Spectroscopic Studies of Nonequilibrium Plasma Generated by Acoustic Cavitation in Aqueous Solutions
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Molecular emissions in multibubble sonoluminescence (MBSL) spectra offer a powerful experimental tool to probe the extreme conditions inside the cavitation bubbles. Emission spectra of OH(C2Σ+-A2Σ+), OH(A2Σ+-X2Π) and NH(A3Π-X3Σ-) systems revealed the formation of nonequilibrium plasma during multibubble cavitation in aqueous solutions saturated with noble gases. In nonequilibrium plasma, the electron temperature (Te) exceeds the vibrational (Tv) and rotational (Tr) temperatures: Te>Tv>Tr≈Tg, where gas temperature (Tg) is usually considered equal to Tr. Spectroscopic analysis highlighted two kinds of plasmas produced by acoustic cavitation. Weakly excited plasma is observed at low ultrasonic frequency (20 kHz) in the presence of Ar and characterized by near-Boltzmann behavior of OH(A2Σ+) and NH(A3Π) species. On the other hand, strongly excited plasma is formed at high frequency ultrasound (200-1000 kHz) and in the presence of Ar or Xe and even at 20 kHz in the presence of Xe. In a strongly excited plasma, the relative population distribution of OH(A2Σ+) vibrational states deviates from Boltzmann equation. This type of nonequilibrium is created by vibration-vibration pumping (Treanor) mechanism. In general, spectroscopic study of MBSL allows to conclude that sonochemistry can be considered as a part of plasma chemistry.