Mechanism of the Ru(bpy)32+ single-bubble sonochemiluminescence in neutral and alkaline aqueous solutions

The paper addresses sonochemiluminescence generated by a single bubble levitating in neutral and alkaline aqueous solutions of Ru(bpy)(3)Cl-2. Generally, this sonochemiluminescence was found to obey the rules identified previously for multibubble sonochemiluminescence and to be described by reactions between the Ru(bpy)(3)(2+) ion and the radical products H-center dot, OH center dot, and e(aq) in solution. These products arise in the bubble upon decay of low-temperature plasma and migrate to the solution as the bubble intensively moves and its spherical shape is distorted as a result of a change in the acoustic pressure above and below the stability threshold. Some radicals can also get into the solution from a stationary bubble. The difference from the multibubble sonochemiluminescence is that luminescence intensity has a higher contribution of the re-emission effect of bubble luminescence by Ru(bpy)(3)(2+) ions. In solutions with low concentrations of the complex (10(-6)-10(-5) M) and the bubble being stationary, this effect provides up to 2/3 of the total ruthenium emission intensity. It is suggested that the hydrated electrons, which increase the yield of sonochemiluminescence, can be generated upon single-bubble sonolysis of neutral solutions rather than only upon conversion of H atoms in alkaline solutions, as is the case for the multibubble sonochemiluminescence.