Maxwellianization of positrons cooling in CF4 and N2 gases

Positron cooling in CF₄ and N₂ gases via inelastic vibrational and rotational (de)excitations is simulated, importantly including elastic positron-positron collisions. For CF₄, it is shown that rotational (de)excitations play no role on the experimental timescale, and further, that in the absence of positron-positron collisions, cooling via excitation of the dipole-active ν3 and ν4 modes alone would lead to a non-Maxwellian positron momentum distribution, in contrast to the observations of experiment. It is shown that the observed Maxwellianization of the distribution may be effected by positron-positron collisions and/or cooling involving the combination of the dipole-inactive ν1 mode with the dipole-active modes. For N₂, rotational excitations alone are sufficient to Maxwellianize the distribution (vibrational effects are negligible).