We review recent attempts to elucidate the phenomenon of sonoluminescence in terms of fundamental principles. We focus mainly on the processes which generate the light, but other relevant facts, such as the bubble dynamics, must also be considered for the understanding of the physics involved. Our emphasis is on single bubble sonoluminescence which in recent years has received much attention, but we also look at some of the excellent work on multiple bubble sonoluminescence and its spectral characteristics for clues. The weakly ionized gas models were recently studied most thoroughly and are remarkably successful when combined with a hydrodynamic bubble model, in terms of reproducing observed spectral shapes, intensities, optical pulse widths and the dependencies of these observables on the experimental parameters. Other radiation models, such as proton tunnelling radiation and the con ned electron model, were not combined with hydrodynamic models and/or have freely adjustable parameters so that their relevance to sonoluminescence studies is at present less critically tested.

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.

Sonoluminescence spectra of the heavy rare gases are calculated by combining the Hilgenfeldt et al. model of sonoluminescence [Phys. Fluids 11, 1318 (1999)] with quantum line-shape calculations of electron-neutral-atom bremsstrahlung spectra [L. Frommhold, Phys. Rev. E 58, 1899 (1998)]. Good agreement between theoretical and experimental spectra is obtained by choosing values of the ambient radius R0 and acoustic pressure amplitude P(a) that are compatible with diffusive equilibrium calculations.

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The dipole surface of H2–He collisional pairs is computed from first principles for 20 intramolecular spacings of the H2 molecule, from 0.6 to 4 Bohr; 11 separations of the H2–He pair, from 2 to 6 Bohr; and five values for the angle subtended by the intramolecular and intermolecular axes. From this dipole surface, the dipole matrix elements are obtained for all possible rotovibrational transitions |vJ〉→|v′J′〉, with v and v′=0,… ,14. Subsequently, the collision-induced emission spectra are computed for frequencies from 100 to 100 000 cm−1, at temperatures of thousands of Kelvin—a range believed to be important for various light sources of high gas densities, such as the atmospheres of “cool” stars, shockwaves, flames, rocket jets, etc. We find that at the lower temperatures considered, radiation is emitted mostly in the fundamental band of H2, while at high temperatures the collision-induced emission spectra extend into the visible, with overtone transitions involving large Δv=v−v′.