CAVITATION THERMOMETRY USING MOLECULAR AND CONTINUUM SONOLUMINESCENCE

The use of molecular and continuum emission spectra from multiple bubb le (MB) and single bubble (SE) sonoluminescence (SL) is explored as a probe of bubble temperature during cavitational collapse. It is propos ed that molecular and continuum SL arise from different chemical pathw ays, which occur during discrete intervals along the cavitational coll apse time line, thus yielding different cavitation temperatures. A cou pled bubble dynamics/chemical kinetic model of cavitational collapse i s developed and used to explore a variety of proposed molecular SL mec hanisms for the C-2(d-->a), CN(B-->X), and OH(A-->X) emissions. Molecu lar SL is shown to arise from chemiluminescent reactions of seed molec ules (e.g., hydrocarbons, N-2, H2O) and their dissociation products, a nd occurs during the early and middle stages of cavitational collapse. This emission is broadly characterized as originating from reactions involving singly or multiply bonded molecular precursors with correspo nding effective emission temperature ranges of approximately 3000-8000 and 8000-25 000 K, respectively. An analysis of an experimentally obs erved CN(B-->X) MBSL spectrum is reported which is consistent with CN emission occurring over a broad distribution of cavitation temperature s ranging from approximately 5000 to 15 000 K. Continuum SL is attribu ted to transitions of electrons produced by high-temperature ionizatio n and confined to voids in the dense fluid formed during the latter st ages of cavitational collapse. The continuum is similar for both SBSL and MBSL, and is characterized by a temperature range of approximate t o 20 000-100 000 K. The observation of significant molecular emission for MBSL, and not for SBSL, is attributed to the broad distribution of initial bubble sizes for MBSL. In SBSL, a single bubble is repetitive ly cycled through collapse and reexpansion, and its collapse is driven well into the continuum emission regime. In MBSL, only a small fracti on of the bubbles will be driven to this level of collapse, while a mu ch larger fraction will attain only the single or multiple bond chemis try regimes. Thus in MBSL the bubble size distribution averaged emissi on will tend to enhance the molecular relative to the continuum emissi on. It is concluded that both SBSL and MBSL are consistent with an adi abatic compressional heating description of bubble collapse.