Spectroscopy of Sonoluminescence and Sonochemistry in Water Saturated with N 2 -Ar Mixtures Temim Ouerhani, Rachel Pflieger,* Warda Ben Messaoud, and Sergey I. Nikitenko Institut de Chimie Sé parative de Marcoule, UMR5257, UM-CEA-CNRS, Centre de Marcoule, BP 17171, 30207 Bagnols-sur-Ce ̀ ze cedex, France * S Supporting Information ABSTRACT: Sonoluminescence spectra in relation with sonochemical activity of water sparged with Ar/N 2 gas mixtures were systematically studied at two ultrasonic frequencies (20 and 359 kHz). At 20 kHz, solely the molecular emission of OH (A 2 Σ + -X 2 Π i ) is observed in addition to a broad continuum typical for multibubble sonoluminescence. On the contrary, at high frequency a second emission band is present around 336 nm which is assigned to the NH (A 3 Π-X 3 Σ - ) system. In addition, the sonolysis of a 0.2 M NH 3 ·H 2 O solution at 359 kHz in the presence of pure Ar yields the emission bands of NH (A 3 Π - X 3 Σ - ) (336 nm) and NH (C 1 Π-A 1 Δ) (322 nm) systems confirming the sonochemical production of NH radicals. The N 2 (C 3 Π u -B 3 Π g ) emission band is absent at both frequencies. This uncommon phenomenon can be explained by the quenching of the N 2 (C 3 Π u ) excited state with water molecules inside the bubbles. The sonoluminescence of NH radicals at 359 kHz indicates more effective intrabubble dissociation of N 2 molecules at high ultrasonic frequency compared to low-frequency (20 kHz) ultrasound. Its absence at 20 kHz may also be related to strong quenching, e.g., by water molecules. The kinetic study of the formation of principal sonochemical products (H 2 ,H 2 O 2 , HNO 3 , HNO 2 ) confirmed the more drastic conditions produced during bubble collapse at higher ultrasonic frequency. 1. INTRODUCTION Sonochemistry, or in other words the chemical effects of ultrasound, originates from acoustic cavitation: the nucleation, growth, and implosive collapse of gas bubbles in liquids submitted to an ultrasonic field. 1 Recent spectroscopic studies of multibubble sonoluminescence (MBSL) in water saturated with noble gases revealed the formation of a nonequilibrium plasma during bubble collapse. 2,3 In principle, MBSL spectros- copy is quite universal: a thorough analysis of the MBSL spectra allows researchers to probe the intrabubble conditions and to identify the chemically reactive species generated inside the cavitation bubbles. 2-4 However, the application of MBSL to better understand the reaction mechanisms occurring under acoustic cavitation is only beginning to emerge. This paper focuses on the study of MBSL and sonochemical reactivity in water saturated with N 2 -Ar gaseous mixtures. The sonochemistry of nitrogen in aqueous solutions was pioneered in 1936 by Schultes and Gohr. 5 They reported the formation of NO 2 - and NO 3 - under the effect of 900 kHz ultrasound in water sparged with a N 2 -O 2 mixture. Much later Misik and Riesz 6 suggested that H 2 O 2 and NO 2 - were the primary products of water sonolysis in the presence of air and that NO 3 - ion resulted from the secondary oxidation of nitrite ion by hydrogen peroxide. According to Wakeford et al., 7 the highly reactive oxygen required for NO x formation from molecular nitrogen would come from the dissociation of oxygen molecules. The occurrence of the latter reaction was confirmed by ultrasonically driven isotopic exchange in the O 2 -H 2 O system. 8 The NO production is supposed to occur by Zeldovich mechanism: 9 − → O ))) 2O 2 (1) + → + N O NO N 2 (2) + → + N O NO O 2 (3) + → + • N OH NO H (4) where the symbol “)))” indicates a reaction initiated by the cavitation event. Then, further oxidation takes place induced by OH • radicals (originated from H 2 O molecules homolytic dissociation) or by O 2 molecules: 10 − → + • HO ))) H OH 2 (5) + → • NO OH HNO 2 (6) + → 2NO O 2NO 2 2 (7) + → • NO OH HNO 2 3 (8) By contrast, the sonochemistry of nitrogen in the absence of oxygen has been much less studied. It was reported that the sonolysis of a H 2 -N 2 mixture in water at 380 11 and 900 kHz 12 led to NH 3 formation suggesting the dissociation of both H 2 and N 2 molecules inside the cavitation bubbles: Received: October 19, 2015 Revised: December 2, 2015 Published: December 8, 2015 Article pubs.acs.org/JPCB © 2015 American Chemical Society 15885 DOI: 10.1021/acs.jpcb.5b10221 J. Phys. Chem. B 2015, 119, 15885-15891