Which light-emission process produces single-bubble sonoluminescence spectra?
In plain words
The flash could come from a hot gas glowing like a light-bulb filament, from free electrons braking near ions in a thin plasma (ionized gas), or from excited atoms and molecules. Which process dominates, and does it change with the gas and the liquid?
Precise statement
Discriminate between optically thin thermal bremsstrahlung and electron-ion recombination in a weakly ionized gas, optically thick blackbody emission from the core, and atomic or molecular line emission, by predicting together the photon number per flash ($\sim 1e5 \text{ to } 1e7$), the spectral shape from $2e-5 \text{ to } 8e-5\,\mathrm{cm}$ wavelength, and the pulse width (in water about $4e-11 \text{ to } 3.5e-10\,\mathrm{s}$, independent of wavelength; in concentrated sulfuric acid longer, of order $1e-9\,\mathrm{s}$), using the measured values for each liquid. The answer must hold for argon and xenon bubbles in both water and sulfuric acid.
What would settle it
A single emission model that reproduces absolute spectra, photon counts and pulse widths across gases and liquids, combined with a diagnostic of the optical thickness of the core.
Status in the literature
Unverified note
The weakly ionized thermal bremsstrahlung picture from the late 1990s fits featureless water spectra, but line emission seen in sulfuric acid since 2005 shows that the dominant process depends on the liquid (not re-verified for 2024-2026).