title: New mechanism for positronium formation on a silicon surface.
abstract: We describe experiments in which positronium (Ps) is emitted from the surface of p-doped Si(100), following positron implantation. The observed emission rate is proportional to a Boltzmann factor exp{-E(A)/kT}, which is dependent on the temperature T of the sample and a characteristic energy E(A)=(0.253±0.004) eV. Surprisingly, however, the Ps emission energy has a constant value of ∼0.16 eV, much greater than kT. This observation suggests the spontaneous emission of energetic Ps from a short-lived metastable state that becomes thermally accessible to available surface electrons once the positron is present. A likely candidate for this entity is an electron-positron state analogous to the surface exciton observed on p-Si(100) c(4×2) by Weinelt et al. [Phys. Rev. Lett. 92, 126801 (2004)].
authors.affiliation: Department of Physics and Astronomy, University of California, Riverside, California 92521-0413, USA.
authors.forename: D B
authors.initials: DB
authors.lastname: Cassidy
authors.forename: T H
authors.initials: TH
authors.lastname: Hisakado
authors.forename: H W K
authors.initials: HW
authors.lastname: Tom
authors.forename: A P
authors.initials: AP
authors.lastname: Mills