We investigate by molecular-dynamics simulation the time evolution of a collision cascade in a condensed Ar solid, initiated by a 1-keV Ar atom. The particle density and velocity in the collision-cascade volume, the kinetic-and potential-energy distributions of the recoiling Ar atoms, and the energy distribution of sputtered particles are monitored. The high pressure building up in the core of the cascade drives the material into the cascade periphery and towards the surface. The surface erupts and hundreds of atoms are emitted, leaving behind a huge crater.PACS numbers: 79.20.NcThe consequences of bombarding a strongly bonded material-such as a metal or semiconductor target -with an energetic atom or ion with energies around 1 keV or higher have been well studied in the last decades. Roughly, the following picture has emerged: The bombarding particle dissipates its energy and momentum, via collisions, to the atoms of the target material; these in their turn collide with other target atoms, etc., and thus a collision cascade is set up. The displacement processes involved lead to the formation of damage (vacancies and interstitial atoms) in the material, while near-surface atoms which acquire sufficient momentum are emitted {sputtered) from the solid. This picture forms the basis of a quantitative theory of damage and sputtering processes, l which presupposes that the number of moving atoms in the relevant stages of the process is small (dilute or linear cascade).There may, however, be situations in which essentially all target atoms in the collision-cascade volume-or in a near-surface part of it-are set in motion: This then is called a dense cascade or a spike. Deviations from the predictions of linear-cascade theory may then be expected, and have been measured using heavy ions and, in particular, molecular ions as projectiles. 2 Spike effects are particularly drastic when bombarding a volatile solid, such as, e.g., a condensed rare-gas target. For these systems, sputtering yields are measured which are up to an order of magnitude higher than the predictions of linear-cascade theory. 3 While at high emission energies, the energy distribution of sputtered particles follows well the predictions of linear-cascade theory, an abundance of particles emitted below the sublimation energy of the material can be detected. 4,5 A number of models have been formulated to explain these findings. 6,7 They range from modifications of linear-cascade theory, in which the value and nature of the surface binding energy is changed, 4 ' 8 to the so-called gas-flow model which assumes that the high energy density in the cascade volume brings the material above the critical point of the liquid-gas phase transition such that the target atoms are free to flow out into the vacuum above the surface. 39 Other models which have been formulated to explain the sputtering from dense cascades comprise the evaporation model, where evaporation from surface regions with high energy density contributes to sputtering, 10,11 and conceptions that a shoc...

A new vehicular traffic flow model based on a stochastic jump process in vehicle acceleration and braking is introduced. It is based on a master equation for the single car probability density in space, velocity and acceleration with an additional vehicular chaos assumption and is derived via a Markovian ansatz for car pairs. This equation is analyzed using simple driver interaction models in the spatial homogeneous case. Velocity distributions in stochastic equilibrium, together with the car density dependence of their moments, i.e. mean velocity and scattering and the fundamental diagram are presented.

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