Observation of a New Phase of Sonoluminescence at Low Partial Pressures

The acoustically driven pulsations of a gas bubble lead to 1$0^6$-fold changes in its volume and the emission of a light flash upon collapse. Mass diffusion between the bubble and the gas dissolved in the surrounding fluid maintains this steady-state bubble motion only at low partial pressures, around 3 Torr. This diffusion-controlled regime is uniquely favorable to sonoluminescence (SL) from hydrogenic gases and polyatomic gases with low adiabatic heating. Our analysis indicates that the previously investigated SL from bubbles at 200 Torr requires a nondiffusive mass flow mechanism.