A schematic drawing of a typical electron energy loss experiment is given in Fig. 1.1. To form a monoenergetic beam of electrons, an electrostatic deflecting system in combination with entrance and exit slits is used to select electrons of well-defined energy from those emitted by an appropriate source. The electrons which pass through the monochromator strike the sample, and a second electrostatic system is used as an analyzer of the energy spectrum of the scattered electrons.
The monochromator and analyzer in some systems are fixed in orientation, and the sample may be rotated, as indicated in the figure. By this means, the distribution of scattered electrons in angle, as well as energy may be sampled. The experiment is very simple in concept, but the construction of the spectrometer involves a number of subtle considerations, if these devices are to achieve energy resolution sufficient to probe vibrational losses, and pass sufficient current at the same time.
Fig. 1.1 Schematic diagram of an electron energy loss experiment. Electrons from a cathode pass through a monochromator, strike the sample, and the energy spectrum of the scattered electrons is probed by a second monochromator.
Analyzers: Either a CMA or a CHA can be used. HREELS almost exclusively employs a CHA.
In an electron energy loss study of surface vibrations, the sample is placed in ultrahigh vacuum. A highly monoenergetic beam of electrons is directed toward the surface, and the energy spectrum and angular distribution of electrons backscattered from the surface are measured. In a typical experiment, the kinetic energy of the incident electrons is in the range of a few electron volts. Under these conditions, the electrons penetrate only the outermost three or four atomic layers of the crystal, so as remarked earlier, the backscattered electrons thus contain information on only the very near vicinity of the surface. As the reader will appreciate shortly, the experimental problem is to achieve high resolution through production of a very mono- energetic and well-collimated electron beam, along with sensitive detectors. This must be done without degrading the signal below the detectable level. At present, the best resolution that may be obtained is 30 cm ' (3.7 meV), although to detect weak signals, it is sometimes necessary to operate with lower resolution.