Evidence for a plasma core during acoustic cavitation in sulfuric acid.

Abstract

Extreme temperatures and pressures are produced through, acoustic cavitation: the formation, growth and collapse of bubbles in a liquid irradiated with high intensity ultrasound. Single bubbles have generally been assumed to give higher temperature conditions than bubble clouds, but confirmation from the single bubble sonoluminescence (SBSL) emission spectra has been problematic because SBSL typically produces featureless emission spectra that reveal little about the intra-cavity physical conditions or chemical processes. Here definitive evidence of the existence of a hot, highly energetic plasma core during SBSL is presented. From a luminescing bubble in sulfuric acid, excited state to excited state emission lines are observed both from noble gas ions (Ar+, Kr+, and Xe+) and from neutral atoms (Ne, Ar, Kr, and Xe). The excited states responsible for these emission lines range from 8.3 eV (for Xe) to 37.1 eV (for Ar+) above the ground state. Observation of emission lines allows for identification of intra-cavity species responsible for light emission; the energy levels and bandshapes of the emitters indicate that the plasma generated during cavitation is comprised of highly energetic atomic and ionic species with Ne as large as 10×D21/cc. Ionization and plasma can also be created during multi-bubble sonoluminescence in sulfuric acid.