Sonoluminescence in solutions of organic aromatic luminophores: Monomer and excimer luminescence of benzene, toluene and p-xylene

Abstract

This paper considers the spectra of glow that emerges during the sonolysis of benzene, toluene and p-xylene solutions in ethylene glycol under the action of a translationally motionless bubble levitating in the ultrasonic standing wave with a frequency of ~27 kHz or a moving bubble (single-bubble, or moving single-bubble sonoluminescence, SBSL, or m-SBSL, respectively). The m-SBSL spectra against the background of the broadband solvent continuum demonstrate the presence of characteristic luminescence bands of luminophores in the range of 270–305 nm (singlet excited monomers) and a band of singlet excited dimers (excimers) of these molecules with a maximum of 320 nm at a luminophores concentration in solution being 10−3-10−2 M. For 1.25∙10−3 M benzene solution, there were also recorded the bands of СН and С2 radicals occurred as a result of decomposing this luminophore. At concentrations higher than 10−2 M, luminophores luminescence is quenched and the m-SBSL spectra reveal only the continuum, the intensity of which also decreases with an increase in the luminophores concentration. This continuum is recorded without any significant change in its shape and intensity, with no bands of luminophores and radicals for the luminophores concentrations specified, and during SBSL. The obtained data are indicative of the penetration of luminophores molecules, in the m-SBSL regime, from the solution into the moving and deformed bubble according to the known sonochemical mechanism to inject solution nanodroplets and also their further collisional excitation in the nonequilibrium intrabubble plasma which repeatedly formed during oscillations of bubble in the acoustic field. The nonequilibrium plasma state determines the presence of luminescence both luminophore molecules (gas temperature about 103 K or less) and radical products of their decomposition (gas temperature is thousands K).