Monochromatised Electron Sources

Monochromatised Electron Sources for two kind of applications have been developed in our institute:

- High-resolution electron energy loss spectroscopy (HREELS) [1]
- Inverse photoemission spectroscopy (IPES) [2]

Investigating the dispersion of phonons and collective charge modes with high surface sensitivity is best achieved with high-resolution electron energy loss spectroscopy (HREELS) [3]. However, a systematic measurement over the whole Brillouin zone can take weeks with standard instruments, which measure the electron intensity sequentially, i.e., at one specific loss energy and one scattering angle at a time. Therefore, we have modified a high-resolution electron source to meet the requirements of commercial hemispherical electron analysers with E(k) imaging capabilities [4]. This allows the parallel detection of electrons in a broad range of momenta. Here, a very high energy and momentum resolution are crucial. The source is composed of two successive monochromator units, small slits and an imaging lens system that fits the geometrical constraints of hemispherical analysers. Our electron source has been licensed to ScientaOmicron and is commercially available as MCES150 [5]. One application example is the exciton dispersion of a single crystal of an organic molecule [6].

For IPES application, electrons are directed on a surfaces and the emitted electrons are analysed. This provides information about the unoccupied states of the surface. The efficiency of this process is however orders of magnitude lower than for direct photoemission. Therefore, a compromise between monochromatic current and energy resolution has to be found. In that case a single monochromator is used with larger slits than for HREELS. We can then achieve several hundreds of nano-ampere with a resolution below 100 meV [2]. We currently produce this kind of source directly in our institute.