Low-energy positron dynamics in small hydrocarbon gases

In the present paper we report a set of parameter-free calculations for the scattering of low-energy positrons off gaseous hydrocarbon molecules like C₂H₂, C₂H₄, C₂H₆ and C₆H₆. The calculations of the corresponding elastic integral cross sections have been carried out within the framework of the symmetry-adapted, single-centre-expansion approach to describe all the bound molecular electrons and the scattered positron, at energies below the thresholds for positronium formation. The corresponding multichannel coupled equations have been solved using a Volterra-type set of integral equations.

The computed cross sections are found to be fairly close to the existing experimental data for all the molecules examined in our work, both in shape and value.

We have also computed the positron annihilation coefficients, Z_eff (T), at the experimentally observed temperature of 300 K and the effect of the breathing vibration in computing the rates of annihilation for different molecular 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; Gianturco and Mukherjee 1997 Phys. Rev. A 56 3638; 2001 Phys. Rev. A 64 024703) that the Z_eff values in such gases are crucially dependent on the inclusion of dynamical coupling between the positron motion and the nuclear vibrations during the annihilation event.