Quantum Turbulence Research
Introduction:
A powerful method to study fluid flow is visualization. Flow visualization in superfluid He-4 is challenging, yet crucial for attaining a detailed understanding of quantum turbulence. Two problems have impeded progress: finding and introducing suitable tracers that are small yet visible; and unambiguous interpretation of the tracer motion. Metastable helium molecules in spin triplet states have been shown to be good tracer candidates. These molecules form tiny bubbles (6 Å in radius) in liquid helium. They are metastable because a radiative transition to the ground state of two free atoms requires a strongly forbidden spin flip. The radiative lifetime of He2 triplet molecules has been measured to be about 13 s.
Due to their small size, the binding energy of the molecules on quantized vortex lines is small. At temperatures above 1K, they are solely entrained by the normal-fluid component without getting trapped on vortices, which makes them ideal tracers for the study of various normal-fluid flows in superfluid He-4 above 1 Kelvin. At temperatures below about 0.2 K, He2 molecules become trapped on quantized vortices. Imaging the trapped molecules at low temperatures will allow clean vortex-line visualization. Quantitative studies of vortex interactions and the decay of quantum turbulence can be performed.
Laser-induced fluorescence:
A laser-induced fluorescence technique has been developed to image He2 molecules. Two infra-red photons at 905 nm can excite a helium molecule from their triplet ground state (a3Σu) to the excited electronic state (d3Σu). Calculations of the branching ratios indicate that about 10% of the excited molecules in d state will decay to the c3Σg state, while the remaining 90% will decay to the b3Πg state, emitting fluorescent red photons at 640 nm. A simple notch interference filter, centered at 640 nm, can be used to prevent unwanted light from reaching the CCD camera to achieve zero background. Molecules in both the c and b states then decay back to the a3Σu triplet ground state, and the process can be repeated (see figure above). However, during the cycling transitions, molecules can fall to long-lived excited vibrational levels of the electronic ground state (the a(1) and a(2) levels), and are out of resonance with the excitation lasers. To recover the molecules lost to the vibrational levels, re-pumping lasers at 1073 nm and 1099 nm can be used to excite the molecules from a(1) and a(2) to the corresponding c states, where the molecules essentially decay back to a(0). Beside the cycling transition scheme, a molecule-tagging visualization scheme has also been developed and tested. This scheme makes use of the long-lived excited vibrational level a(1) to tag a small group of molecules among a large number of them so as to study specifically the motion of the tagged molecules. To do this, a pump laser pulse at 910 nm is used to illuminate the He2 molecules in the triplet ground state.
The pump pulse tags molecules by driving the population from the a(0) to the excited state c(0) and relying on redistribution of the c(0) population into the long-lived a(1) excited vibrational level via nonradiative transitions which occur naturally in a few nanoseconds. About 4% of the molecules decay to the a(1) from the c(0) state. An probe laser at 925 nm can then be used to image only the tagged molecules at a given delay time by driving the molecules from a(1) to the d state and inducing 640 nm fluorescence via the d to b transition. In the case that molecules are dispersed in the fluid, this tagging method will allow us to select the molecules at a desired location and study the local flow field. The motion of a normal-fluid jet impinging on the center of a copper disc imaged by laser-induced fluorescence of the entrained He2 molecules is shown in the figure on right.
Current experiments:
To be added...
