Evidence for Landau's critical velocity in superfluid helium nanodroplets from wave packet dynamics of attached potassium dimers

TL;DR

This study uses femtosecond spectroscopy and quantum modeling to demonstrate the Landau critical velocity in superfluid helium nanodroplets by observing vibrational damping of attached potassium dimers.

Contribution

It provides the first direct experimental evidence for Landau's critical velocity in superfluid helium nanodroplets through wave packet dynamics analysis.

Findings

Damping of vibrational dynamics due to helium environment

Agreement with models neglecting damping below critical velocity

First direct evidence of Landau critical velocity in nanodroplets

Abstract

Femtosecond pump-probe spectroscopy has been used to study vibrational dynamics of potassium dimers attached to superfluid helium nanodroplets. Comparing the measured data with theoretical results based on dissipative quantum dynamics we propose that the most important effect of the helium environment is a general damping of the vibrational dynamics as a result of the interaction between dimer and collective degrees of freedom of the helium droplet. The calculations allow us to explain crucial experimental findings that are unobserved in gas-phase measurements. Remarkably, best agreement with experiment is found for a model where we neglect damping once a wave packet moves below a critical velocity. In this way the results provide first direct evidence for the Landau critical velocity in superfluid nanodroplets.