Superfluidity is a many-body quantum effect observed in liquid helium. Macroscopic superfluidity can be probed by means of a torsional pendulum immersed in liquid helium; changes in the temperature of the system from the lambda point of helium (2.17K) down to 1K lead to an increase in the moment of inertia of the pendulum [1]. Its microscopic analogue would be obtained by replacing the torsional pendulum with a rotating molecule; however, the realization of a bulk experiment is very difficult since at the low temperatures of liquid helium all foreign molecules would freeze at the container walls.

In order to overcome this difficulty we use a corona discharge to excite short lived helium excimers in Rydberg states. Such excimers emit fluorescence and we were able to resolve rotational lines in its spectrum, thereby sensitively probing the environment. Rotational resolution has been obtained in molecule-doped helium droplets before, but here we are able to measure temperature and pressure dependence for the first time.

Preliminary results show the rotationally resolved excimer fluorescence can be observed in helium at 1 bar and at 300K, 77K and 4K. These preliminary measurements also show that the effective moment of inertia of the excimers change depending on the temperature. In order to investigate if the density effect can be observed at room temperature we observed rotationally resolved lines from 1 bar until 25 bar. We did not observe a changing moment of inertia in this range at 300K.

[1] Andronikashvili, E.: 1971, `A direct observation of two kinds of motion in liquid helium II’. In: translated by Z.M. Galasiewicz (ed.): Helium 4. Oxford: Pergamon, pp. 154-165.

[2] J. P. Toennies and A. F. Vilesov, Angew. Chemie Int. Ed. 43, 2622 (2004).