Evidence for a plasma core during multibubble sonoluminescence in sulfuric acid

Multibubble sonoluminescence arises from the cavitation of bubbles owing to high-intensity ultrasonic irradiation of liquids. The spectrum of multibubble sonoluminescence in concentrated sulfuric acid sparged with Ar, at relatively low acoustic power, consists of a broad continuum extending into the UV with SO and Ar emission lines on top of this continuum. The observation of the Ar lines indicates that an optically opaque plasma is probably generated inside the bubble during cavitation. There are three distinct light-emitting morphologies for multibubble sonoluminescence from an ultrasonic horn; as the acoustic intensity is increased, the emission changes from filamentous, to bulbous, to cone shaped, and it is only from the filamentous emission that strong atomic and molecular emissions are observed.