LASER-INDUCED BREAKDOWN IN LIQUID HE4

The threshold power for laser-induced breakdown in liquid helium was studied as a function of temperature between 4 and 1.1 K. Above the λ point, it was found to be temperature-independent. In He II, however, the threshold increases rapidly by one order of magnitude on cooling between the λ temperature and 1.5 K. This behavior is interpreted in terms of a rapidly increasing mobility of free electrons in the energy range of several electron volts if the liquid is cooled below the λ point. © 1969 The American Physical Society.