Single-Bubble Sonoluminescence of Colloidal Suspensions of Europium(II) Salt Nanoparticles

Colloidal suspensions of EuCl₂, EuBr₂, and EuSO₄ nanoparticles (<50 nm) in dodecane and EuSO₄ in 70% H₂SO₄ were synthesized. Moving single-bubble sonoluminescence (m-SBSL) spectra were obtained for a bubble performing radial oscillations in these suspensions and translational motions at the antinode of a standing ultrasonic wave with a frequency of about 27 kHz. In these spectra (at a spectral resolution of 10 nm), the sono-excited luminescence bands of the Eu²⁺ ion were detected for the first time, coinciding in the shape and position of the maxima (404, 413, and 377 nm for EuCl₂, EuBr₂, and EuSO₄, respectively) with the bands of Eu²⁺ located in a crystalline environment in the photoluminescence spectra of nanoparticles of europium salts in suspensions. The detected sonoluminescence of Eu²⁺ arises due to the injection of nanoparticles into a bubble deformed during motion and excitation of a lanthanide ion at the periphery of the bubble volume during collisions of nanoparticles with charged particles, mainly electrons, coming from a hot nonequilibrium plasma, which periodically arises during bubble compression. Evidence for the excitation of the europium ion in the bubble is the absence of its luminescence bands in the SBSL spectra of the translationally immobile bubble, in which nanoparticles are unlikely to enter. The nanoparticles that enter the bubble also undergo decomposition in the plasma into fragments, in particular, with the formation of Eu, Eu⁺ in the excited state. The atomic lines of these fragments were recorded for the first time in the m-SBSL spectrum with a resolution of 1 nm for a suspension of EuSO₄ nanoparticles in 70% H₂SO₄. The resulting m-SBSL spectra will add to the library of characteristic spectra of objects of sonoluminescent spectroscopic analysis and will make it possible to identify and determine the content of Eu or Eu²⁺ in these objects.