Quantum Rotation of HCN and DCN in Helium-4

TL;DR

This paper models the rotational spectra of HCN and DCN molecules in superfluid helium-4, revealing strong coupling effects at higher rotational states and matching experimental effective moments of inertia.

Contribution

It combines Diffusion Monte Carlo and Correlated Basis Function theory to accurately predict rotational spectra and coupling phenomena in helium-4.

Findings

Agreement with experimental effective moments of inertia

Large shifts and anomalies in J=2 and J=3 spectral lines

Emergence of roton-maxon bands and secondary peaks

Abstract

We present calculations of rotational absorption spectra of the molecules HCN and DCN in superfluid helium-4, using a combination of the Diffusion Monte Carlo method for ground state properties and an analytic many-body method (Correlated Basis Function theory) for the excited states. Our results agree with the experimentally determined effective moment of inertia which has been obtained from the $J=0\to 1$ spectral transition. The correlated basis function analysis shows that, unlike heavy rotors such as OCS, the J=2 and higher rotational excitations of HCN and DCN have high enough energy to strongly couple to rotons, leading to large shifts of the lines and accordingly to anomalous large spectroscopic distortion constants, to the emergence of roton-maxon bands, and to secondary peaks in the absorption spectra for J=2 and J=3.