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Measuring the normal-∞uid velocity in super∞uid liquid Helium-4 using metastable helium molecules

Wei Guo

Published Jan 1, 2009• 0 citations
OpenAlex

Abstract

We show in two demonstration experiments that metastable helium molecules can be visualized and used as tracers to quantitatively study the ∞ow of the normal component in super∞uid liquid 4 He using a laser-induced-∞uorescence technique. The methods we developed are useful in studying the role of the normal ∞uid component in quantum turbulence. A proposed experiment on imaging the normal-∞uid velocity pro- flle during the transition of quantum turbulence in a counter∞ow channel is discussed.

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We show in two demonstration experiments that metastable helium molecules can be visualized and used as tracers to quantitatively study the ∞ow of the normal component in super∞uid liquid 4 He using a laser-induced-∞uorescence technique. The methods we developed are useful in studying the role of the normal ∞uid component in quantum turbulence. A proposed experiment on imaging the normal-∞uid velocity pro- flle during the transition of quantum turbulence in a counter∞ow channel is discussed.
Metastable helium molecules generated in a discharge near a sharp tungsten tip immersed in superfluid 4He are imaged using a laser-induced-fluorescence technique. By pulsing the tip, a small cloud of He(2*) molecules is produced. 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 first excited vibrational level of the triplet ground state. Subsequent imaging of the tagged molecules with an expanded 925 nm probe laser pulse allows us to measure the flow velocity of the jet. The techniques we developed provide new tools in quantitatively studying the normal fluid flow in superfluid helium.
We describe a technique, using thin lines of triplet-state He2∗\mathrm{He}_{2}^{*}He2∗​ molecular tracers created by femtosecond-laser field ionization of helium atoms, for visualizing the flow of the normal fluid in superfluid 4He^{4}\mathrm{He}4He, together with its application to thermal counterflow in a channel. We show that, at relatively small velocities, where the superfluid is already turbulent, the flow of the normal fluid remains laminar, but with a distorted velocity profile, while at a higher velocity there is a transition to turbulence. The form of the structure function in this turbulent state differs significantly from that found in types of conventional turbulence. This visualization technique also promises to be applicable to other fluid dynamical problems involving cryogenic helium.
We present data that show a cycling transition can be used to detect and image metastable He2 triplet molecules in superfluid helium. We demonstrate that limitations on the cycling efficiency due to the vibrational structure of the molecule can be mitigated by the use of repumping lasers. Images of the molecules obtained using the method are also shown. This technique gives rise to a new kind of ionizing radiation detector. The use of He2 triplet molecules as tracer particles in the superfluid promises to be a powerful tool for visualization of both quantum and classical turbulence in liquid helium.
We describe an approach to detecting ionizing radiation that combines the special properties of superfluid helium with the sensitivity of quantum optics techniques. Ionization in liquid helium results in the copious production of metastable He2 molecules, which can be detected by laser-induced fluorescence. Each molecule can be probed many times using a cycling transition, resulting in the detection of individual molecules with high signal to noise. This technique could be used to detect neutrinos, weakly interacting massive particles, and ultracold neutrons, and to image superfluid flow in liquid 4He.
We study the time evolution of the cloud of Dy atoms dispersed in superfluid helium and investigate the possibility of using these atoms as tracers for the imaging of superfluid-helium flows. The atoms are produced by pulsed laser ablation and are visualized via their laser-induced fluorescence. It turns out that the observation time is limited by the optical pumping of the atomic population into a nonabsorbing metastable state which is also responsible for the saturation of the fluorescence yield observed earlier.
Metastable He2∗{\mathrm{He}}_{2}^{*}He2∗​ excimer molecules have been utilized as tracer particles of the normal component in superfluid 4He^{4}\mathrm{He}4He (He II) which can be imaged via laser-induced fluorescence. These excimer molecules form tiny bubbles in He II and can bind to quantized vortices at sufficiently low temperatures, thereby allowing for direct visualization of vortex dynamics in an inviscid superfluid. However, the ${a}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}\ensuremath{\rightarrow}{c}^{3}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ optical absorption line, which is responsible for the fluorescence imaging of the He2∗{\mathrm{He}}_{2}^{*}He2∗​ molecules, is controlled by fluctuations on the bubble shape, and its exact line profile is not known at low temperatures. In this paper, we present a bubble model for evaluating the surface fluctuation eigenmodes of the excimers in He II. The line profile of the ${a}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}\ensuremath{\rightarrow}{c}^{3}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ transition is calculated at different temperatures by considering both the zero-point and thermal fluctuations on the bubble shape. We show that as the temperature drops from 2 K to 20 mK, the peak absorption strength is enhanced by a factor of about five, accompanying a blueshift of the peak location by about 2 nm. A double-peak line profile due to the rotational levels of the molecular core can be resolved. This bubble model also allows us to evaluate the stiffness of the He2∗{\mathrm{He}}_{2}^{*}He2∗​ bubbles and hence their diffusion constant in He II due to scattering off thermal phonons. Our results will aid the design of future experiments on imaging quantized vortices in He II using He2∗{\mathrm{He}}_{2}^{*}He2∗​ tracers.