Auger-Elektronen-Spektroskopie (AES)

With Auger spectroscopy it is possible to investigate surfaces with regard to their chemical composition. The sample is bombarded with an electron beam (ca.1-3 keV), and electrons of full inner shells are excited to higher, free energy levels of the atom surface by impacts. The atom can now, for example, return to the ground state by transmitting this energy to one of its enveloping electrons, which is then able to leave the atom (Auger electron). The electrons thus emitted have characteristic energies for each element, since different elements have different energies. If an energy spectra of the Auger electrons is observed (Auger spectrum), the surface composition can easily be determined by means of the Auger peaks and comparative graphs. An Auger spectrometer with cylindrical mirror analyzer (CMA) with secondary electron multipliers is used in our experiment setup. The following figure shows the structure of a two-stage CMA Auger spectrometer:

A voltage is applied between the inner and outer cylinders, so that only electrons of a certain energy range can pass through the two cylinders (the paths of these electrons are shown by dashed lines); All others collide against the cylinder walls and can not penetrate to the secondary electron multiplier. If you vary the voltage, you can go through a whole energy spectrum. Since the probability of an Auger electron leaving the solid directly is very small, the peaks in the energy spectrum are also very small. In the solid state secondary processes, which account for the greatest part of the energy spectrum, are much more important. In order to be able to detect the Auger peaks as well as possible, the lock-in measurement technique is used here as well. In addition, an alternating voltage is applied to the cylinders, which simultaneously serves as a reference for a lock-in amplifier. You then only measure the proportion of the same frequency and get a signal derived from the energy.