Article

# Metastable Helium Molecules as Tracers in Superfluid He-4

Physics Department, Yale University, New Haven, CT 06515, USA.
(Impact Factor: 7.51). 07/2009; 102(23):235301. DOI: 10.1103/PhysRevLett.102.235301
Source: arXiv

ABSTRACT 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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Available from: John David Wright, Oct 09, 2014
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• "However, new techniques for visualizing vortex tangles in the zero temperature limit are needed. One promising route is to use helium excimer molecules that can be created in situ [58]. It was found recently that these molecules become trapped on vortices below 0.2 K [59] raising the possibility of imaging vortex tangles through laser-induced fluorescence. "
##### Article: Dynamics of quantum turbulence of different spectra.
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ABSTRACT: Turbulence in a superfluid in the zero-temperature limit consists of a dynamic tangle of quantized vortex filaments. Different types of turbulence are possible depending on the level of correlations in the orientation of vortex lines. We provide an overview of turbulence in superfluid (4)He with a particular focus on recent experiments probing the decay of turbulence in the zero-temperature regime below 0.5 K. We describe extensive measurements of the vortex line density during the free decay of different types of turbulence: ultraquantum and quasiclassical turbulence in both stationary and rotating containers. The observed decays and the effective dissipation as a function of temperature are compared with theoretical models and numerical simulations.
Proceedings of the National Academy of Sciences 03/2014; 111 Suppl 1:4691-8. DOI:10.1073/pnas.1312544110 · 9.81 Impact Factor
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• "They also provide complementary spectroscopic data [4] [5] for quantitative investigation of the structural changes induced in the fluid by the electronic excitation and of the de-excitation channels [3] [6] [7]. Moreover, they potentially provide high sensitivity and time resolution for improved detection, imaging, or tracking of dissolved He * 2 (currently performed using light-induced fluorescence techniques [8] [9]). So far, He * 2 absorption has been probed at low temperatures with fairly low resolution using broad lamps and monochromators [6] [10] or with sophisticated lasers [11] and systematic studies involve only the most prominent lines [7]. "
##### Article: Using a cw 465 nm Laser to Probe Metastable He 2 Molecules
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ABSTRACT: A cw 465 nm laser has been built for sensitive detection of helium metastable molecules by resonant absorption on the transition band. The frequency-mixing radiation is obtained from commercial laser diodes in a periodically-poled KTP non-linear crystal. The 1.3-nm tuning range includes the main rotational lines of 3He2, 4He2, and 3He-4He dimers. Measurements of absolute molecular densities down to a few 109 cm−3 are reported in low pressure (1–400 mbar) room temperature He gas excited by a weak rf discharge. Unsophisticated detection techniques provide signals with good signal-to-noise ratios thanks to the narrow absorption linewidths (a few GHz, due to Doppler and moderate collisional broadenings) in the fully resolved spectrum. Prospects for use or upgrade of this blue laser to probe the broadened and shifted molecular lines in condensed He are discussed.
Journal of Low Temperature Physics 01/2010; 158(1):339-345. DOI:10.1007/s10909-009-0050-5 · 1.04 Impact Factor
• ##### Article: Visualization Technique for Determining the Structure Functions of Normal-Fluid Turbulence in Superfluid Helium-4
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ABSTRACT: Metastable helium molecules have been shown to be ideal tracers for the study of normal-fluid flow in superfluid 4He above 1 K. These molecules can be visualized using a laser-induced-fluorescence technique. In order to extract quantitative information of the flow field, practical techniques need to be developed to study the density evolution of the molecules with a well defined initial distribution. In an on-going experiment, we use a focused femtosecond laser pulse to create a thin line of $\mathrm{He}^{*}_{2}$ molecules in liquid helium. Such a line of molecules can be easily imaged. Studying the drift and distortion of the molecular line in a turbulent flow shall allow us to measure the instantaneous flow velocity field and hence determine the structure functions of the normal-fluid turbulence.
Journal of Low Temperature Physics 06/2012; 171(5-6). DOI:10.1007/s10909-012-0708-2 · 1.04 Impact Factor