All in a ball: Neutral molecular clusters consisting of a few thousand molecules can be seen as tiny snow balls; if they are thrown fast enough onto a surface, they are able to pick up biomolecules such as insulin from that surface. Since they break down and evaporate during and after the collision, bare biomolecular ions are available for mass spectrometry after such an energetic throw.

Large-scale classical molecular-dynamics simulations of (H2O)n (n=1032,4094) collisions with graphite have been carried out. The clusters have an initial internal temperature of 180 K and collide with an incident velocity in the normal direction between 200 and 1000 m/s. The 1032-clusters are trapped on the surface and completely disintegrate by evaporation. The 4094-clusters are found to partly survive the surface impact provided that the surface is sufficiently hot. These clusters are trapped on the surface for up to 50 ps before leaving the surface under strong evaporation of small fragments. The time spent on the surface is too short for full equilibration to occur, which limits the fragmentation of the clusters. The size of the emitted fragment is roughly 30% of the incident cluster size. The cluster emission mechanism is found to be very sensitive to the rate of the surface-induced heating and thus to the surface temperature. The incident cluster velocity is less critical for the outcome of the collision process but influences the time spent on the surface. The trends seen in the simulations agree well with recent experimental data for collisions of large water clusters with graphite [Chem. Phys. Lett. 329, 200 (2000)].

Sulfur dioxide clusters (SO2) x of mean size 〈x〉 = 1.7 × 103 collide at a velocity of 1.6 km s-1 with a metal surface that has been pretreated with dilute salt solutions containing Li+, Na+, K+, Cl-, and Br- ions in different concentrations. In the scattered gas plume, positive and negative cluster fragments of the formal composition (SO2) x (Li+, Na+, K+, SO2 -) are detected. While the amount of charge observed in the various cationic channels correlates with the concentration of the respective cations in the solutions, no cluster fragments carrying chloride or bromide anions have been observed. This indicates, that the observed free charge carriers are not formed by a direct pickup of ions from the surface. Based on the assumption that the physical state for alkali adsorbates on metal surfaces is independent of the charge state of the adsorbing precursor, the findings are explained in terms of a known charge separation effect in cluster surface collisions involving neutrally deposited alkali adsorbates.

The fragmentation dynamics of (H2O)1032 clusters colliding with a repulsive surface at incident velocities of 1753 m/s and 2909 m/s, corresponding to kinetic energies of 0.5 and 1.5 times the cluster binding energy, has been examined in a classical molecular dynamics simulations study. The results show a large anisotropy in the energy redistribution inside the cluster upon impact, which leads to asymmetric fragmentation, starting in the leading part of the cluster. The low-mass region of the fragment size distribution can be described by a power law with an exponent close to −1.6, and the range of this region increases with increasing incident velocity. The formed fragments have rather uniform internal temperatures close to the standard boiling point of water, but the translational energy of the monomers formed upon collision is much larger, pointing at the asymmetric energy distribution inside the cluster. The angular distributions of fragment mass and fragment kinetic energy peak at grazing exit angles. For the investigated conditions, the dynamics is insensitive to the details of the initial structure of the cluster.

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