Comparison of sonoluminescence and water vapor plasma spectra

An experimental study was carried out to find the mechanism of OH spectra generation through sonoluminescence. Line spectra of OH, H, and O were observed very clearly in water vapor plasma generated by 2.45-GHz microwave irradiation under the pressure of 40 hPa; however, broadband continua were not detected. Multibubble sonoluminescence (MBSL) and single-bubble sonoluminescence (SBSL) have continua, and OH spectra were detected occasionally in MBSL. However, OH spectra were not detected in SBSL and higher temperature MBSL. The sources of emission in MBSL and SBSL were considered to be identical and differences in spectra of OH bands were attributable to the higher temperature. Experimental results obtained using low temperature water vapor plasma confirm that OH spectra depend on the bubble’s interior plasma temperature: that is, at low temperatures, spectra of OH (308 nm), Ha, Hb, and O are clearly visible in water vapor plasma.