A standing acoustic wave is known to cause a gas microbubble to successively expand and contract. And in a sufficiently intense field, the bubble will implode, heat to temperatures several times that of the Sun’s surface, and emit a brief burst of light known as sonoluminescence (SL). But the intrabubble conditions that give rise to SL are largely a mystery. New experiments by Seth Putterman (UCLA) and coworkers suggest that at least in some cases, SL is attributable to the formation of a dense plasma. Each time the team fired a 3-ns laser pulse at a sonoluminescing xenon bubble, they observed a bright spot near the bubble’s edge, as seen in this CCD image. (Blue depicts brighter regions; black depicts darker ones.) The implication is that the bubble was opaque enough to absorb the pulse on entry. That finding, coupled with the fact that absorption in the bubble is mediated mainly by interactions between free electrons and ions, led the team to conclude that nearly 20% of the bubble’s Xe atoms had ionized. The result comes as a surprise: Existing SL models predict an ionization fraction closer to 1%. Putterman and colleagues think the disparity may be due largely to charge screening, which isn’t accounted for in the existing models but can lower an atom’s effective ionization energy. (S. Khalid et al., Phys. Rev. Lett. 108, 104302, 2012.)—Ashley G. Smart
