Image: Phys. Rev. Lett. 86, 4934 (2001)

Sonoluminescence¿the physical phenomenon by which sound turns into light¿is as mystifying as a magic trick. Despite 70 years of trying, scientists still cannot fully explain how a bubble of air in water focuses acoustic energy a trillionfold to spit out picosecond bursts of ultraviolet radiation. Initially physicists attributed the flashes to friction. In the late 1980s, though, they came to see that bubbles in a sound wave's path expanded and rapidly collapsed¿heating the gas inside them to temperatures hotter than the sun's surface. This collapse and heat, they determined, created a glowing plasma.

In this week's issue of Physical Review Letters, Gary A. Williams and his colleagues from the University of California at Los Angeles present evidence that lends further support to that theory. The researchers set out to explain earlier observations that the spectra of light from a single bubble lacked an emission line¿for the molecule OH¿seen from multiple bubbles. Because of the discrepancy, some had suggested that different physical mechanisms were at work and that there were, in essence, two kinds of sonoluminescence. But Williams's group proved that isn't the case, creating larger-than-usual single bubbles whose spectra included the missing emission.

Although they don't know why, the researchers say that bubble size alone seems to predict the OH line and suggest that, compared with smaller single bubbles that collapse symmetrically (top right), larger bubbles in multibubble systems are unstable (bottom right). The team further fitted the spectra to a blackbody radiation curve and showed that it corresponded to plasma at a temperature of about 8,000 degrees Kelvin. "It's a nice connecting together of the underlying physical phenomena," Ken Suslick of the University of Illinois in Urbana-Champaign told Physical Review Focus. "And the ability to recognize the OH emission line is pretty cool."

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