Rotational cooling of molecular gases by positron impact at vanishing collision energies

The behaviour of the rotationally inelastic cross sections for ultracold positrons injected in molecular gases is examined computationally by considering the ambient molecules to be rotationally 'hot' (j = 10) and by letting the relative translational energies tend to zero. The size of the corresponding state-to-state transitions are obtained from an exact quantum calculation carried out in the space-frame representation, using non-empirical model interactions between e(+) and the target test molecules (H-2, N-2, O-2, Li-2). The structural features of the molecules are related, via the dynamical coupling effects, to the size and efficiency of the cooling process along the above series of target gases. It is shown that positron projectiles can provide a possible route to the cooling of molecular rotations for simple homonuclear diatomics within the framework of ultracold dynamical conditions.