The collision of a low-energy positron, which impinges on a crystalline surface, with a valenceelectron may result in the emission of a spatially separated time-correlated electron–positronpair. We present a method for calculating the cross section for this positron surface reactionchannel, which we briefly refer to as (p, ep) in analogy to electron-induced pair emission(e, 2e). The two-particle final state is represented by a product of an electron and a positrondiffraction state coupled by a 'correlation factor', which accounts for the screened Coulombinteraction. The electron–solid and positron–solid quasi-particle potentials are based onfirst-principles calculations within density functional theory. Numerical (p, ep) results arepresented for Cu(111) and compared to their (e, 2e) counterparts. Energy distributions forconstant emission angles reflect, to a large extent, the valence electron density of states.In equal-energy (p, ep) angular distributions, the Coulomb interaction produces a centralaccumulation zone—in contrast to a depletion zone for (e, 2e)—the relative weight and theextension of which are subject to 'matrix element effects'. At larger angles sharp features arisefrom single-particle surface resonances.
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