A Quest to Find the Mechanism for the Formation of Excited State Metal Atoms During Acoustic Cavitation

Line emission from excited state metal atoms (e.g., Na*) can be observed in sonoluminescence (SL) spectra when aqueous solutions containing metal salts, such as NaCl, are sonicated. With a view to understanding the metal ion reduction process, we have investigated the influence of various experimental parameters, such as, counter ions, dissolved gases, the presence of volatile solutes, ultrasound frequency, etc. on the intensity of emission from Na*. An identical emission intensity from Na* was observed from aqueous solutions containing 1 M sodium ions with chloride or bromide or iodide as counter ions. However, the emission intensity was affected by counter ions, such as, alkyl sulfonates, alkyl carboxylates. The presence of such surface active counter ions enhanced the Na* emission intensity significantly. We postulate that reduction of Na and excitation of Na atoms occur in a hot shell around the collapsing bubbles.