Sonochemistry and sonoluminescence: effects of external pressure

The oxidation of iodide by 1 MHz continuous ultrasound and the sonoluminescence in water created by pulsed ultrasound were investigated in the pressure range from 0.7 to 3 bar. At low intensities, the chemical yield and the luminescence intensity decrease with increasing pressure. At higher intensities the yields increase moderately with increasing pressure. There is a critical intensity above which a steep decrease in the chemical and luminescence yields occurs. This critical intensity is strongly shifted to higher values with increasing pressure. Thus, by applying higher pressures at intensities above the critical intensity, the chemical yields and the luminescence intensity are increased by a factor of more than 10. The effects are not dependent on the nature of the gas under which the irradiation occurs. The effects are discussed in terms of the current theories of cavitation and gas bubble dynamics. The effects can be rationalized by assuming that there is a decreased rate of bubble formation with increasing pressure but that the probability that a particular bubble initiates chemical reactions increases with increasing pressure