  Sonoluminescence is the phenomena of light emission from a collapsing gas
bubble in a liquid. Theoretical explanations of this extreme energy focusing
are controversial and difficult to validate experimentally. We propose to use
molecular dynamics simulations of the collapsing gas bubble to clarify the
energy focusing mechanism, and provide insight into the mechanism of light
emission.
  In this paper, we model the interior of a collapsing noble gas bubble as a
hard sphere gas driven by a spherical piston boundary moving according to the
Rayleigh-Plesset equation. We also include a simple treatment of ionization
effects in the gas at high temperatures. By using fast, tree-based algorithms,
we can exactly follow the dynamics of million particle systems during the
collapse. Our results clearly show strong energy focusing within the bubble,
including the formation of shocks, strong ionization, and temperatures in the
range of 50,000---500,000 degrees Kelvin. Our calculations show that the
gas-liquid boundary interaction has a strong effect on the internal gas
dynamics. We also estimate the duration of the light pulse from our model,
which predicts that it scales linearly with the ambient bubble radius.
  As the number of particles in a physical sonoluminescing bubble is within the
foreseeable capability of molecular dynamics simulations we also propose that
fine scale sonoluminescence experiments can be viewed as excellent test
problems for advancing the art of molecular dynamics.
