4.3.2 Surface Charging

Surface charging was an issue for insulating surfaces when using high beam intensities (\( \approx \)10\( ^{9} \) particles s\( ^{-1} \)cm\( ^{-2} \)). Due to the positive primary ions used an insulating surface would charge up positively. Surface charging could easily be identified by comparing the spatial distribution of the reflected neutral and the reflected negatively charged particles. A positively charged surface would bend the maxima in the angular distribution for the negatively charged particles towards the surface. This was an important problem mainly for MgO(100). By placing a filament emitting electrons close to the surface, local surface charges could be removed. However this procedure made it impossible to accurately determine the secondary electron yield for this surfaces. The additional free electrons could not interfere with the scattering measurements because electrons with energies lower than 150eV were hindered from reaching the MCPs by the first grid of the retarding potential analyzer RPA set to a potential of -150V.

March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland