Plasma formation and temperature measurement during single-bubble cavitation


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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Figure 1: SBSL spectra from 85% H2SO4(aq.) and pure water regassed with Xe and Ar (solid lines); apparent fits to blackbody spectra are given as dashed lines.
Figure 2: SBSL spectra from 85% H2SO4(aq.).
Figure 3: Emission temperatures of SBSL of 85% H2SO4(aq.) regassed with Ar/Ne mixtures (acoustic pressure 3 bar) are shown as a function of the thermal conductivity of the gas mixtures.
Figure 4: Vibronic progressions in SBSL spectra.


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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.

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Correspondence to Kenneth S. Suslick.

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Flannigan, D., Suslick, K. Plasma formation and temperature measurement during single-bubble cavitation. Nature 434, 52–55 (2005).

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