Measurement of pressure and density inside a single sonoluminescing bubble

The average pressure inside a sonoluminescing bubble in sulfuric acid has been determined by two independent techniques: (1) plasma diagnostics applied to Ar atom emission lines, and (2) light scattering measurements of bubble radius vs time. For dimly luminescing bubbles, both methods yield intracavity pressures similar to 1500 bar. Upon stronger acoustic driving of the bubble, the sonoluminescence intensity increases 10 000-fold, spectral lines are no longer resolved, and radius vs time measurements yield internal pressures > 3700 bar. Implications for a hot inner core are discussed.