Berlin 2008 – scientific programme

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O: Fachverband Oberflächenphysik

O 6: Metal Substrates: Clean Surfaces

O 6.2: Talk

Monday, February 25, 2008, 11:30–11:45, MA 005

Diffraction and crystal symmetry in electron pair emission from surfaces — •Frank O. Schumann, Carsten Winkler, and Jürgen Kirschner — Max-Planck-Institut f\"{u}r Mikrostrukturphysik, Weinberg 2 , 06120 Halle, Germany

We have investigated the electron pair emission from a Cu(111) surface excited with a primary electron beam of 30.7 eV kinetic energy via a coincidence time-of-flight experiment. The movement of two interacting particles can be described within the so-called two-body problem, where it is shown that the motion of the center-of-mass can be separated from the relative motion. We adopt this notation in our presentation of the data. If we focus on the motion of the center-of-mass, we compute the sum of the in-plane momentum of the pair. Plotting the coincidence intensity as a function of the sum momentum gives a two-dimensional distribution, which displays intensity peaks, if a reciprocal lattice vector is added to the sum momentum of this electron pair and not to the individual electrons. This highlights the fact that the pair as the whole experiences diffraction at the surface. This in turn means that the electrons must be interacting. The interaction within the pair is described by the relative motion. The intensity distribution of this motion reveals the six-fold symmetry of the surface. Both features are only observed, if the valence band electron comes from the vicinity of the Fermi level, where the Cu(111) surface exhibits a Shockley surface state. For an explanation of our observations this electronic state is important.