Nonequilibrium Vibrational Excitation of OH Radicals Generated During Multibubble Cavitation in Water Abdoul Aziz Ndiaye, † Rachel Pflieger, † Bertrand Siboulet, † John Molina, ‡ Jean-Franc ̧ ois Dufre ̂ che, † and Sergey I. Nikitenko* ,† † Institute for Separation Chemistry of Marcoule (ICSM), UMR 5257 - CEA-CNRS-UMII-ENSCM, BP 17171, 30207 Bagnols sur Cè ze, Cedex, France ‡ UPMC-Universite ́ Paris 06, UMR 7195, PECSA, F-75005 Paris, France * S Supporting Information 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. ■ INTRODUCTION The OH • radicals are important reaction intermediates in a large variety of advanced oxidation processes initiated by acoustic cavitation in aqueous solutions. 1,2 These species are produced during the violent implosion of gas-filled micro- bubbles in liquids submitted to power ultrasound. In water saturated with noble gases, acoustic cavitation is accompanied not only by the generation of chemically reactive species but also by light emission, named sonoluminescence (SL). 3 Spectroscopic analysis of the SL spectra helps to better understand the origin of the extreme conditions inside the cavitation bubbles. Despite numerous studies, the real nature of SL is still an open question. The multibubble SL spectra in water saturated with argon are composed of the emission lines of excited OH • radicals and a broad continuum ranging from UV to near-infrared (NIR) spectral ranges, which probably results from the superposition of several emission bands: H + OH • recombination, water molecule de-excitation, and OH- (B 2 Σ + -A 2 Σ + ) emission. 4 Recently, Pflieger et al. 5 reported SL from OH(A 2 Σ + ) and OH(C 2 Σ + ) excited states in water saturated with noble gases at various ultrasonic frequencies. These results clearly showed the strong effects of gas nature and ultrasound frequency on the relative intensities of OH(A 2 Σ + -X 2 Π i ) (0-0) and (1-1) transitions. Moreover, the observation of OH(C 2 Σ + -A 2 Σ + ) emission in the presence of Kr and Xe revealed nonthermal plasma formation during multibubble cavitation in water. 5 The spectroscopic analysis of OH(A 2 Σ + -X 2 Π i ) emission lines appears to be a very useful tool to study the non-Boltzmann behavior of the plasma generated within the cavitation bubbles. However, the vibrational transitions in SL spectra are still poorly investigated and are mostly only used to identify excited species. The major difficulty in the quantification of SL spectral data is that OH(A 2 Σ + -X 2 Π i ) emission yields dense overlapping vibrational structures. Rotational structures of these emission lines are not observed in SL most probably due to strong Doppler, collisional, or van der Waals broadening. This paper focuses on the development of an original approach where OH(A 2 Σ + - X 2 Π i ) emission lines in multibubble SL spectra are deconvoluted in order to probe the intrabubble conditions via the determination of the vibrational population distribution of the OH(A 2 Σ + ) state for various ultrasonic frequencies. ■ EXPERIMENTAL SECTION The multifrequency ultrasonic device for SL measurements is shown in the Supporting Information (Figure 1SI). In brief, the thermostatted cylindrical reactor was mounted on top of a high- frequency transducer (25 cm 2 ) providing power ultrasound at Received: February 29, 2012 Revised: May 3, 2012 Published: May 3, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 4860 dx.doi.org/10.1021/jp301989b | J. Phys. Chem. A 2012, 116, 4860-4867