Plasma formation and temperature measurement during single-bubble cavitation

Single-bubble sonoluminescence (SBSL1-5) results from the extreme temperatures and pressures achieved during bubble compression; calculations have predicted(6,7) the existence of a hot, optically opaque plasma core(8) with consequent bremsstrahlung radiation(9,10). Recent controversial reports(11,12) claim the observation of neutrons from deuterium - deuterium fusion during acoustic cavitation(11,12.) 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 spectra(13) 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 (O-2(+)) 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 emitters(14-16). The emissive excited states observed from both Ar and O-2(+) 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 O-2 is more than twice its bond dissociation energy, so O-2(+) 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.