Positron annihilation from F centers of alkali halide crystals

Two-photon annihilation of a positron from an F center in an alkali halide crystal has been studied theoretically by accurately solving with the integral transform (generator coordinate method) the two-particle (electron-positron) problem for two different model crystal potentials (an effective Coulomb potential and the Krumhansl-Schwarz cavity potential) which are often used in the theoretical description of F centers. Calculations are presented of the stability and relevant expectation values of the positron-F-center system, the two-photon annihilation lifetime τ and the angular correlation distribution N(theta) for sixteen different alkali halides. Comparison is made with available experimental data and previous, less accurate calculations. It is demonstrated that a proper description of electron-positron correlation in the wave function is necessary in order to obtain meaningful results. We have also established a range of effective charges, Z∗ , of the defect that can capture an electron-positron pair in a stable state. For the Krumhansl-Schwarz potential, the present highly accurate calculations show that all sixteen crystals have stable electron-positron states in the defect