A low-energy electron diffraction data acquisition system for very low electron doses based upon a slow scan charge coupled device camera

We have developed a video low-energy electron diffraction (LEED) system on the basis of a slow scan charge coupled device (CCD) camera which is capable of collecting LEED IV data at very low electron doses quickly and therefore enables us to study extremely beam sensitive surface structures which have not been accessible to LEED IV analysis before. The slow scan CCD camera allows separating the relatively short data acquisition process from the more lengthy digitizing, storage, and data analysis processes. Typical total effective exposure times can therefore be reduced to about 200–300 s (1 s per energy point) at a primary beam current of 100 nA which corresponds to a total dose of about 12 e per adsorbate particle; further decrease is possible. The total measurement time for collecting a complete set of LEED images is of the order of 30–40 min which assures the exclusion of contamination effects, even for sensitive layers. The IV curves are then extracted from the digitally stored images off-line which allows collecting the intensities of all visible spots simultaneously with a high reliability in tracing beams, even for very dense LEED patterns.