DEFINING THE UNKNOWNS OF SONOLUMINESCENCE

As the intensity of a standing sound wave is increased the pulsations of a bubble of gas trapped at a velocity node attain sufficient amplit ude so as to emit picosecond hashes of light with a broadband spectrum that increases into the ultraviolet. The acoustic resonator can be tu ned so that the flashes of light occur with a clocklike regularity: on e flash for each cycle of sound with a jitter in the time between flas hes that is also measured in picoseconds. This phenomenon (sonolumines cence or ''SL'') is remarkable because it is the only means of generat ing picosecond flashes of light that does not use a laser and the inpu t acoustic energy density must be concentrated by twelve orders of mag nitude in order to produce light. Light scattering measurements indica te that the bubble wall is collapsing at more than 4 times the ambient speed of sound in the gas just prior to the light emitting moment whe n the gas has been compressed to a density determined by its van der W aals, hard core. Experiments indicate that the collapse is remarkably spherical, water is the best fluid for SL, some noble gas is essential for stable SL, and that the light intensity increases as the ambient temperature is lowered. In the extremely stable experimental configura tion consisting of an air bubble in water, measurements indicate that the bubble chooses an ambient radius that is not explained by mass dif fusion. Experiments have not yet been able to map out the complete spe ctrum because above 6 eV it is obscured by the cutoff imposed by water , and furthermore experiments have only determined an upper bound on t he hash widths. In addition to the above puzzles, the theory for the l ight emitting mechanism is still open. The scenario of a supersonic bu bble collapse launching an imploding shock wave which ionizes the bubb le contents so as to cause it to emit Bremsstrahlung radiation is the best candidate theory but it has not been shown how to extract from it the richness of this phenomenon. Most exciting is the issue of whethe r SL is a classical effect or whether Planck's constant should be invo ked to explain how energy which enters a medium at the macroscopic sca le holds together and focuses so as to be emitted at the microscopic s cale.