DROPLETS FORMED IN HELIUM FREE-JET EXPANSIONS FROM STATES NEAR THE CRITICAL-POINT

Droplets formed in He-4 free jets expanded from source stagnation stat es in the vicinity of the critical point (T-c=5.2 K, P-c=2.3 bar) are investigated using a mass-spectrometer time-of-flight (TOF) technique. Depending on the source conditions, three different TOF peaks are ide ntified: (a) atoms, (b) droplets formed by condensation from the gas p hase, and (c) droplets formed by disintegration of the liquid phase. T he latter show the lowest ever observed He-4 cluster speeds, about 50 m/s, at a source pressure of 1.5 bar and a source temperature of 4-5 K , just below the critical pressure and temperature. The TOF distributi ons at the critical point are very broad and this may be due to critic al-point fluctuations during the averaging period (2-10 min) of the in dividual measurements. (C) 1997 American Institute of Physics.