Temperature dependence of positron surface trapping: Evidence for Rydberg precursor states

Theory of positrons in solids and on solid surfaces

“…The positrons are implanted in the solid and part of them diffuse back to the surface, where they can be trapped or emitted as free positrons or as positronium atoms. From the measured positronium yields Kong et al (1990) concluded that the positron trapping rate is strongly enhanced for Ag(111) towards low temperatures, whereas for the Ag(100) it increases only moderately as temperature decreases. Kong et al (1990) explained the result for Ag(111) using a model in which a positron is first trapped by a mirrorpotential-induced Rydberg state in a phonon-excitation process.…”

“…From the measured positronium yields Kong et al (1990) concluded that the positron trapping rate is strongly enhanced for Ag(111) towards low temperatures, whereas for the Ag(100) it increases only moderately as temperature decreases. Kong et al (1990) explained the result for Ag(111) using a model in which a positron is first trapped by a mirrorpotential-induced Rydberg state in a phonon-excitation process. The positron then makes the transition to the ground state via an electron-hole creation process.…”

“…The main interest of Neilson et al (1986) was, however, the calculation of the energy losses of positrons reemitted from solids to vacuum. Kong et al (1990) studied the temperature dependence of positron trapping on Ag(111) and Ag(100) surfaces both experimentally and theoretically. The positrons are implanted in the solid and part of them diffuse back to the surface, where they can be trapped or emitted as free positrons or as positronium atoms.…”

“…This two-step model is analogous to that used for explaining positron trapping into negatively charged vacancies in semiconductors. According to Kong et al (1990), the two-step model is possible for Ag(111), because its work function for positrons is negative. The work function for Ag(100) is positive, and Kong et al (1990) explains that trapping via Rydberg states is not possible because the Rydberg states are higher in energy than the initial positron state in the bulk Ag.…”

“…According to Kong et al (1990), the two-step model is possible for Ag(111), because its work function for positrons is negative. The work function for Ag(100) is positive, and Kong et al (1990) explains that trapping via Rydberg states is not possible because the Rydberg states are higher in energy than the initial positron state in the bulk Ag. In this case only the direct trapping via an electron-hole process to the ground state is possible, and Kong et al (1990) showed that this process results in a weak temperature dependence.…”