Abstract
Spectra of single-bubble sonoluminescence (SBSL) in aqueous luminol solutions were measured under acid and basic conditions in order to study the mechanism of sonochemiluminescence (SCL). A rectangular quartz glass cell of 56×56×70 mm^3 internal dimensions was used as the SBSL cell. The bottom of the cell was attached to a bolted Langevin-type transducer, which was driven by a function generator through an amplifier at 24.5 kHz. SBSL spectra were recorded by a spectroscope and ICCD system through a quartz optical fiber with 0.1 s exposures and 300 cycles accumulation. The results show that in the case of a stable bubble, which is normal SBSL case, the SBSL emission was absorbed by luminol in the excitation spectrum range, but no emission was observed in the fluorescence spectrum range. The absorbance depends on pH of the solutions. In the case of dancing bubbles in the solution which dissolved more than 1 % argon, where an unstable bubble grew and ejected daughter bubbles repeatedly, the fluorescence was observed at pH=11.2 but not at pH=2.5. These results are consistent with the pH dependence of multibubble SCL. It is found that the SCL occurs only at unstable collapse of bubbles. Therefore, the dynamic behavior of bubbles is important in sonochemistry related to reactions of radicals produced from cavitation collapse.
