Collision-induced IR emission spectra of impact-heated rare-gas clusters

Emission spectra of mixed rare gas clusters, heated by impact with a hard surface at hypersonic velocities, are shown to extend into the near-IR and visible regimes. The emission is due to the transient dipole that arises during the collision of dissimilar atoms. The simulations are for a cluster that remains in the electronic ground state throughout the collision and use classical dynamics to determine the positions of the atoms vs time. The spectrum is computed as the Fourier transform of the (quantum mechanical) time rate of change of the dipole of the cluster. The time dependence of the dipole velocity is obtained by replacing the positions of the atoms by the computed classical functions of time. Taking the Fourier transform of the dipole velocity rather than of the dipole itself introduces a quantal correction with the result that the computed spectrum satisfies the oscillator sum rule. Binary collisions make the major contribution to the spectrum and there are hardly any caging effects. The spectral density of emitted photons is found to be thermal with a temperature that scales linearly with the impact velocity. Using the oscillator sum rules, this temperature is related to the deformation energy of the electronic charge cloud of the cluster. The hot cluster shatters and the fragments are in translational thermal equilibrium with a mean energy that scales linearly with the energy of impact. The temperature of the emitted light is, therefore, significantly lower than the translational temperature.