Microscopic two-fluid theory of rotational constants of the OCS-H$_2$ complex in $^4$He droplets

TL;DR

This paper develops a microscopic quantum model using local two-fluid theory and path integral Monte Carlo simulations to accurately predict the rotational constants of the OCS-H$_2$ complex in helium droplets, aligning well with experimental data.

Contribution

It introduces a novel microscopic approach combining two-fluid theory and path integrals to analyze rotational constants of OCS-H$_2$ in helium droplets, accounting for local helium density effects.

Findings

Calculated moments of inertia agree with experimental values.

H$_2$ induces a non-superfluid helium density affecting rotation.

Model provides insights for larger OCS-H$_2$ complexes.

Abstract

We present a microscopic quantum analysis for rotational constants of the OCS-H$_2$ complex in helium droplets using the local two-fluid theory in conjunction with path integral Monte Carlo simulations. Rotational constants are derived from effective moments of inertia calculated assuming that motion of the H$_2$ molecule and the local non-superfluid helium density is rigidly coupled to the molecular rotation of OCS and employing path integral methods to sample the corresponding H$_2$ and helium densities. The rigid coupling assumption for H$_2$-OCS is calibrated by comparison with exact calculations of the free OCS-H$_2$ complex. The presence of the H$_2$ molecule is found to induce a small local non-superfluid helium density in the second solvation shell which makes a non-negligible contribution to the moment of inertia of the complex in helium. The resulting moments of inertia for the OCS-H$_2$ complex embedded in a cluster of 63 helium atoms are found to be in good agreement with experimentally measured values in large helium droplets. Implications for analysis of rotational constants of larger complexes of OCS with multiple H$_2$ molecules in helium are discussed.