Modified surface nanoscale explosion: Effects of initial condition and charge flow

Molecular dynamics (MD) simulations have been performed to study surface na noscale explosion due to slow highly charged ion (HCI)-surface interactions . In order to understand the interplay between the mechanisms for surface m odification and the dynamical consequences of the explosion, a new simulati on model is formulated to include the electronic degrees of freedom in an e mpirical manner. In this model, surface ionization occurs at a finite rate and surface charges are allowed to flow into the substrate at various rates simultaneously. In one of the simulations based on the simultaneous ioniza tion and charge migration (SICM) model, 100 excitations (positively charged surface ions) occur during the first 24 fs, which is longer than the in-th e-substrate neutralization time of the HCI (approximately 10 fs) deduced fr om experimental measurements. At the same time, positively charged surface ions are allowed to migrate away from the center region at an average speed of approximately 40 Angstrom per picosecond. Compared to the results from pure Coulomb explosion in which charge exchange between surface atoms and s urface ion is not allowed, the strength of the nano-explosion is not weaken ed but somewhat enhanced. When the time interval for ionization is reduced to instant charging but. with other conditions unchanged, little influence on the formation of a crater was found between the two cases. The finite ti me interval for building up the charged region only postponed the formation of the repulsive center by approximately 25 fs and slightly lowered the pe ak value of the Coulomb repulsion. The explosion strength starts to decreas e, however, as the speed of the charge flow in the substrate increases. In a test simulation, an estimation of a lower bound of surface damage as a fu nction of surface energy deposition is provided by monitoring the dynamics according to the energetics of the systems. Dynamical consequences of these surface processes are studied by a comprehensive analysis of energetics, t emperature, pressure, and structural information. We also discuss the relev ance of the current model to HCI-surface experiments as well as to future m odeling and simulations.