Studies on sonoluminescence in the form of resonance radiation

Abstract

Recent studies have shown that measured flash width from single and multibubble sonoluminescence are in the subnanosecond or even picosecond regime. Here we provide conclusive experimental evidence for the existence of nanosecond multibubble sonoluminescence. This has become possible by our ability to find a medium from which exclusive sodium-D line resonance radiation as a form of sonoluminescence is possible. The measured flash width of this emission is found to be in the range of tens of nanoseconds and sensitively dependent on experimental parameters. Our finding is important since all the earlier pulse width measurements have been limited to emission with physical source or species responsible for observed optical radiation not being clearly identified. Statistical study on large nanosecond SL optical pulses shows the effect of gas thermal conductivity and the ratio of specific heats. We propose that the presently observed resonance radiation originates from a ‘soft’ collapse as analyzed by Kamath, Prosperetti and Egolfopoulos [J. Acoust. Soc. Am., 94, 248 (1993)]. This is supported by our computation using mathematical formulation of Prosperetti, Crum and Commander [J. Acoust. Soc. Am. 83, 502 (1988)]. Synthetic optical SL pulse generated from bubble dynamics and sodium-D line excitation computation agrees well with the experimentally observed SL optical pulse shape. Other features of the optical pulse like its dependence on gas thermal conductivity are also predicted and the general trends are in good agreement with experimental observations.