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) syste m 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 quic kly 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 dat a analysis processes. Typical total effective exposure times can there fore 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 meas urement time for collecting a complete set of LEED images is of the or der 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 intensiti es of all visible spots simultaneously with a high reliability in trac ing beams, even for very dense LEED patterns. (C) 1995 American Instit ute of Physics.