Driving high threshold chemical reactions during the compression interlude in cluster surface impact

A comparative study of cluster-surface collisions: Molecular-dynamics simulations of (H2O)1000 and (SO2)1000

A classical molecular-dynamics study of (H2O)1000 and (SO2)1000 clusters impacting with velocities between 6 x 10(2) and 8 x 10(3) ms at normal incidence on a repulsive target is presented. Using the ratio of total kinetic energy to total binding energy of the cluster as a scaling parameter, a general description of the fragmentation dynamics as well as the final fragment size distributions is achieved for the different systems. With increasing ratio, the angular distribution of the emitted monomers rapidly shifts from isotropic to anisotropic. At the highest investigated velocities, a tendency to recover more isotropic distributions is observed. Comparable transient compression of the impacting cluster is reached, on the other hand, for the same, unscaled collision velocities in both systems. For both H2O and SO2 the obtained internal temperatures of the cluster fragments are found to be independent of impact energy and close to the boiling temperature of the respective systems.

Mechanical Simulation of the Pressure and the Relaxation to Thermal Equilibrium of a Hot and Dense Rare Gas Cluster

A cold atomic cluster can be very rapidly heated and compressed by a hypersonic impact at a hard surface. The impact can be simulated by computing a classical trajectory for the motion of the atoms. By suddenly confining the hot and dense cluster within a rigid container, it is possible to monitor the time evolution of the force acting on the faces of the container. It is found that the pressure computed this way very rapidly decays to a time-independent value. After a somewhat longer time, this value reproduces the value for the pressure computed as the sum of the kinetic and internal pressures. This agreement is expected for a system in equilibrium. These observations support the conclusion that there is a fast relaxation to thermal equilibrium in these essentially hard-sphere systems. The deviation from equilibrium is primarily due to the propagation of shock waves within the cluster. The equilibrium pressure can reach up to the megabar range.

A mechanical representation of entropy for a large finite system

The temporal evolution of the entropy of a mechanical system as described by a single trajectory is computed using the Clausius [Philos. Mag. 40, 122 (1868)] equality. This requires computing the maximal work that can be done by the system and comparing it to the actual work performed. A single trajectory suffices to determine the entropy when it is "typical," meaning that average values of mechanical variables will not be different when computed using trajectories with different initial conditions. The results are illustrated for small rare gas clusters heated and compressed by an impact at a hard surface.

Evanescent high pressure during hypersonic cluster-surface impact characterized by the virial theorem

Matter under extreme conditions can be generated by a collision of a hypersonic cluster with a surface. The ultra-high-pressure interlude lasts only briefly from the impact until the cluster shatters. We discuss the theoretical characterization of the pressure using the virial theorem and develop a constrained molecular-dynamics procedure to compute it. The simulations show that for rare-gas clusters the pressures reach the megabar range. The contribution to the pressure from momentum transfer is comparable in magnitude and is of the same sign as that ("the internal pressure") due to repulsive interatomic forces. The scaling of the pressure with the reduced mechanical variables is derived and validated with reference to the simulations.