Impact of vibrational Feshbach resonances on positron thermalization: a Monte Carlo simulation study

Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mover> <mml:mi>Z</mml:mi> <mml:mo stretchy="false">¯</mml:mo> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . Cross sections for elastic, vibrational, rotational, and annihilation processes are included in the simulation, calculated using the zero-range potential 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mover> <mml:mi>Z</mml:mi> <mml:mo stretchy="false">¯</mml:mo> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> 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.