Film Thickness Dependence of Heat Transmission into Helium

A heater is pulsed at the interface between a sapphire crystal and an adsorbed 4He film. For short pulses, the amount of heat absorbed by the film saturates when the film thickness reaches a few layers, just as observed in phonon reflection experiments. For longer pulses, however, there is little difference between thin film and vacuum results, while substantial heat is transferred to the bulk liquid.