Transition from molecular complex to quantum solvation in OCS(He)_N

TL;DR

This study uses quantum Monte Carlo methods to analyze how the rotational properties of OCS molecules change as they are solvated by helium clusters, revealing a transition from molecular complex to quantum solvation.

Contribution

It introduces spectral evolution quantum Monte Carlo techniques to study excited states in helium clusters, providing new insights into the transition from molecular complex to quantum solvated molecule.

Findings

Rotational constants decrease then increase with cluster size, indicating a transition.

The transition point marks a change from floppy to more rigid molecular behavior.

Results agree with experimental data on large helium clusters.

Abstract

We present quantum calculations of the rotational energy levels and spectroscopic rotational constants of the linear OCS molecule in variable size clusters of 4He, using spectral evolution quantum Monte Carlo methods that allow excited states to be accessed without nodal constraints. The rotational constants of OCS are found to decrease monotonically from the gas phase value as the number of helium atoms increases to N=6, after which the average constant increases to saturation at a value in excellent agreement with experimental measurements made on significantly larger clusters (N>1000). The minimum is shown to indicate a transition from a molecular complex to a quantum solvated molecule, with the former characterized by floppy but near rigid behavior, while the latter is characterized by non-zero permutation exchanges and a smaller extent of rigid coupling.