On luminescence and sonochemical reactions in liquid ammonia induced by ultrasound

Light emission phenomena in the single- and multibubble sonolysis of liquid ammonia and solutions of salts (NaCl, AgNO3, EuCl3, and SmCl3) in liquid ammonia at −70 оС were considered for the first time. It was found that the key mechanisms of glow excitation found for other liquids with known sonoluminescence are also implemented in this case. Under the action of ultrasound, light flashes arise in liquid NH3, caused by the formation of nonequilibrium plasma in the cavitation bubbles due to their periodic compression and strong heating and emission of photons by the plasma, characterized of a spectrally broad, structureless continuum in the 240–800 nm range. In addition, especially in solutions containing additives, there is sonoluminescence of these additives, solvent molecules, or the products of sonochemical reactions of these molecules and additives, after they get into bubbles to give individual emitters during collisional excitation in the bubble plasma. For example, sodium atoms, undergo such collisional excitation, as follows from the presence of 589 nm line in the sonoluminescence spectrum of an NaCl solution in NH3. Apart from the bubble sonoluminescence, plasma generation may give rise to glow outside the bubbles, caused by chemiluminescent reactions of the primary sonolysis products that arise in the plasma and get into the liquid. In the case of moving single-bubble sonolysis of NH3 solutions, solvated electron (es–) was shown to be present among these primary products. In particular, it can reduce trivalent lanthanide ions in solution, thus generating light-emitting electronically excited divalent ions, as was found, for example, for europium (450 nm) and samarium (755 nm). In the presence of Ag+ ions, acting as es– acceptors, this sonochemiluminescence is quenched.