Light emitting processes in sonoluminescence
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Radiative processes of interest in single bubble sonoluminescence (SBSL) studies, such as electron-atom and electron-ion bremsstrahlung; electron-atom polarization bremsstrahlung; recombination radiation; radiative attachment of electrons to hydrogen and oxygen atoms; and charge exchange, ion-atom bremsstrahlung and ion-atom association radiation in helium, are considered and their relative importance is discussed. We argue that processes involving strictly neutral species, e.g., collision-induced emission, are most likely insignificant for the continuum emission of SBSL. However, we include the band emission of excited OH radicals which has been observed recently in spectra of very dim SBSL bubbles. All radiative processes are investigated in the context of a hydrodynamical model of sonoluminescent rare gas bubbles in water which accounts for time variations of the water vapor content; chemical reactions; and the ionization of the rare gas and the H2O dissociation products. Peak temperatures exceed 10,000 K at densities of a few hundred amagat (≈ 1028 particles per m3 ). The gas mixture in the bubble is weakly ionized. Spectral shapes, spectral intensities and durations of the light pulses for helium, neon, argon, krypton and xenon bubbles are computed. We generally obtain good agreement with observations for photon numbers and pulse durations. Some calculated spectral profiles agree, however, less well with the experiment, especially in the case of low water temperatures and for helium bubbles. We show that by allowing the bubble to heat somewhat non-isotropically, agreement of observed and computed spectral profiles may be obtained, even in the case of helium bubbles at freezing water temperatures. Our model also correctly reproduces experimental evidence that at low driving frequencies, despite a more violent bubble collapse, light emission is comparable to that at higher driving frequencies. We discuss why measurements could not detect sonoluminescence emission in the microwave window of water and describe our understanding of the OH band spectra emitted by very dim SBSL bubbles.