Positronium formation in collisions of positron with water and methane molecules

Abstract Positronium formation through electron capture in positron-molecule collisions is studied theoretically in the framework of the boundary corrected distorted-wave approximation at intermediate and high impact energies. In this approximation, distortion related to the Coulomb continuum states of the positron and the active electron in the field of residual target are included in the final channel. Here, many electron target is reduced to one active electron target and the other electrons remain frozen in their initial orbital during collision. The description of molecular wavefunctions are expressed in the two different ways: the first one is a linear combination of atomic orbitals (LCAO) based on the complete neglect of differential overlap (CNDO) method for the effective electron occupation of each atomic orbital and the second one is one center molecular wavefunctions called Moccia wave function. Our aim is to study the effect of target description on positronium formation at intermediate and high energy positron scattering with molecules. Total cross sections (TCS) as well as angular-differential cross sections (DCS) are calculated. Contributions to the total cross sections from the strongly bound and weakly bound orbitals of the molecule have also been explicitly analyzed. In the angular DCS, we find that the probabilities of positronium formation is minimum near about 25 ∘ . The results are compared with the other theories and available experimental measurements. In general, overall agreement is found particularly at high energies where this approach is expected to be more accurate.