Two-dimensional angular correlation of electron-positron annihilation on a simple metal surface

Isotropic theoretical angular correlation of annihilation radiation spectra for positrons trapped at an Al surface?

“…It is worthwhile to mention here that the experimental anisotropic ACAR from a Cu(121) surface, measured by Howell et a1 (1985), seems to confirm rather than contradict the present formalism. It should be remembered that copper is a transition metal and that the contribution of d electrons to the annihilation is significant, Recently it has been found, both theoretically and experimentally (Singh et a1 1986, Jarlborg and Singh 1987, Sormann 1987,1988, Daniuk 1989a, that in transition metals the effect of deenhancement occurs (this effect was first predicted by Fujiwara et al (1972)). The energydependent electron-positron local enhancement factors E appeared to be decreasing functions of energy for some energy bands (especially d bands).…”

“…one order of magnitude longer than the experimental one.) Therefore, although IPM could lead to less or more isotropic surface ACAR spectra, depending on the choice of electron and positron wavefunctions (Lou Yongming 1988, Brown et a1 1988a, nevertheless the positron lifetime obtained within IPM is always overestimated and therefore IPM results cannot be treated as reliable. In contrast, the lifetime resulting from the enhanced model balances between its (overestimated) IPM value and its (underestimated) one corresponding to the use of bulk local partial (equations ( 12) and (13a)) or total (formula (13b)) annihilation rates.…”

“…The effective calculations of ACAR from an A1 surface performed within the model of a positron trapped in the potential well are based either on &he independent-particles model (IPM) (Rozenfeld et a1 1983, Lou Yongming 1988 or on the mixed-density approximation (MDA) (Nieminen and Manninen 1974, Brown et a1 1987, 1988a. It should be mentioned here that the MDA (introduced by Arponen et a1 1973), which turned out to be very efficient for investigating positron annihilation at lattice defects, seems to be rather controversial in the case of positron interaction with a metal surface because of the infinite size of vacuum, treated as a large void.…”

Calculation of the angular correlation and the lifetime of positron surface states on metal surfaces

“…where n is the electron density and kf is the Fermi momentum. For the enhancement form r(n) we use [4] a revised Brandt-Reinheimer form where the density parameter r, is equal to (4nn/3)-lI3 and rsc is a cut-off parameter in the low-density region. When r,, is set equal to infinity, r ( n ) in equation ( 25) becomes the Brandt-Reinheimer form [27].…”

“…Using the positron wavefunction given by Barton [3], we calculate the two-dimensional angular correlation momentum distribution and the lifetime of positron-electron 0953-8984/89/182977 + 12 $02.50 @ 1989 IOP Publishing Ltd 2978 Yongming Lou et a1 annihilation from A1(110), A1(100), Al(111) and Cu(121) surfaces. We also adopt a revised Brandt-Reinheimer enhancement factor [4] in an effort to calculate the positron lifetime at these surfaces. Our results agree very well with experimental data.…”