Binary-encounter dipole model for positron-impact direct ionization

In the present work we study the applicability of the binary-encounter-dipole (BED) model and its simpler version the binary-encounter-Bethe model for positron-impact direct ionization. We show that ignoring the exchange and interference effects in the theory of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] yields simple analytical formulas with no fitting parameters that can be used to estimate cross sections over a wide energy range except near-threshold region. To correct this deficiency we combine BED theory with the Wannier-type threshold law derived by Klar [J. Phys. B 14, 4165 (1981)] for positron-impact ionization. We show that such combination is necessary in order to predict cross sections at low positron energies where strong polarization-correlation effects are present during collision and the positronium formation is a dominant scattering process. The present theory is tested for a wide range of targets including helium (He), atomic hydrogen (H), neon (Ne), argon (Ar), molecular hydrogen (H-2), nitrogen (N-2), oxygen (O-2), carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4). An extensive comparison with available experiments and theories is done.