Sonoluminescence from alkali-earth metal salts in sulfuric acid solutions

Sonoluminescence (SL) from alkali-earth metal salts of calcium, strontium and barium sulfate in concentrated sulfuric acid solutions was observed. The emission regions of the line and band spectra associated with the alkali-earth metals were spatially separated from the continuum, similarly to those of alkali metals. The spectra in the emission region of alkali-earth metal salts showed excited neutral atom emissions and metal hydroxide molecular emissions, which were similar to those in flame. However, the excited emission lines of monovalent cations were also observed. The emissions of monovalent cations were never observed in alkali metal salt solutions. Furthermore, the relative intensity of emission from the excited monovalent cation seemed to be much higher than that in flame. On the other hand, the emissions of divalent cations were not observed. The results imply that the excitation mechanism of monovalent cation may not be thermal excitation but electron impact excitation or other excitations.