Nonequilibrium Vibrational
Excitation of OH Radicals
Generated During Multibubble Cavitation in Water
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Abstract
The sonoluminescence (SL) spectra of OH(A<sup>2</sup>Σ<sup>+</sup>) excited state produced during the sonolysis
of water sparged
with argon were measured and analyzed at various ultrasonic frequencies
(20, 204, 362, 609, and 1057 kHz) in order to determine the intrabubble
conditions created by multibubble cavitation. The relative populations
of the OH(<i>A</i><sup>2</sup>Σ<sup>+</sup>) <i>v′</i> = 1–4 vibrational states as well as the
vibronic temperatures (<i>T</i><sub>v</sub>, <i>T</i><sub>e</sub>) have been calculated after deconvolution of the SL
spectra. The results of this study provide evidence for nonequilibrium
plasma formation during sonolysis of water in the presence of argon.
At low ultrasonic frequency (20 kHz), a weakly excited plasma with
Brau vibrational distribution is formed (<i>T</i><sub>e</sub> ∼ 0.7 eV and <i>T</i><sub>v</sub> ∼ 5000
K). By contrast, at high-frequency ultrasound, the plasma inside the
collapsing bubbles exhibits Treanor behavior typical for strong vibrational
excitation. The <i>T</i><sub>e</sub> and <i>T</i><sub><i>v</i></sub> values increase with ultrasonic frequency,
reaching <i>T</i><sub>e</sub> ∼ 1 eV and <i>T</i><sub>v</sub> ∼ 9800 K at 1057 kHz