THEORY OF COLLECTIVE EXCITATION-ASSISTED POSITRONIUM FORMATION ON METAL SURFACES BEYOND A FIXED CHARGE APPROXIMATION

摘要

In conclusion, the positronium formation probability per unit time (transition rate) has been calculated using a unitary transformation on the second quantized Hamiltonian for a positron with a specific energy scattered from a metal surface. The charge transfer between the metal surface and the positron, by which positronium formation occurs, was assumed to be assisted by surface plasmon excitation. The kinetic energy of the positron approaching the surface was taken into account in order to guarantee the conservation of energy during the scattering and positronium formation. The application of the theory on aluminum surface shows that the positronium formation probability decreases sharply with distance from the surface. It increases for lower incoming positron energy, and it has a maximum value for incoming positron with an angle that is related to the hydro dynamical model for surface plasmons dispersion. Comparison with the results for ion neutralization at aluminum surface via surface plasmon excitation shows that the same overall dependence on distance from surface is followed in both cases. However, ion neutralization is two orders of magnitudes higher than positronium formation. Also, the hump in transition rate at small distances from surface (～ 1) for ion neutralization does not appear for positronium formation.