Roton-Rotation Coupling of Acetylene in $^4$He

TL;DR

This study investigates how acetylene's rotational spectra are affected by superfluid helium-4, revealing strong coupling with helium excitations and predicting anomalous spectral features using advanced computational methods.

Contribution

It introduces combined CBF/DMC and PICF computational approaches to analyze rotational spectra of acetylene in superfluid helium-4, highlighting the coupling with helium excitations.

Findings

Large distortion constants predicted

Reduced rotational constants observed

Higher rotational excitations show additional peaks

Abstract

Rotational absorption spectra of acetylene in superfluid $^4$He are calculated using a combined correlated basis function-diffusion Monte Carlo method (CBF/DMC) as well as a numerically exact path integral correlation function approach (PICF). Both approaches predict anomalously large distortion constants in addition to reduced rotational constants, and the higher rotational excitations are seen to be accompanied by an additional peak and absorption band. The CBF analysis shows that these phenomena are due to strong coupling of the higher rotational states with the roton and maxon excitations of $^4$He, and the assocated divergence of the helium density of states in this region.