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arXiv:2001.05625 · Physical review. B./Physical review. B

Shape fluctuations and optical transition of He2* excimer tracers in superfluid He4

Wei Guo, Andrei I. Golov

Published Feb 25, 2020• 4 citations
arXivHTMLPDF

Abstract

Metastable ${\mathrm{He}}_{2}^{*}$ excimer molecules have been utilized as tracer particles of the normal component in superfluid $^{4}\mathrm{He}$ (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 ${\mathrm{He}}_{2}^{*}$ 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 ${\mathrm{He}}_{2}^{*}$ 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 ${\mathrm{He}}_{2}^{*}$ tracers.

DOI
10.1103/physrevb.101.064515

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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 a3\ensuremathΣu+\ensuremath→c3\ensuremathΣg+{a}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}\ensuremath{\rightarrow}{c}^{3}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}a3\ensuremathΣu+​\ensuremath→c3\ensuremathΣg+​ 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 a3\ensuremathΣu+\ensuremath→c3\ensuremathΣg+{a}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}\ensuremath{\rightarrow}{c}^{3}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}a3\ensuremathΣu+​\ensuremath→c3\ensuremathΣg+​ 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.
We have studied the interaction of metastable 4He2∗^{4}\mathrm{He}_{2}^{*}4He2∗​ excimer molecules with quantized vortices in superfluid 4He^{4}\mathrm{He}4He in the zero temperature limit. The vortices were generated by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be $96\ifmmode\pm\else\textpm\fi{}6\text{ }\text{ }\mathrm{nm}$ at a pressure of 0.1 bar and $27\ifmmode\pm\else\textpm\fi{}5\text{ }\text{ }\mathrm{nm}$ at 5.0 bar. We have also demonstrated that a moving tangle of vortices can carry the molecules through the superfluid helium.
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.
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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.
Recent experiments on quantum turbulence in superfluid helium made use of small tracer particles to track the motion of quantized vortices, determine velocity statistics and visualize vortex reconnections. A problem with this visualization technique is that it may change the turbulent flow which it visualizes. To address this problem, we derive and solve the equations of motion of a quantized vortex ring which contains a given number of particles trapped on the vortex core, hence derive a quantitative criterion to determine in which measure small particles trapped in quantized vortices affect vortex motion. Finally we apply the criterion to a recent experiment.