Physical processes in positronium formation at metal surfaces

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Abstract
The rate of positronium formation (pickup) near a metal surface undergoing bombardment with medium energy positrons is calculated within an approximate model. The formation process proceeds in three stages: slowdown (during which the positron is thermalized due to inelastic positron-conduction-electron and positron-phonon scattering), diffusion (which brings the positron from the characteristic stopping distance to the surface), and electron capture (which takes place at the surface). The process is formulated in terms of a phenomenological Boltzmann-equation approach. We then calculate the range and the microscopic features of the positron diffusion. It is shown that the thermal positron may be regarded as undergoing an isotropic random walk. The role of the surface potential in impeding the positronium formation is discussed and a picture evolves of the positron making several attempts at tunneling through the attractive surface barrier before escaping with a captured electron. Our final results for the pickup fraction are in reasonable qualitative agreement with recent experimental results of Mills.

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