Spectral Fingerprints in Sonoluminescence from Electron-Driven Excitation

Sonoluminescence (SL) converts acoustic or impulsive mechanical forcing into sub-microsecond light emission from collapsing bubbles. Across regimes (single-bubble SL,multibubble SL, and laser-induced single-bubble collapse), spectra frequently show a broad-band continuum but can also display reproducible line/band “fingerprints” tied to specificemitters (notably OH* band emission near 310 nm and alkali lines such as Na* near 589 nm).We develop and defend a minimal thesis: the photons at the flash moment are producedprimarily by electron-driven processes, with spectral fingerprints arising from electron-impactexcitation of identifiable chemical species created at collapse; solute-enriched alkali emis-sion emerges when interfacial availability is high; and continuum dominance increases withacoustic drive amplitude. We formalize this thesis with a compact collisional–radiative (CR)model coupled to a minimal electron-energy distribution closure, derive line-to-continuumscaling laws against drive amplitude, dissolved-gas composition, ionic strength, and soluteconcentration, and provide a falsifiable closure program (gated spectroscopy + continuum sub-traction + parameter sweeps) capable of distinguishing electron-impact excitation dominancefrom purely thermal, featureless interpretations. The framework is consistent with reportedOH-band observations in collapsing single bubbles and in MBSL, Na-line observations insaline solutions and seawater, time-resolved spectral evolution in SBSL, and precursor alkaliline emission attributable to radiation trapping.