Laser-Induced Breakdown in LiquidHe4

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 $\lambda$ 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 $\lambda$ 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 $\lambda$ point.