Positron cooling via inelastic collisions in CF$_4$ and N$_2$ gases

TL;DR

This study simulates positron cooling in CF$_4$ and N$_2$ gases, highlighting the roles of vibrational excitations and positron interactions in achieving thermalization and matching experimental results.

Contribution

It introduces a detailed simulation including positron-positron interactions and vibrational modes, providing new insights into positron cooling mechanisms in these gases.

Findings

Inclusion of the $ u_1$ mode improves thermalization accuracy.

Positron-positron interactions aid in Maxwellianization.

Simulation results agree well with experimental data.

Abstract

Positron cooling via inelastic collisions in CF$_4$ and N$_2$ gases is simulated, including positron-positron interactions. Owing to the molecular symmetries, cooling is assumed to be chiefly due to energy loss via vibrational (rotational) excitations for CF$_4$ (N$_2$). For CF$_4$, it is found that the inclusion of the dipole-inactive $\nu_1$ mode, in addition to the dipole-active modes $\nu_3$ and $\nu_4$, can provide room-temperature thermalization and an accurate cooling timescale. Combination cooling enabled by the $\nu_1$ mode, and positron-positron interactions both contribute to the Maxwellianization of the positron momentum distribution. For both gases the evolution of the positron temperature is found to be in excellent agreement with experiment.