Free and hindered-rotation of helium excimers in liquid helium via a bulk experiment

Superfluidity is a many-body quantum effect observed for the first time in liquid helium. In the context of modern nanoscience, a natural question is whether superfluidity exists at the nanoscale and if so, under what conditions it occurs. Superfluidity can be probed by means of a torsional pendulum immersed in liquid helium: a decrease in the moment of inertia of the pendulum was observed during the superfluid transition. By replacing the torsional pendulum with a carbonyl sulfide molecule embedded in helium droplets, Grebenev and coworkers explored superfluidity at the nanoscale. They established that 60 4He atoms is the threshold to observe superfluidity. The thermodynamic conditions necessary for this transition could not be ascertained in Grebenev's work since in the helium droplet technique the transition into the superfluid state is impossible to control. One possible way around this experimental limitation is to perform a bulk experiment and embed short-lived helium excimers because all other molecules would freeze. The excimers are in Rydberg states and emit fluorescence sensitive to their environment. In this work, helium excimers have been produced in bulk liquid helium using corona discharges. A wide range of the phase diagram of helium has been probed via fluorescence spectroscopy of Rydberg excimers for the first time: molecular transitions in gaseous, supercritical, vapor and normal liquid phases have been studied systematically. Depending on the thermodynamic conditions, sharp as well as broadened spectra have been observed. The linewidths and lineshifts of a transition of interest have been interpreted on the basis of a model that considers emission from two kinds of excimer: on the one hand, excimers embedded in voids and fully solvated in liquid helium exhibiting hindered rotation, and on the other hand, excimers residing in larger gas pockets within the liquid helium, exhibiting free rotation. The relative contributions of each species of excimer have been estimated in a ratio of approximately 1:5. Hindered transitions were identified for pressures and temperatures in the vapor phase, before helium liquefies. These points in the phase diagram show the formation of clusters between excimers and ground-state helium atoms, indicating that the He_2 -He interaction is stronger than that of He-He.