MASS-SPECTRA AND TIME-OF-FLIGHT DISTRIBUTIONS OF HELIUM CLUSTER BEAMS

Liquid helium clusters are produced by expanding gaseous 4He into a vacuum from a cold source with temperatures between 5 and 20 K at stagnation pressures from P0 = 8 to 20 bar and are studied by time-of-flight (TOF) and mass spectrometry. At low temperatures, T0 < 12 K, the mass spectra show several anomalies which can be attributed to pick-up of residual gases. At T0 < 10K, there is evidence for a very intense peak at m = 16 amu which is attributed to He4 +. Depending on the temperatures, the TOF spectra reveal ions with three different velocities. These TOF observations are analyzed using isentropic lines in the known phase diagram of 4He, which take into account deviations from ideal gas behavior. Three qualitatively different expansion regimes are identified: (I) the expansion proceeds through a region on the high temperature side of the critical point, (II) the expansion passes through or near the critical point, and (III) the expansion passes through a region on the low temperature side of the critical point. The mass spectra, peak velocities and speed ratios, when analyzed with the aid of the phase diagram, indicate that (a) two of the TOF peaks are due to clusters, (b) the fastest cluster peak is due to clusters formed by condensation of gas phase atoms, and (c) the slowest cluster peak is due to either separation into two phases (regime II) or disintegration of a liquid phase (regime III). Measured conversions of initial enthalpy into free jet kinetic energy suggest that the cluster temperature undergoes a sharp drop to a very low temperature approaching 0 K at T 0 < 6.5 K where the expansion isentrope intersects the liquid-vapor line upstream from the source orifice. © 1990 American Institute of Physics.