Nature of sonoluminescence: Noble gas radiation excited by hot electrons in cold water

It was proposed before that single bubble sonoluminescence (SBSL) may be ca used by strong electric fields occurring in water near the surface of colla psing gas bubbles because of the flexoelectric effect involving polarizatio n resulting from a gradient of pressure. Here we show that these fields can indeed provoke dynamic electric breakdown in a micron-size region near the bubble and consider the scenario of the SBSL. The scenario is (i) at the l ast stage of incomplete collapse of the bubble, the gradient of pressure in water near the bubble surface has such a value and a sign that the electri c field arising from the flexoelectric effect exceeds the threshold field o f the dynamic electrical breakdown of water and is directed to the bubble c enter; (ii) mobile electrons are generated because of thermal ionization of water molecules near the bubble surface; (iii) these electrons are acceler ated in "cold" water by the strong electric fields; (iv) these hot electron s transfer noble gas atoms dissolved in water to high-energy excited states and optical transitions between these states produce SBSL UV flashes in th e transparency window of water; (v) the breakdown can be repented several t imes and the power and duration of the UV flash are determined by the multi plicity of the breakdowns. The SBSL spectrum is found to resemble a black-b ody spectrum where temperature is given by the effective temperature of the hot electrons. The pulse energy and some other characteristics of the SBSL are found to be in agreement with the experimental data when realistic est imates are made.