Single-bubble sonoluminescence of suspensions in dodecane of porous SiO2 nanoparticles with lanthanide ions

Single-bubble sonoluminescence of colloidal suspensions in dodecane of SiO2 nanoparticles containing Ln3+ ions (Ce3+, Pr3+, Gd3+, Tb3+) was investigated. Sonoluminescence is achieved by irradiating the suspension with ultrasound (27 kHz) in a spherical standing-wave resonator. As found for all suspensions, the luminescence spectra of an immobile bubble contains a structureless continuum with no characteristic bands or Ln3+ lines, which is associated with the low probability of nanoparticle penetration into the bubble. For suspensions with Tb3+ or Gd3+ ions, the narrow bands (quasi-lines of f-f transitions) of these ions are recorded in the spectra of a bubble moving near the resonator’s centre. This is due to the entry of nanoparticles with Ln3+ ions into a bubble deformed during the motion and collisional electronic excitation of these ions in nonequilibrium bubble plasma. For suspensions with Ce3+ or Pr3+ ions, in the moving single-bubble sonoluminescence spectra, the bands corresponding to luminescent d-f transitions of these ions were not found despite their entry into the bubble similar to that of Tb3+ and Gd3+. This result is explained by much lower probability of collisional excitation during the electronic intershell 4f-5d transition in Ln3+ ions compared to the probability of such excitation upon the intrashell 4f-4f transition.