Spectroscopy of Sonoluminescence and Sonochemistry in Water Saturated with N₂–Ar Mixtures

Abstract

Sonoluminescence spectra in relation with sonochemical activity of water sparged with Ar/N₂ gas mixtures were systematically studied at two ultrasonic frequencies (20 and 359 kHz). At 20 kHz, solely the molecular emission of OH (A²Σ⁺–X²Π_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 (A³Π–X³Σ^–) system. In addition, the sonolysis of a 0.2 M NH₃·H₂O solution at 359 kHz in the presence of pure Ar yields the emission bands of NH (A³Π – X³Σ^–) (336 nm) and NH (C¹Π–A¹Δ) (322 nm) systems confirming the sonochemical production of NH radicals. The N₂ (C³Π_u–B³Π_g) emission band is absent at both frequencies. This uncommon phenomenon can be explained by the quenching of the N₂ (C³Π_u) excited state with water molecules inside the bubbles. The sonoluminescence of NH radicals at 359 kHz indicates more effective intrabubble dissociation of N₂ 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 (H₂, H₂O₂, HNO₃, HNO₂) confirmed the more drastic conditions produced during bubble collapse at higher ultrasonic frequency