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.

Experimental studies of helium droplets

High-energy cluster—surface collisions

The physical properties of vapor deposited amorphous solid water, (H2O]as, are known to depend strongly on the details of preparation. Various parameters were found to have an influence, for example the rate of deposition, the substrate temperature and so on (1,2). We observed a further parameter to be important for the preparation of (H2O)as which has not been discussed yet: supersonic flow of the water vapor and condensation of the vapor to clusters already in the gas phase (3). The usual procedure for preparing (H2O)as is to admit water vapor through a capillary info a high vacuum system and condense the vapor on a cryoplate. The exit of the capillary can act as a nozzle and from water vapor pressure and nozzle diameter it can be calculated that for the usual deposition conditions supersonic flow and possibly formation of clusters in the gas phase will occur. In contrast, a baffle between nozzle exit axnd cryoplate has the effect to break up supersonic flow and give water vapor consisting only of monomers. The amorphous deposits prepared either way look very different: supersonic flow gives a white deposit with a very rugged surface, whereas baffled flow gives transparent deposits with a smooth surface.

Polymorphism in Vapor Deposited Amorphous Solid Water

We have carried out stochastic trajectory simulations of the scattering of argon clusters, Arn, n=5 to 26, from a Pt(111) surface. At incident energies of 0.1 and 0.5 eV per atom, the clusters fragment almost completely into individual atoms. Some atoms remain trapped on the surface, but the majority scatter. For the larger clusters, the angular distributions of the scattered atoms peak near the surface tangent, and are almost independent of the incident angle. The mean kinetic energy of atoms is largest for those scattered near the surface tangent, and decreases as the scattering angle approaches the surface normal. These trends are in qualitative accord with the results of experimental studies of cluster scattering from surfaces. A multiple binary collision mechanism involving atom–atom collisions within the cluster as well as atom–surface collisions is responsible for this behavior.

Dynamics of cluster scattering from surfaces

The transition from intact clusters to shattered small fragments is discussed for high velocity impact of cold atomic clusters at hard surfaces. The distribution of fragment sizes is computed as one of maximal entropy subject to conservation of matter and energy. The entropy is computed using a graph‐theoretical approach for counting the number of possible isomers for a cluster of given size. As the impact velocity is increased, there is an onset of fragmentation into small fragments and the heat capacity is found to be discontinuous at that point. The results are compared with molecular dynamics simulations for rare gas clusters. Throughout the emphasis is on the special conditions that prevail during the ultrafast compression stage of the high energy cluster.

Fragment size distribution in cluster impact: Shattering versus evaporation by a statistical approach

Clusters of helium, hydrogen, and nitrogen are reflected at a polished stainless steel plate at temperatures ranging from 80 to 550 K. The incident clusters contain on the average about 1.5×105 atoms of helium or molecules of hydrogen or 104 molecules of nitrogen, as measured by time‐of‐flight mass spectrometry. The angular distributions of the average sizes, the velocities, and the molecular intensities of the reflected cluster beams show that in the investigated range of reflector temperatures the reflection of the helium clusters corresponds to the hydrogen cluster reflection at higher reflector temperatures while the nitrogen cluster reflection corresponds to the hydrogen cluster reflection at lower reflector temperatures. The transition between the two regimes of reflection as observed with hydrogen clusters is marked by an optimum reflector temperature leading to a maximum intensity of the reflected beam, a minimum loss of clustered material, and a distinct angular separation of incident cluster sizes. At a grazing incidence angle of 84.3° the measured optimum reflector temperature for the hydrogen cluster reflection is 215 K and increases with decreasing angle of incidence. The two regimes of high‐temperature and low‐temperature reflection of clusters exhibit close phenomenological relationship to the regimes of thermal and structure dominated scattering of atoms from single crystal planes. In both cases the transition region is characterized by a maximum angle of reflection and a minimum divergence of the reflected beam. The features of high‐temperature cluster reflection are explained by a semiempirical model based on the evaporation recoil of the cluster molecules ablating after contact with the comparatively hot reflector surface.

G. Krieg

Papers

Reflection of clusters of helium, hydrogen, and nitrogen as function of the reflector temperature