Positron states and annihilation characteristics at the (100), (110), and (111) surfaces of alkali metals

Abstract

In this paper we present results of theoretical studies of positron states and annihilation characteristics at the clean surfaces of alkali metals. Positron surface states and positron work functions have been computed for the ͑100͒, ͑110͒, and ͑111͒ surfaces of Li, Na, K, Rb, and Cs using the modified superimposed-atom method to account for discrete-lattice effects, and the results are compared with those obtained for the transition-metal surfaces. Stable positron surface states are found in all cases, with the Li states lying about 0.5 eV below the bulk positron band, and other alkali metals having positron surface states a few hundredths of an eV below the bulk bands. The results for the positronium activation energy and positronium work function for the clean surfaces of alkali metals are presented as well. Surface and bulk state lifetimes and probabilities for a positron trapped in a surface state to annihilate with relevant core-level electrons are also computed and compared with available experimental data.

Positron-induced emission of electron–positron pairs from solid surfaces

“…For the interaction of the positron with the semi-infinite crystal, we first constructed a shape-unrestricted ground state potential V + tot as the sum of a Coulomb potential, which is the sign-reversed Coulomb potential V − coul ( r ) from the electron ground state calculation and an attractive positron-electron correlation potential V + corr ( r ). The latter was constructed similarly as described in detail in previous work [18,19], but based on the charge density ρ( r ) from our self-consistent FLAPW film calculation. Inside the solid, we employed, in local density approximation V + corr (ρ( r )), correlation energies of a positron in an electron gas of density ρ from a many-body calculation [20] in a convenient parametrized form [21].…”