Low-energy positron dynamics in small hydrocarbon gases

Abstract

In the present paper we report a set of parameter-free calculations for the scatteringof low-energy positrons off gaseous hydrocarbon molecules like C2H2, C2H4, C2H6 andC6H6.The calculations of the corresponding elastic integral cross sections have been carriedout within the framework of the symmetry-adapted, single-centre-expansionapproach to describe all the bound molecular electrons and the scattered positron,at energies below the thresholds for positronium formation. The correspondingmultichannel coupled equations have been solved using a Volterra-type set ofintegral equations.The computed cross sections are found to be fairly close to the existingexperimental data for all the molecules examined in our work, both in shape andvalue.We have also computed the positron annihilation coefficients,Zeff (T),at the experimentally observed temperature of 300 K and the effect of thebreathing vibration in computing the rates of annihilation for differentmolecular geometries is examined for the case of gaseous C2H2.The present results, among the first non-empirical calculations for polyatomic gases,confirm our earlier findings (Gianturco et al 2001 Phys. Rev. A 64 32715; Gianturcoand Mukherjee 1997 Phys. Rev. A 56 3638; 2001 Phys. Rev. A 64 024703) that theZeffvalues in such gases are crucially dependent on the inclusion of dynamicalcoupling between the positron motion and the nuclear vibrations during theannihilation event.