Molecular dynamics study of non-equilibrium energy transport from a cylindrical track - I. Test of "spike" models

Molecular Dynamics (MD) calculations were carried out to describe the kinetic energy transport in a low temperature, condensed-gas solid following electronic excitation of a narrow cylindrical region by fast ions. Solid Ar is used as a model material. The atoms in a cylindrical region of an Ar sample are each given a kinetic energy, which they might obtain by non-radiative radiative relaxation processes, and the evolution of the sample is followed in time. Whereas a uniformly energized sample is found to equilibrate in a mean collision time, equilibration competes with radial transport of energy from the cylindrically excited region. The radial evolution of the deposited energy is compared to that predicted by thermal spike models because of their extensive use. Both analytic and finite difference calculations are examined. Although the numerical model, using a realistic conductivity, gives the best comparison, none of the thermal spike models very accurately describe the radial energy profile with time. Initially, energy is collisionaly transported along nearest neighbor directions. Very rapidly a melt front forms, which has nearly constant radial extent, and a pressure wave moves outward in the region not melted, carrying off part of the total energy deposited. In paper II these results are applied to electronic sputtering of condensed-gas solids. (C) 1998 Elsevier Science B.V. All rights reserved.