Observation of Ps(<i>n</i>=2) from well-characterized metal surfaces in ultrahigh vacuum

Abstract

The production of the n=2 state of positronium by slow-positron bombardment is observed from well-characterized metal surfaces of Cu and W under UHV conditions. The maximum fractions observed are (2.73\ifmmode\pm\else\textpm\fi{}0.13)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ for Cu and (3.70\ifmmode\pm\else\textpm\fi{}0.53)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ for W. Measurements made as a function of incident-positron energy show a substantial difference in the incident-energy dependence for the Ps(n=1) and Ps(n=2) fractions, with W exhibiting a threshold for Ps(n=2) formation at an energy near 5 eV. These observations are consistent with Ps(n=2) formation as a result of charge capture at the surface by backscattered nonthermalized positrons. An understanding of the Ps(n=2) emission mechanism is important for improving the yields of excited-state production for fine-structure measurements.