Mode coupling and multiquantum vibrational excitations in Feshbach-resonant positron annihilation in molecules

TL;DR

This paper investigates how anharmonic vibrational mode coupling affects positron annihilation rates in molecules, revealing both enhancement and suppression effects through theoretical calculations and experimental comparisons.

Contribution

It introduces a detailed theoretical analysis of anharmonic vibrational effects on positron annihilation, including multiquantum excitations and mode coupling impacts.

Findings

Mode coupling can enhance positron annihilation rates.

Anharmonic effects enable multiquantum vibrational excitations.

Experimental data supports the theoretical predictions.

Abstract

The dominant mechanism of low-energy positron annihilation in polyatomic molecules is through positron capture in vibrational Feshbach resonances (VFR). In this paper we investigate theoretically the effect of anharmonic terms in the vibrational Hamiltonian on the positron annihilation rates. Such interactions enable positron capture in VFRs associated with multiquantum vibrational excitations, leading to enhanced annihilation. Mode coupling can also lead to faster depopulation of VFRs, thereby reducing their contribution to the annihlation rates. To analyze this complex picture, we use coupled-cluster methods to calculate the anharmonic vibrational spectra and dipole transition amplitudes for chloroform, chloroform-$d_1$, 1,1-dichloroethylene, and methanol, and use these data to compute positron resonant annihilation rates for these molecules. Theoretical predictions are compared with the annihilation rates measured as a function of incident positron energy. The results demonstrate the importance of mode coupling in both enhancement and suppression of the VFR. There is also experimental evidence for the direct excitation of multimode VFR. Their contribution is analyzed using a statistical approach, with an outlook towards more accurate treatment of this phenomenon.