Effect of molecular constitution and conformation on positron binding and annihilation in alkanes

TL;DR

This study uses a model-potential approach to analyze how molecular structure and conformation influence positron binding and annihilation in alkanes, revealing temperature-dependent effects and potential for probing molecular isomerization.

Contribution

The paper introduces a detailed calculation of positron binding energies in alkanes considering molecular conformation, highlighting the impact of structure on annihilation rates and resonance features.

Findings

More compact alkanes have higher positron binding energies.

Thermally averaged binding energies increase with temperature for larger alkanes.

Conformational changes affect vibrational Feshbach resonances in positron annihilation.

Abstract

The model-potential approach previously developed by the authors to study positron interactions with molecules is used to calculate the positron binding energy for $n$-alkanes (C$_n$H$_{2n+2}$) and the corresponding cycloalkanes (C$_n$H$_{2n}$). For $n$-alkanes, the dependence of the binding energy on the conformation of the molecule is investigated, with more compact structures showing greater binding energies. As a result, thermally averaged binding energies for larger alkanes ($n\gtrsim 9$) show a strong temperature dependence in the range of 100-600 K. This suggests that positron resonant annihilation can be used as a probe of rotational (trans-gauche) isomerization of $n$-alkanes. In particular, the presence of different conformers leads to shifts and broadening of vibrational Feshbach resonances in the annihilation rate, as observed with a trap-based low-energy positron beam.