The scattering of slow positrons by CH4and NH3

The fixed-nuclei approximation is used to investigate the collisions between slow positrons and methane molecules. A static potential and various models for the polarisation potential are used to calculate total elastic cross sections. There is fair agreement between recent experimental measurements and the results from these models. The authors also evaluate Z eff , the effective number of annihilation electrons of the molecule, taking into account the effect of distortion in the molecular orbitals due to the incoming positrons. They obtain, e.g. for CH 4 , 99.5 compared with an experimental value of 140-155. They argue that there is no need to postulate the formation of positron-CH 4 complexes to explain the high value of Z eff . They also calculate Z eff for NH 3 where the experimental value is 1300. A calculation ignoring charge-cloud distortion gives 241.