Non-Boltzmann population distributions during single-bubble sonoluminescence.

Single-bubble sonoluminescence (SBSL) spectra from aqueous sulfuric acid solutions containing dissolved neon show widely varying emission despite being similar in chemical composition. From a 65 wt % solution, emission from hydroxyl radicals is observed, with the rovibronic progression being well-described by a single temperature of 7600 K. From an 80 wt % solution, however, emission spectra reveal vibrationally hot sulfur monoxide (SO; Tv = 2400 K) that is also rotationally cold (Tr = 280 K). Further, the SO vibrational population distribution is best-described by a non-Boltzmann distribution. Excited neon atom emission observed from the 80 wt % solution gives an estimated temperature of only 3400 K, indicative of emission from a cool outer shell at the interfacial region. The neon atom excited-state population is also best-described by a non-Boltzmann distribution. These observations are consistent with SBSL emission having both a spatial and temporal component, and the implications for these effects are discussed.