A COMPUTER-CONTROLLED LOW-ENERGY ELECTRON DIFFRACTOMETER FOR SURFACE CRYSTALLOGRAPHY

A new diffractometer has been developed for LEED crystallography. It uses three grid electron optics in front of a large phosphor coated hemispherical glass screen which forms part of the vacuum wall. The LEED pattern on this screen is viewed and intensity data acquired by a computer controlled TV camera. The same computer controls the whole experiment. A predictor-corrector algorithm is used to track the diffracted beams as the primary energy is changed. System evaluation has used a Cu(111) surface and R factor analysis has shown the systematic experimental errors to lead to uncertainty in the fractional surface layer spacing of the order of plus or minus 1%. The instrument is thus about twice as accurate and a hundred times faster than conventional LEED systems.