BISTABLE HEATING OF SOLID SAMPLES IMMERSED IN LIQUID-HELIUM

A bistable heating of samples is found in liquid helium. Using the str ong temperature dependence of the no-phonon doublet of the 4T1(G) --> 6A1(S) emission of the ZnSe:Cr+ centre, the temperature can be determi ned in the excited volume. Temperature jumps of about 5 K are recorded . The cause for the bistable crystal heating lies in the transition fr om the nucleate boiling to the film boiling state of the helium. In th is process, the heat transfer is drastically reduced and thus the samp le is further heated.