Multi-bubble sonoluminescence of phosphoric acid

Abstract

Multi-bubble sonoluminescence spectra of 85% H3PO4 and the dependences of sonoluminescence intensity on the acid concentration and temperature are obtained. The spectra contain a weakly structured 300–600-nm band formed by the superposition of radiation from several emitters (presumably, oxygencontaining products of acid sonolysis, viz., PO, HOPO, and PO2). Weak luminescence at a wavelength exceeding 600 nm can be due to emission from excited O* and Ar* atoms. The shape of the fundamental band changes upon a transition from multi-bubble sonolysis to sonolysis in the setup for one-bubble sonoluminescence, in which several clusters of cavitation bubbles are formed in a spherical flask at ultrasonic frequencies multiple of the first acoustic resonance frequency (multi-cluster sonoluminescence). The form of the temperature dependence of the sonoluminescence intensity depends on the detection regime: for natural heating of 85% acid under the action of ultrasound, a curve with a luminescence peak at 40°C is observed, while in detection with preliminary thermostating “over points,” only an inflection exists on a monotonic curve describing a decrease of intensity upon heating. An analogous curve for acids with a lower viscosity (hydrochloric and nitric acids) has neither a peak nor inflection irrespective of the detection regime. It is concluded that the viscosity of phosphoric acid plays a decisive role in the evolution of cavitation and in obtaining intense sonoluminescence.