Scintillation activation of luminescence of terbium sulfate suspensions in aromatic hydrocarbons under sonication

Abstract

Sonotriboluminescence of terbium sulfate suspensions in aromatic hydrocarbons (benzene, toluene, or p-xylene) containing 1.5·10−2 mol L−1 of p-terphenyl in atmospheric air was investigated. An intense luminescence band of p-terphenyl was detected in the UV region (λmax = 350 nm). The luminescence spectrum coincided with the photo- and X-ray-excited luminescence spectra of analogous homogeneous solutions of p-terphenyl containing no terbium salt. p-Terphenyl suppresses weak fluorescence bands of aromatic molecules in the 270–320 nm range, which are present in the sonotriboluminescence spectra of suspensions recorded in the absence of p-terphenyl. However, it does not affect luminescence of the Tb3+ ion, which is also detected in the sonotriboluminescence spectra of the suspensions. The presence of p-terphenyl does not affect the multibubble sonoluminescence intensity and spectra of aromatic hydrocarbons, in which no fluorescence bands are visible. It was found that the intensity of p-terphenyl band increase relative to that of aromatic hydrocarbons in the series: radioluminescence excited by α-particles → radioluminescence excited by X-ray → sonotriboluminescence of the suspensions. It was concluded that excitation of aromatic hydrocarbon molecules by the sonication of suspensions is due to the action of charged particles, most likely, low-energy (up to tens of eV) electrons. The electrons are generated upon collisions and destruction of crystals initiated by shock waves generated by ultrasound. When electrons get into the liquid, they excite aromatic hydrocarbon molecules via collisions. The excitation energy is nonradiatively transferred to the activator (p-terphenyl) molecules and emitted. This process is identical to radioluminescence in organic scintillators. Thus, the ultrasonic tribological impact is actually accompanied by radiolysis of suspensions.