High Resolution Electron Energy Loss Spectroscopy - High Resolution Electron Energy Loss Spectrometer

High Resolution Electron Energy Loss Spectrometer

As the electrons used for HREELS are low energetic they do not only have a very short free mean path length in the sample materials but also under normal atmospheric conditions. Therefore one has to set up the spectrometer in UHV. The spectrometer is in general a computer simulated design that optimizes the resolution while keeping an acceptable electron flux.

The electrons are generated in an electron source, by heating a tungsten cathode, which is encapsulated by a negatively charged so called repeller that prevents stray electrons from coming into the detector unit. The electrons can leave the source only through a lens system, like e.g. a slot lens system consisting of several slits all on different potential. The purpose of this system is to focus the electrons on the entrance of the monochromator unit, to get a high initial electron flux.

The monochromator is usually a concentric hemispherical analyser (CHA). In more sensitive setups an additional pre-monochromator is used. The task of the monochromator is to reduce the energy of the passing electrons to some eV due to the help of electron lenses. It further lets only those electrons pass which have the chosen initial energy. To achieve a good resolution it is already important to have incident electrons of a well defined energy one normally chooses a resolution of for the monochromator. This means, the electrons leaving the monochromator with e.g. 10 eV have an energy accurate to 10−1 eV. The beam’s flux is then in the orders of 10−8 A to 10−10 A. The radii of the CHA are in the order of several 10 mm. And the deflector electrodes have a saw tooth profile to backscatter electrons which are reflected from the walls in order to reduce the background of electrons with the wrong E_i. The electrons are then focused by a lens system onto the sample. These lenses are, in contrary to those of the emitter system very flexible, as it is important is to get a good focus on the sample. To enable measurements of angular distributions all those elements are mounted on a rotate able table with the axis cantered at the sample.Its negative charge causes the electron beam to broaden. What can be prevented by charging the top and bottom plates of the CHA deflectors negative. What again causes a change in the deflection angle and has to be considered when designing the experiment.

In the scattering process at the sample the electrons can loose energies from several 10−2 eV up to a few electron volt. The scattered electron beam which is of around 10−3 lower flux than the incident beam then enters, the analyzer, another CHA.

The analyzer CHA again allows only electrons of certain energies to pass to the analyzing unit, a channel electron multiplier (CEM). For this analyzing CHA the same facts are valid as for the monochromator. Except that a higher resolution as in the monochromator is wanted. Hence the radial dimensions of this CHA are mostly bigger by like a factor 2. Due to aberrations of the lens systems the beam has also broadened. To sustain a high enough electron flux to the analyzer the apertures are also about a factor 2 bigger. To make the analysis more accurate, especially to reduce the background of in the deflector scattered electrons often two analyzers are used, or additional apertures are added behind the analyzers as scattered electrons of the wrong energy normally leave the CHAs under large angles. In this way energy losses of 10−2 eV to 10 eV can be detected with accuracies of about 10−2 eV.