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

Ionizing radiation events in superfluid helium result in the copious production of metastable He2 molecules which have a 13 s radiative lifetime in liquid helium. In this thesis, we present results on using lasers to detect and image the molecules by driving them through multiple fluorescence-emitting transitions during their lifetime. We have determined that a single laser pulse at 905 nm can drive a molecule into an excited state where it will emit a detectable red photon at 640 nm. We demonstrate that additional continuous-wave repumping lasers can be used to control the vibrational state population of the molecules. We have shown that each molecule can be made to emit over 100 photons in its lifetime. This has allowed us to image individual tracks of molecules produced in Compton scattering events. The technique gives rise to a new method for detecting ionization events in superfluid helium with applications in the detection of gamma rays, ultracold neutrons, and WIMP dark matter. It also promises to be a powerful tool for visualizing fluid flows and vortices in superfluid helium.