Electron-Stimulated Sonochemiluminescence of Ce3+ Ions in Liquid Ammonia

Abstract

This is the first study of the light emission during the sonolysis of liquid ammonia and of solutions of cerium and silver salts in this solvent at –70°C. A Ce³⁺ luminescence band with a maximum at 400 nm in the moving singlebubble sonoluminescence spectrum generated by ultrasound in a solution of 5·10^–4 M Ce(SO₄)₂ in ammonia was discovered. This band was found in addition to the bubble luminescence continuum usual for liquids (240–800 nm in ammonia). The quenching of this band in the presence of AgNO₃ in ammonia solution indicates that electron acceptor Ag⁺ ions suppress the probable sonochemiluminescence reaction Ce⁴⁺ + \({e}_{s}^{-}\) → η(Ce³⁺)^* + (1 − η) Ce³⁺, (Ce³⁺)^* → Ce³⁺ + hν responsible for the detected Ce³⁺ glow. The concentration of Ce4+ in the solution is diminished when it is reduced in this reaction and the sonoluminescence intensity of the Ce³⁺ band also decreases during the sonolysis but then reaches a constant level a few minutes after its onset. This behavior is attributed to the attainment of a dynamic equilibrium between the rates of cerium ion reduction and oxidation: Ce⁴⁺ → Ce³⁺ and Ce³⁺ → Ce⁴⁺, respectively. The one-electron oxidizing agent in the latter reaction is presumably the NH₂ radical, which is another primary product of ammonia sonolysis, in addition to a solvated electron (\({e}_{s}^{-}\)). The formation of Ce³⁺ cations during the sonolysis of a Ce(SO₄)₂ solution was confirmed by the detection of Ce³⁺ photoluminescence. The excitation yield in the sonochemiluminescence reaction η = 2·10^–6.