Trace Detection of Metastable Helium Molecules in Superfluid Helium by Laser-Induced Fluorescence

McKinsey, D. N.; Lippincott, W. H.; Nikkel, J.; Rellergert, Wade G.

doi:10.1103/physrevlett.95.111101

Cited by 32 publications

(36 citation statements)

References 43 publications

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“…The radiative lifetime of the triplet molecules has been calculated to be 18 s in vacuum [6] and measured to be 13 s in liquid helium [7]. Here we present data supporting our previous proposal [8] to detect and image the triplet molecules by driving them through multiple fluorescenceemitting transitions during their lifetime.…”

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confidence: 89%

Detection and Imaging ofHe2Molecules in Superfluid Helium

et al. 2008

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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.

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confidence: 89%

Detection and Imaging ofHe2Molecules in Superfluid Helium

et al. 2008

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“…In Fig. 4, we plot the square of the mean width of the molecule line w 2 ðtÞ as a function of time t. The broadening of the molecule line occurs too rapidly for it to be explained by ordinary diffusion at the experimental temperature (1.95 K) [13]. We deduce that it is caused by turbulent diffusion [23], thus confirming that the flow of the normal fluid is turbulent.…”

Section: Prl 105 045301 (2010) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 59%

“…A pulsed laser at 905 nm (repetition rate 500 Hz) illuminated the needle apexes [see Fig. 5(b)] to drive the molecules to produce fluorescent light through a cycling transition [13][14][15][16]. Typical images of a molecule cluster at a heat flux of 119 mW=cm 2 are shown in Fig.…”

Section: Prl 105 045301 (2010) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

“…In addition to micron-sized tracers, 3 He atoms have been used to study the flow in superfluid 4 He by using the neutron radiography technique, which requires a finely collimated neutron beam and the ability to raster the neutron beam through the region of interest [11]. Recently, we have demonstrated that metastable He Ã 2 triplet molecules, with a radiative lifetime of about 13 s in liquid helium [12], can be imaged by using a laser-induced-fluorescence technique, which involves only tabletop laser systems [13][14][15][16]. These molecules can act as tracers that follow the motion of the normal fluid.…”

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Laminar, turbulent, or doubly turbulent?

Barenghi¹

2010

Physics

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Heat is transferred in superfluid 4 He via a process known as thermal counterflow. It has been known for many years that above a critical heat current the superfluid component in this counterflow becomes turbulent. It has been suspected that the normal-fluid component may become turbulent as well, but experimental verification is difficult without a technique for visualizing the flow. Here we report a series of visualization studies on the normal-fluid component in a thermal counterflow performed by imaging the motion of seeded metastable helium molecules using a laser-induced-fluorescence technique. We present evidence that the flow of the normal fluid is indeed turbulent at relatively large velocities. Thermal counterflow in which both components are turbulent presents us with a theoretically challenging type of turbulent behavior that is new to physics. DOI: 10.1103/PhysRevLett.105.045301 PACS numbers: 67.25.dk, 29.40.Gx, 47.27.Ài The superfluid phase of liquid 4 He exhibits two-fluid behavior [1]: a normal fluid, carrying all the thermal energy, coexists with a superfluid component. The proportion of superfluid falls from unity to zero as the temperature T rises from zero to the transition [1]. In a thermal counterflow, the normal-fluid velocity v n is related to the heat flux q by q ¼ STv n ;( 1) where is the total density and S is the entropy per unit mass [1]. Above a certain critical heat flux, superflow is known to become turbulent. This quantum turbulence takes the form of a disorganized tangle of quantized vortex lines [2,3]. A mutual friction force between the two fluids arises through the interaction between the quantized vortices and the normal fluid [4]. This type of quantum turbulence is maintained by the relative motion of the two fluids, through processes that are reasonably well understood [2,3]. Features in the observed relation between vortex density and heat flux suggest that the normal fluid may also become turbulent, and mutual friction has been shown theoretically to induce an instability in the laminar flow of the normal fluid [5]. Satisfactory evidence for such normalfluid turbulence can come only from a visualization of the normal-fluid flow. Techniques for such visualization have recently started to be developed. Turbulence in both fluids has been observed in other types of flow in liquid helium [6], such as that behind a moving grid [7]. But in those cases the two fluids are not forced to have any relative motion and behave like a single classical fluid, exhibiting a Kolmogorov energy spectrum [1]. Simultaneous turbulence in both fluids in a counterflow must be different, and it would be a type of turbulence that is new to physics. Past experiments on the visualization of thermal counterflow have used micron-sized tracer particles formed from polymer spheres or solid hydrogen, and they have been based on either particle image velocimetry [8] or particle tracking techniques [9]. The particle image velocimetry data obtained at large heat fluxes are hard to interpret since micron-s...

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“…IV, we evaluate the diffusion of the He * 2 molecules in He II using the calculated bubble stiffness and compare the result with experimental observations. A brief summary is given in Section V. In order to image the He * 2 molecules in the a 3 Σ + u triplet ground state, McKinsey's group first developed a cyclingtransition LIF technique 29,30,42 . The concept of this technique is shown schematically in Fig.…”

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confidence: 99%

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

Guo

Golov

2020

Phys. Rev. B

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Metastable He * 2 excimer molecules have been utilized as tracer particles of the normal component in superfluid 4 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 Σ + u →c 3 Σ + g optical absorption line, which is responsible for the fluorescence imaging of the 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 Σ + u →c 3 Σ + 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 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 He * 2 tracers.

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Trace Detection of Metastable Helium Molecules in Superfluid Helium by Laser-Induced Fluorescence

Cited by 32 publications

References 43 publications

Detection and Imaging ofHe2Molecules in Superfluid Helium

Detection and Imaging ofHe2Molecules in Superfluid Helium

Laminar, turbulent, or doubly turbulent?

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

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