Measurement of the Positron Surface-State Lifetime for Al

The decay rate has been determined for positrons annihilating with electrons while localized in the image-induced potential well at the (110) surface of a clean well-annealed single crystal of Al at 300 K. The lifetime associated with positrons in this surface state was found to be 580 \ifmmode\pm\else\textpm\fi{} 10 psec. This value, which has never before been directly measured, is in disagreement with present theories. Changes in the lifetime spectra were also found from ion sputtering and from exposure of the surface to various amounts of oxygen.