Single-Bubble Cavitation Spectra Indicate Plasma Formation

Experimental investigations into the spectra of sonoluminescence have offered evidence to support the existence of a plasma core during the implosive collapse of bubbles in liquids irradiated with ultrasound. Kenneth S. Suslick and David J. Flannigan of the University of Illinois at Urbana-Champaign report in the March 3 issue of Nature that the spectra of cavitating bubbles filled with argon, xenon, sulfur monoxide or oxygen in sulfuric acid are indicative of temperatures as high as 15,000 K and of dissociation and ionization by collisions with high-energy electrons in a plasma.

Sonoluminescence occurs in bubbles exposed to an acoustic wave in a liquid. In the work, the researchers employed a piezoelectric element attached to a quartz flask to induce cavitation, and a monochromator and CCD detector to collect the spectral data. The choice of sulfuric acid yielded sonoluminescence intensities more than three orders of magnitude greater than in previous experiments, enabling the collection of more detailed spectra from single bubbles.

The spectra of single argon bubbles fit well with the calculated emission spectrum of atomic argon heated to 15,200 K and suggested the population of excited states indicative of collisions of the argon with high-energy particles from a plasma core. Similarly, the spectra of single oxygen bubbles revealed dioxygenyl cation emissions. Because more than 18 eV of energy is required to form the excited ion, the investigators propose that high-energy electrons from a plasma are responsible.