Shape fluctuations and optical transition of He${}_2^{\ast }$ excimer tracers in superfluid ${}^4$He

Abstract

Metastable He${}_2^{\ast }$ 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{\Sigma }_u^{+}$$\to$$c^3{\Sigma }_g^{+}$ optical absorption line, which is responsible for the fluorescence imaging of the He${}_2^{\ast }$ 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 $a^3{\Sigma }_u^{+}\to c^3{\Sigma }_g^{+}$ 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 He${}_2^{\ast }$ 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${}_2^{\ast }$ tracers.

11 pages, 8 figures

References in corpus (12)

• Emergence of turbulence in an oscillating Bose-Einstein condensate
• Emergence of a Turbulent Cascade in a Quantum Gas
• Giant Vortex Clusters in a Two-Dimensional Quantum Fluid
• Characterization of reconnecting vortices in superfluid helium
• Velocity Statistics Distinguish Quantum Turbulence from Classical Turbulence
• Order from chaos: Observation of large-scale flow from turbulence in a two-dimensional superfluid
• Visualization study of counterflow in superfluid helium-4 using metastable helium molecules
• Detection and Imaging of He_2 Molecules in Superfluid Helium
• Dissipation in quantum turbulence in superfluid ${}^4$He above 1K
• Determination of the effective kinematic viscosity for the decay of quasiclassical turbulence in superfluid ${}^4$He
• Visualizing Pure Quantum Turbulence in Superfluid ${}^3$He: Andreev Reflection and its Spectral Properties
• Observation of Crossover from Ballistic to Diffusion Regime for Excimer Molecules in Superfluid ${}^4$He

Cited by in corpus (1)

• A cryogenic-helium pipe flow facility with unique double-line molecular tagging velocimetry capability