An efficient lattice based search and optimization procedure has been developed and used with various assumed pair potentials to find minimal energy structures on an icosahedrally derived lattice. These were then taken as initial configurations and allowed to relax freely under the Lennard-Jones pair potential to the adjacent energy minimum. The initial configurations and relaxed energies of the most tightly bound structures found for each N are presented for the range 13≤N≤147. While the energies obtained are rigorously only upper bounds to the absolute minimal energy of an N-atom Lennard-Jones cluster, they appear to be less than or equal to that of any other structures proposed previously. They are believed to be the most tightly bound structures of the multilayer icosahedral type, and to be reasonable candidates for the absolutely minimal energy structures in this size range. The most tightly bound configuration found was the truncated icosahedral structure at N=135.

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Liquid helium clusters are produced by expanding gaseous 4 He 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 He+4 . 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 4 He, 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 T0 <6.5 K where the expansion isentrope intersects the liquid–vapor line upstream from the source orifice.

Clusters are produced by expanding high pressure (P0≤20 bar), low temperature (T0≥5 K) helium gas through a 5 μm nozzle into a vacuum. The neutral beam time-of-flight distribution has three peaks which we associate with distinct groups of large and small clusters, and atoms. The beam is ionized by electron impact and the resulting time resolved charged fragment mass distribution reveals in addition to previously observed anomalies (‘‘magic numbers’’) a new strong He+4 signal at high source pressures and low temperatures. The dependence of the various charged and neutral metastable fragment currents on the bombarding electron energy reveals that each has a unique appearance potential. A comparison with the calculated energy required for an electron to create various electronic excitations in the interior of a large cluster indicates that the production and dynamical evolution of metastable 3S1 atomic and a 3Σ+u molecular excitations plays a significant role in the formation of charged fragments from large clusters, but that the production of detectable metastable cluster fragments apparently proceeds via decay of high lying excitonic states. The strong He+4 signal does not appear until the incident electron has about enough energy to create two metastable 3S1 excitations in a cluster. Thus we propose that this ion signal results from the recombination of a pair of a 3Σ+u molecular excitons in or on a large cluster, or possibly from the dynamical evolution of a metastable spin quartet bound hole-exciton pair.

In this report I will review experimental studies of free helium droplets, with the exception of spectroscopic studies of helium droplets that contain impurities. This particular topic, as well as theoretical studies of helium droplets, will be reviewed separately elsewhere in this issue.