Metastable helium molecules as tracers in superfluid liquid $^{4}$He

TL;DR

This paper introduces a novel laser-induced fluorescence technique to visualize and measure the motion of the normal fluid component in superfluid helium using metastable helium molecules as tracers.

Contribution

It demonstrates for the first time the ability to trace the normal-fluid component in superfluid helium with angstrom-sized particles, enabling detailed fluid dynamics studies.

Findings

Successfully imaged helium molecules in superfluid helium.

Measured normal-fluid velocity in heat-induced counterflow.

Generated and tracked normal-fluid jets using laser techniques.

Abstract

Metastable helium molecules generated in a discharge near a sharp tungsten tip operated in either pulsed mode or continuous field-emission mode in superfluid liquid $^{4}$He are imaged using a laser-induced-fluorescence technique. By pulsing the tip, a small cloud of He$_{2}^{*}$ molecules is produced. At 2.0 K, the molecules in the liquid follow the motion of the normal fluid. We can determine the normal-fluid velocity in a heat-induced counterflow by tracing the position of a single molecule cloud. As we run the tip in continuous field-emission mode, a normal-fluid jet from the tip is generated and molecules are entrained in the jet. A focused 910 nm pump laser pulse is used to drive a small group of molecules to the vibrational $a(1)$ state. Subsequent imaging of the tagged $a(1)$ molecules with an expanded 925 nm probe laser pulse allows us to measure the velocity of the normal fluid. The techniques we developed demonstrate for the first time the ability to trace the normal-fluid component in superfluid helium using angstrom-sized particles.