically scattered electrons whose energy has been decreased by amounts corresponding to characteristic absorption frequencies in the solid. At lower energies, the reflected beam is monitored for the same transitions. Bulk and surface plasmons are the principal features of these spectra.
CEELS - needs electrons of energy sufficient to ionize core electrons. Usually studied in the double differential mode (like AES). Signals are characteristic of atomic core energy levels.
EELS and CEELS are less common. So here we just give brief description of them, and we give a more comprehesive description of HREEELS.
HREELS - This is the most important electron loss spectroscopy. A new variant, TREELS (Time-Resolved Electron Energy Loss Spectroscopy - the "high resolution" is still inferred) allows one to monitor these signals in real time to study kinetic events.
The core of HREELS experimental apparatus:
The high energy resolution in the incident beam is achieved by monochromatizing a thermionic electron beam with a CHA. As such, HREELS employs to CHA's, each pointed at the surface. The first one forms the narrow energy electron beam that excites the surface adsorbates and the second studies the loss energies of the scattered electrons. The alignment of these devices is very demanding and achieving the ultra narrow linewidths needed to compete with RAIRS is very challenging.
Why do we use HREELS?
The incident electrons are of quite low energy (a few eV) and the losses are in the meV range. The primary focus of study for this technique is vibrational structure of the surface and especially of adsorbates on that surface. As such, it is a competitive technique with RAIRS. RAIRS has the advantage of significantly greater energy resolution, but HREELS can study vibrational features of energy right down to a few meV of O (depending upon the width of the incident beam).
What we can learn from it?
Electrons with energy in the range of a few electron volts sample only a few atomic layers. As they approach or exit from the crystal, they interact with the vibrational modes of the crystal surface, or possibly with other elementary excitations localized there. The energy spectrum of electrons back-reflected from the surface is thus a rich source of information on its dynamics; as we know from the well-developed fields of vibrational spectroscopy of molecules and solids, the dynamical properties of an entity, along with selection rules, offer insight into its basic structural features. Also, the vibrational modes of molecules adsorbed on the surface provide one with direct information on the nature of the chemical bonds between the molecule and the substrate.