Intermolecular forces and correlations mediated by a phonon bath

TL;DR

This paper introduces the biangulon quasiparticle, describing two molecules in a bosonic bath, revealing how bath-mediated interactions lead to molecular alignment and unique spectral features, with potential experimental signatures.

Contribution

It proposes the biangulon quasiparticle as a novel concept, extending angulon theory to describe correlated two-molecule systems in a bath.

Findings

Biangulon exhibits shifted angulon instabilities.

Strong molecular alignment occurs in the ground state.

Spectral signatures indicate resonant angular momentum transfer.

Abstract

Inspired by the possibility to experimentally manipulate and enhance chemical reactivity in helium nanodroplets, we investigate the effective interaction and the resulting correlations between two diatomic molecules immersed in a bath of bosons. By analogy with the bipolaron, we introduce the \emph{biangulon} quasiparticle describing two rotating molecules that align with respect to each other due to the effective attractive interaction mediated by the excitations of the bath. We study this system in different parameter regimes and apply several theoretical approaches to describe its properties. Using a Born-Oppenheimer approximation, we investigate the dependence of the effective intermolecular interaction on the rotational state of the two molecules. In the strong-coupling regime, a product-state ansatz shows that the molecules tend to have a strong alignment in the ground state. To investigate the system in the weak-coupling regime, we apply a one-phonon excitation variational ansatz, which allows us to access the energy spectrum. In comparison to the angulon quasiparticle, the biangulon shows shifted angulon instabilities and an additional spectral instability, where resonant angular momentum transfer between the molecules and the bath takes place. These features are proposed as an experimentally observable signature for the formation of the biangulon quasiparticle. Finally, by using products of single angulon and bare impurity wave functions as basis states, we introduce a diagonalization scheme that allows us to describe the transition from two separated angulons to a biangulon as a function of the distance between the two molecules.