STUDY OF POSITRON SURFACE-STATES ON THE ALKALI-METAL-COVERED TRANSITION-METAL SURFACE

We present a first-principles study of annihilation probabilities of s urface trapped positrons with core electrons at the Cu(100) surface wi th sub-monolayers of Cs adsorbed. Image-potential-induced positron sur face states are calculated using a corrugated-mirror model in a full t hree-dimensional geometry. These states are studied for various arrang ements of Cs atoms below and above the critical alkali-metal coverage of approximately 0.7 physical monolayer. Computer simulations of the d eposition 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 wi th Cu 3p electrons observed experimentally results from a metallizatio n of the Cs islands. This causes a shift in the localization of the po sitron bound state from the Cs/Cu interface to the vacuum side of the Cs overlayer, with the corresponding abrupt decrease in the positron a nnihilation probability with Cu electrons. Annihilation probabilities for positrons with Cs electrons are also computed and compared with ex periment.