Effects of rare-gases on MBSL spectrum of K atom emission

Sonoluminescence from alkali-metal salt solutions has been studied because emission mechanism from non-volatile alkali-metal ions has been interested. We measured multi-bubble sonoluminescence spectra from KCl solutions saturated with Ar, Xe and He gases at temperatures in the range of 15 40 °C at the frequency of 148 kHz. For Arsaturated solutions, the spectral line width of K atom emission, which broadened asymmetrically to the red side, was independent of temperature whereas the K line intensity decreased with increasing temperature. These results showed that an amount of water vapor does not affect on the K-line width but on the line intensity. The results for Xesaturated solution indicated that the spectrum of K atom emission is composed of two peaks, unshifted narrow line and shifted broad line. We also observed in some ultrasound-irradiation condition that each K line was clearly separated into two peaks. In contrast to the cases of Ar and Xe saturation, the spectrum in He-saturated solution showed symmetrically-broadened doublet of K line which are shifted to blue side by 0.21 nm. These results strongly suggest that the excited K atoms are perturbed by rare gase inside bubbles. The rare-gas effect observed is in good agreement with results by gas-phase spectroscopy. We conclude that K atom emission occurs in gas phase inside bubbles.