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Multibubble sonoluminescence (MBSL) spectra were collected from solutions containing volatile metal-containing compounds in silicone oil saturated with helium or argon. Ultrasonic irradiation of these solutions at 20 kHz and 90 W/cm 2 led to emission from excited states of the free metal atoms. The widths and peak positions of these lines varied as a function of the gas used to saturate the solution, indicating that the emission was from the gas phase of the bubble. High-resolution MBSL spectra showed that the pressure within the bubble at the point of emission was on the order of 300 bar in argon-saturated silicone oil, which is consistent with simple adiabatic compression during cavitation.
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McNamara et al. (2003) studied this question.
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