Nonequilibrium Vibrational Excitation of OH Radicals Generated During Multibubble Cavitation in Water

Abstract

The sonoluminescence (SL) spectra of OH(A(2)Σ(+)) 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(A(2)Σ(+)) v' = 1-4 vibrational states as well as the vibronic temperatures (T(v), T(e)) 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 (T(e) ~ 0.7 eV and T(v) ~ 5000 K). By contrast, at high-frequency ultrasound, the plasma inside the collapsing bubbles exhibits Treanor behavior typical for strong vibrational excitation. The T(e) and T(v) values increase with ultrasonic frequency, reaching T(e) ~ 1 eV and T(v) ~ 9800 K at 1057 kHz.

Plasma Formation during Acoustic Cavitation: Toward a New Paradigm for Sonochemistry

“…It was concluded that the observation of OH(C 2 Σ + -A 2 Σ + ) emission indicates the formation of nonthermal plasma inside the cavitation bubble [44]. Spectroscopic analysis of OH(A 2 Σ + -X 2 Π i ) vibrational transitions supports this hypothesis [22]. The relative populations of OH(A 2 Σ + ) ] = 1-4 vibrational states have been calculated using an optical thin plasma model.…”

“…• recombination, water molecule de-excitation, and OH(B 2 Σ + -A 2 Σ) emission [22,44]. Figure 1 shows the experimental SL spectra in argon measured at different ultrasonic frequencies normalized on the most intense OH( (0-6; 0-7; 0-8; 0-9)…”

“…The electron temperature ( ) often significantly exceeds the vibrational ( V ), rotational ( ), and translational ( 0 ) temperatures: > V > ≈ 0 [39]. Table 2 summarizes the optimized values of and V estimated from the simulated MBSL spectra [22]. It shows that the nonequilibrium plasma generated during the sonolysis of water sparged with argon obeys the classical inequality > V .…”

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…In this instance the electron temperature (T e ) is much greater than the vibrational (T v ), rotational (T r ) and translational (T g ) temperatures in the order T e > T v > T r ≈ T g [81]. This moves away from an adiabatic model and indicates that, rather than considering the effects of temperature inside the bubble from the viewpoint of a single value, there is in fact a multi-temperature environment governed by differing particle energies and degrees of freedom.…”

“…At lower frequencies, radicals inside the bubbles have more time to follow pathways analogous to flame reactions, whereas at high frequencies, diffusion into the bulk solution increases and more hydroxyl radicals are released into the liquid [167]. Spectral emissions from multibubble sonoluminescence in argon atmosphere have shown that vibrational excitations of the plasma inside the bubble can change with frequency [81]. A disparity in the population of vibrational levels can have significant effects on the reactions taking place for several sonochemical systems [80].…”