Plasma formation and temperature measurement during single-bubble cavitation

The phenomenon known as single-bubble sonoluminescence (SBSL) has been the focus of intense investigation since it was discovered 15 years ago, leading to predictions that extreme temperatures are reached within the cavity at extreme compression. Conditions, some have controversially claimed, that could even lead to nuclear fusion. The lack of features in the typical SBSL spectrum has made it difficult to establish what is happening within the bubble. But now, using concentrated sulphuric acid as the medium subjected to acoustic treatment, Flannigan and Suslick have obtained the most intense sonoluminescence yet seen. This provides plenty of spectral information — most importantly, evidence for temperatures as high as 15,000 K — indicating that the collapsed bubble has a hot plasma core. 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.