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O: Fachverband Oberflächenphysik

O 60: Poster Session III (Methods: Atomic and electronic structure; Methods: electronic structure theory; Methods: Molecular simulations and statistical mechanics; Methods: Sanning probe techniques; Methods: other (experimental); Methods: other (theory) )

O 60.29: Poster

Mittwoch, 24. März 2010, 17:45–20:30, Poster B2

Analysis of optical systems, caustic formation and contrast depth in mirror electron microscopy — •Sergej Nepijko and Gerd Schönhense — Institute of Physics, University of Mainz, 55099 Mainz, Germany

The conditions of realization of positive and negative contrasts in electron mirror microscope are shown. The contrast depth is analyzed as well, that is the sensitivity of electron mirror microscope to disorders of homogeneity on the object (local magnetic and electric fields, surface relief). Because of these, electron trajectories are distorted and electrons acquire additional velocity components in radial and azimuthal directions. This leads to the shift of the observed point on the screen and, as a consequence, to an image contrast. Since the electron energy, when reflected, is close to zero, electrons are influenced by heterogeneities for a long time. It causes high sensitivity to heterogeneities, up to the crossing of electron trajectories (caustics are generated). The conditions of caustic generation due to local electric or magnetic fields are analyzed: (i) the larger the distance r from the centre of the image, the easier the caustic is generated, (ii) after generation, the caustic expands with growth of distance r from the centre of the image, (iii) the value of azimuthal linear shift S=γ r (γ - azimuthal angular shift) reverses sign when passing through the centre (r=0). A dark wedge will be the continuation of the bright wedge opposite the centre. The caustic disappears in the centre of the screen. The contrast increases with growth of r. This differs from the contrast due to electric fields, which does not depend on r.