Single-bubble sonoluminescence (SBSL1,2,3,4,5) results from the extreme temperatures and pressures achieved during bubble compression; calculations have predicted6,7 the existence of a hot, optically opaque plasma core8 with consequent bremsstrahlung radiation9,10. Recent controversial reports11,12 claim the observation of neutrons from deuterium–deuterium fusion during acoustic cavitation11,12. 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 spectra13 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 H2SO4 solutions. Both the Ar and SO emission permit spectroscopic temperature determinations, as accomplished for multi-bubble sonoluminescence with other emitters14,15,16. 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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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.
The authors declare that they have no competing financial interests.
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Flannigan, D., Suslick, K. Plasma formation and temperature measurement during single-bubble cavitation. Nature 434, 52–55 (2005). https://doi.org/10.1038/nature03361
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