Detecting and imaging He2 molecules in superfluid helium by laser -induced fluorescence

Wataru Kobayashi

Wade G. Rellergert, S. B. Cahn, A. Garvan +3 more

Andrey F. Vilesov

Daniel N. McKinsey, W. H. Lippincott, James A. Nikkel +1 more

We describe an approach to detecting ionizing radiation that combines the special properties of superfluid helium with the sensitivity of quantum optics techniques. Ionization in liquid helium results in the copious production of metastable He2 molecules, which can be detected by laser-induced fluorescence. Each molecule can be probed many times using a cycling transition, resulting in the detection of individual molecules with high signal to noise. This technique could be used to detect neutrinos, weakly interacting massive particles, and ultracold neutrons, and to image superfluid flow in liquid 4He.

Wade G. Rellergert, S. B. Cahn, A. Garvan +4 more

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.

Wei Guo, John D. Wright, S. B. Cahn +2 more

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.