High resolution electron energy loss spectroscopy (HREELS)

Principle of the HREELS measurement

The Experiment

A schematic drawing of an HREELS experiment is given in the figure above (click on the figure for a larger view). The electron source is a filament tip, which emits electrons with a thermal energy distribution of some hundred meV. With the monochromator it is possible to lower this distribution to about 3 meV (in modern spectrometers to about 0.5 meV).

The electrons are then scattered at the sample surface. Some of them are sacattered inelastically, which results in a change in their kinetic energy (and their momentum as well). Possible interactions are mainly the excitations of vibrations of adsorbate molecules, vibrations of the upper layers of the substrate (surface phonons), and vibrations of the electrons in the substrate or in films or islands on the substrate surface (plasmons).

With a tilting analyser it is possible to scan the energy of the reflected electrons angle resolved. Excitations with a momentum parallel to the surface will lead to a change of the angle of the reflected electrons in respect to the elastically scattered electrons. The inset shows a schematic spectrum with elastically scattered electrons with an energy loss of 0 meV and inelastically scattered electrons at two loss energies.

The Spectrometer - moved to CAM

The here used spectrometer is a Leybold-Heraeus ELS22 with two double-127deflectors. The drawing shows the setup and a diagramm of the kinetic energy of the electrons in the different parts of the spectrometer. The energy in the scattering chamber (Eprim) can be varied from a few eV for to several hundred eV. Higher energies are required for the measurement of exititations with a momentum parallel to the surface.

The experiment is mounted in an ultra-high vacuum chamber with a base pressure of about 5*10-11 mbar. The chamber is equipped for sample preparation (sputtergun) and provides a LEED system to analyse the surface quality. An evaporator (Omicron EFM3) allows to evaporate metal films of a thickness from 0.1 monolayer to several hundred nanometers.