Positron binding to metal surfaces

Experiment and theory are shown to contradict each other about the image-induced binding energy B of positrons (e⁺) at metal surfaces, with (i) experimental values ( approximately 0.1 au) inferred from thermally activated positronium (Ps) emission, assuming that the barrier between the e⁺ and Ps channels is no higher than dictated by energy conservation; and (ii) calculated values (<or approximately=0.05 au) from the hydrodynamic model of the metallic plasma (with cut-off), embodying long-range correlations and collective effects. Higher theoretical values ( approximately 0.08 au) previously claimed for the same model can result only from a cut-off procedure which, through widespread, is physically unreasonable for a half-space (as opposed to an unbounded medium), truncating sums over normal modes but not affecting the amplitudes and frequencies. The author shows that this is not a true cut-off in Fourier space; it fails to limit wavenumbers, or to describe the physically expected delocalising effects of a cut-off. The quantum theory of the model is developed compatibility with a true wavenumber cut-off. By using the e⁺ workfunction phi ₊ as input, he by-passes the difficulty of calculating ab initio the important repulsive effects from the metal interior and from the surface dipole layer.