Impact of Vibrational Feshbach Resonances on Positron thermalization: A Monte Carlo Simulation Study

In this article, we investigate the impact of Vibrational Feshbach Resonances (VFRs) on positron thermalization and annihilation in molecular gases using a Monte Carlo collision simulation approach. The Kr 2 dimer, a benchmark system with a well-characterized VFR, is employed to analyze how resonances affect the positron speed distribution (PSD) and the thermally averaged annihilation parameter Zeff . Cross sections for elastic, vibrational, rotational, and annihilation processes are included in the simulation, calculated using the Zero-Range Potential (ZRP) approximation. By comparing the resonant model with a non-resonant counterpart, we demonstrate that VFRs can strongly distort the PSD, leading to significant deviations from Maxwell-Boltzmann statistics and substantial overestimation of Zeff if equilibrium is assumed. Despite having little effect on the thermalization rate, resonances greatly influence the positron lifetime and annihilation probability. These findings highlight the necessity of accounting for non-equilibrium effects in positron interaction models, particularly for molecular systems exhibiting sharp vibrational resonances.