Positron annihilation in simple metal vacancies

The interaction between positrons and vacancies in simple metals Al, Li, Na, K are investigated, as well as characteristic changes in the lifetimes and angular distributions of annihilation quanta following the capture of positrons by vacancies. In the framework of a homogeneous model of a metal, self-consistent states of positrons in vacancies and capture energies are found. When calculating the capture energy, one used the assumption on a new localized positron state in simple metals-pseudopositronium (PPs). The calculations have shown that it is energetically beneficial for a positron to be captured by a vacancy with the capture energy of the order of one electron-volt in the case of monovalent alkali metals and of the order of three electron-volts in the case of tervalent aluminium. In this case the characteristic size of the positron distribution in a vacancy decreases, which results in the smearing of the angular distribution of annihilation quanta. The positron lifetime in the vacancy increases. The results agree qualitatively with the experiment