Excited rotational states of molecules in a superfluid

TL;DR

This study combines experimental and theoretical methods to investigate excited rotational states of molecules in helium nanodroplets, revealing how superfluid surroundings influence molecular rotation through angular momentum transfer.

Contribution

It introduces a simple quantum model for molecular rotation in superfluid helium and validates it with experimental spectral data, extending understanding of molecule-superfluid interactions.

Findings

Good agreement between calculated and measured spectral lines

Rotational states extend beyond initial population at low temperature

Superfluid effects can be described by angular momentum transfer

Abstract

We combine experimental and theoretical approaches to explore excited rotational states of molecules embedded in helium nanodroplets using CS$_2$ and I$_2$ as examples. Laser-induced nonadiabatic molecular alignment is employed to measure spectral lines for rotational states extending beyond those initially populated at the 0.37 K droplet temperature. We construct a simple quantum mechanical model, based on a linear rotor coupled to a single-mode bosonic bath, to determine the rotational energy structure in its entirety. The calculated and measured spectral lines are in good agreement. We show that the effect of the surrounding superfluid on molecular rotation can be rationalized by a single quantity -- the angular momentum, transferred from the molecule to the droplet.