Observation of Crossover from Ballistic to Diffusion Regime for Excimer Molecules in Superfluid $^4$He

We have measured the temperature dependence of the time of flight of helium excimer molecules He2* in superfluid 4He and find that the molecules behave ballistically below 100mK and exhibit Brownian motion above 200 mK. In the intermediate temperature range the transport cannot be described by either of the models.