The classical trajectory study of e⁺+A to A⁺+e^-+e⁺ reaction near the threshold

Numerical investigations of the positron-atom ionisational collisions near the threshold have been carried out within the classical trajectory method. It is found that for an infinitely heavy target the threshold law reads: I approximately E^2.49, whereas recent analytical calculations predict a value of 2.65 for the exponent. However, in the case of a hydrogen-atom target, the value 1.64 has been obtained, indicating that the resting target mass assumption, as employed in all previous analytical investigations of e^+or--A processes near the ionisation threshold breaks down in the case of the positron-light-atom collisions. The energy distribution of the outgoing particles appears non-zero and uniform in larger parts of available energy regions: e⁺(E/2, E), e^-(0, E/2), but turns out, surprisingly, to be zero around E/2. The calculated energy dependence of the mutual angle is consistent with the analytically predicted theta _+- approximately E^1/4 behaviour.