Light emission of sonoluminescent bubbles containing a rare gas and water vapor

We present numerical simulations of sonoluminescent rare-gas bubbles in water, which account for (i) time variations of the water vapor content, (ii) chemical reactions, and (iii) the ionization of the rare gas and the H2O dissociation products. Peak temperatures exceed 10 000 K at densities of a few hundred amagat ( approximately 10(28) particles per m(3)). The gas mixture in the bubble is weakly ionized. Our model accounts for the light emission by electron-atom, electron-ion, and ion-atom bremsstrahlung, recombination radiation, and radiative attachment of electrons to hydrogen and oxygen atoms, which are all more or less important for single bubble sonoluminescence. Spectral shapes, spectral intensities, and durations of the light pulses are computed for helium, argon, and xenon bubbles. We generally obtain good agreement with the observations for photon numbers and pulse durations. Some calculated spectral profiles agree, however, less well with observations, especially in the case of the low water temperature and for helium bubbles. We try to identify the reasons why computed and observed spectral profiles might discernibly differ when all other computed features considered here seem to be quite consistent with observations. We show that by allowing the bubble to heat somewhat nonisotropically, agreement between observed and computed spectral profiles may be obtained, even in the case of helium bubbles at freezing water temperatures. In this case, charge exchange radiation and related processes involving helium atoms and ions become important.