Luminescence of sodium atoms in aqueous solution during sonolysis in moving-single-bubble regime

Emission from excited sodium atoms (Na 589 nm) under the conditions of single-bubble sonoluminescence (SBSL) in the moving-bubble regime has been observed in aqueous NaCl solutions at concentrations within 2.5–6 mol/L. Atomic line emission in the gas phase was caused by the penetration of metal salt species inside the bubble, followed by the transition of Na atom to an excited state. Saturation with argon and a low temperature (from -10 to -15$^\circ$C) of solution are factors that favor the SBSL of metal in solution of a nonvolatile salt. The results confirm the validity of the model of microdrop injection that explains the appearance of metal inside a bubble, its excitation, and subsequent atomic line emission during SBSL as related to deformations of the moving bubble.