Fast translational thermalization of extreme disequilibrium induced by cluster impact

Abstract

Impact heating of cold molecular clusters moving at high velocities dissipates extreme amounts of energy (often more than several eV per atom) in very short times. Molecular dynamics simulations of larger rare gas clusters show that this excess energy is thermalized in 100 fs or less, depending on cluster size and impact velocity. Dissipation is also extensive for smaller clusters but these shatter before being fully thermalized. A simple analytical hard sphere model that recovers this behavior is discussed. The model attributes the ultrafast relaxation to the random orientation of the interatomic distance before the collision. A perfectly ordered array of atoms is indeed found not to relax. Such an array also allows for a dispersion-free propagation of a shock front. The route to equilibrium is therefore the efficient mixing in phase space caused by the velocity components after the collision having a random part. The implications for the maximum entropy description of cluster impact induced chemistry, for the production of electronically excited and ionic species and for electron emission are discussed.