Abstract
Intense orange Na* emission was observed in different spatial locations from blue emission during multibubble sonoluminescence in sulfuric acid. The color change from blue to orange was observed along the streamer in the filamentous structure of a bubble cloud. By stroboscopic observation, the Na* emission seemed to occur when a large bubble ejected tiny bubbles at the positions toward a pressure node after bubble coalescence around a pressure antinode. The intensity of Na* emission in the sulfuric acid case increased at lower frequency in contrast with the water case. By comparing a high-resolution Na* spectrum of sulfuric acid with that of water, the widths of the spectra were almost the same, except for something being superimposed in the water case. The estimations of the temperature and pressure inside the Na* emission bubbles at 28 and 150 kHz were 1900 K and 100 atm and 2200 K and 150 atm, respectively.