A calculation of the annihilation characteristics of positrons trapped at metal surfaces

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Abstract
A positron trapped at a simple metal surface is treated within the jellium model. The positron lifetime and angular correlation (ACAR) spectrum for the surface state are calculated and compared with recent measurements. Results for the lifetime are good but the anisotropy of the calculated ACAR spectra is larger than that observed. It is suggested that surface defect trapping may be responsible for the isotropy of the experimental data. Results obtained for the angular correlation in the Independent Particle Model (IPM) and in the Mixed Density Approximation (MDA) are in close agreement, supporting the use of MDA in future calculations. The anisotropy of the calculated MDA spectra is shown to be insensitive to the selvedge width of the electron density but is affected by an attempt to represent the screening of the positron through a shift in the image plane of the positron potential.