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Spatiotemporal Separation of Na-Atom Emission from Continuum Emission in Sonoluminescence

The spectrum of multi-bubble sonoluminescence (MBSL) from alkali-metal aqueous solutions consists of continuum emission and line emission from alkali-metal atoms. We performed spatiotemporal separation of the two types of emission to investigate origin of the emissions. Images of MBSL at 137 kHz from NaCl solution taken with a digital camera were analyzed to obtain red and blue components. The red component corresponds to the Na-atom emission (Na D lines), and the blue one to the continuum emission. The spatial range of the Na-atom emission in the images extended wider than that of the continuum emission. Spectrally divided optical pulses of sonoluminescence were observed using a high-sampling-rate oscilloscope. Timing distributions of the sonoluminescence (SL) pulses from both emissions were compared and the distribution from Na-atom emission was found to be wider than that from the continuum emission. The results indicate the following conclusions. The Na-atom emission originates from those bubbles that have a wide radius distribution and low collapse temperature. The continuum emission originates from those bubbles that have a narrow radius distribution and high collapse temperature.