Study of positron surface states on the alkali-metal-covered transition-metal surface

We present a first-principles study of annihilation probabilities of surface trapped positrons with core electrons at the Cu(100) surface with sub-monolayers of Cs adsorbed. Image-potential-induced positron surface states are calculated using a corrugated-mirror model in a full three-dimensional geometry. These states are studied for various arrangements of Cs atoms below and above the critical alkali-metal coverage of approximately 0.7 physical monolayer. Computer simulations of the deposition of Cs atoms are used to show that hexagonal close-packed Cs islands are formed at about 0.7 physical monolayer coverage of Cs. It is found that the abrupt decrease in the positron annihilation rate with Cu 3p electrons observed experimentally results from a metallization of the Cs islands. This causes a shift in the localization of the positron bound state from the CsCu interface to the vacuum side of the Cs overlayer, with the corresponding abrupt decrease in the positron annihilation probability with Cu electrons. Annihilation probabilities for positrons with Cs electrons are also computed and compared with experiment.