Experimental results for single-bubble sonoluminescence of air bubbles at very low frequency f = 7.1 kHz are presented: In contrast to the predictions of a recent model [S. Hilgenfeldt and D. Lohse, Phys. Rev. Lett. 82, 1036 (1999)], the bubbles are only as bright (10(4)-10(5) photons per pulse) and the pulses as long (approximately 150 ps) as at f = 20 kHz. We can theoretically account for this effect by incorporating water vapor into the model: During the rapid bubble collapse a large amount of water vapor is trapped inside the bubble, resulting in an increased heat capacity and hence lower temperatures, i.e., hindering upscaling. At this low frequency water vapor also dominates the light emission process.

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A streak camera with high spatial and temporal resolution was used for imaging the dynamics of the violent collapse in single-bubble sonoluminescence. The high pressure in the last phase of the bubble collapse leads to the emission of a shock wave, which is launched with a shock velocity of almost 4000 m/s. The shock amplitude decays much faster than approximately 1/r. From the strongly nonlinear propagation the pressure in the vicinity of the bubble can be calculated to be in the range of 40-60 kbar.

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The dynamics of a single-air bubble trapped in a resonant sound field in water has been characterized by Mie scattering and a Streak camera with high spatial and temporal resolution. The streak images show that in the endphase of the cavitation collapse the scattered light intensity is no function of the bubble radius anymore. In the last nanoseconds around minimum bubble radius most of the light is scattered at the highly compressed water surrounding the bubble and not at the bubble wall. This leads to a minimum in the scattered light intensity about 700 ps before the sonoluminescence pulse is emitted. And neglecting this changes leads to a strong overestimation of the bubble-wall velocity. In the reexpansion phase the high spatial resolution of the streak camera allows one to distinguish between the light scattered at the bubble wall and the light scattered at the outgoing shock wave.