Single-bubble sonoluminescence (SBSL) results from the extreme temperatures and pressures achieved during bubble compression; calculations have predicted the existence of a hot, optically opaque plasma core with consequent bremsstrahlung radiation. Recent controversial reports claim the observation of neutrons from deuterium-deuterium fusion during acoustic cavitation. However, there has been previously no strong experimental evidence for the existence of a plasma during single- or multi-bubble sonoluminescence. SBSL typically produces featureless emission spectra that reveal little about the intra-cavity physical conditions or chemical processes. Here we report observations of atomic (Ar) emission and extensive molecular (SO) and ionic (O2+) progressions in SBSL spectra from concentrated aqueous H2SO 4 solutions. Both the Ar and SO emission permit spectroscopic temperature determinations, as accomplished for multi-bubble sonoluminescence with other emitters. The emissive excited states observed from both Ar and O2+ are inconsistent with any thermal process. The Ar excited states involved are extremely high in energy (>13 eV) and cannot be thermally populated at the measured Ar emission temperatures (4,000-15,000 K); the ionization energy of O2 is more than twice its bond dissociation energy, so O2+ likewise cannot be thermally produced. We therefore conclude that these emitting species must originate from collisions with high-energy electrons, ions or particles from a hot plasma core.
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Acknowledgements This work was supported by the National Science Foundation and the US Defense Advanced Research Projects Agency. We acknowledge conversations with F. Grieser on the mechanism of Ar atom emission, and with L. A. Crum, D. Lohse, W. C. Moss and S. J. Putterman.
Acknowledgements S.D.H. thanks J. Neureuther and M. Lazarova for their assistance in the Galactic Centre transient monitoring programme. We thank D. Chakrabarty, G. Denn, C. Dermer, W. Erickson, R. Remillard, K. Weiler and K. Wood for discussions. The Very Large Array (VLA) is operated by the National Radio Astronomy Observatory (NRAO), which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities. Basic research in radio astronomy is supported at Sweet Briar College by funding from the Jeffress Memorial Trust and Research Corporation. Basic research in radio and X-ray astronomy at the NRL is supported by the Office of Naval Research.