Spectrum of Luminescence from Laser-induced Bubbles in Water and Cryogenic Liquids

The spectrum of the luminescence pulse from collapsing laser-induced bubbles in water and cryogenic liquids has been measured. For small bubbles in water the spectrum approximates that of a black-body at about 7800 K, while for bubbles whose maximum radius is greater than 1 mm the OH* molecular band at 310 nm becomes apparent, a feature which is also seen in multi-bubble sonoluminescence. The larger bubbles also show considerable instabilities in the vicinity of the collapse minimum, where jets and fission events are observed. In the liquid nitrogen and liquid argon the the luminescence is found to consist of discrete lines from chromium atoms, which are compressed in the bubble collapse to a temperature of about 4500 K. The chromium apparently results from the vaporization by the laser of stainless steel microparticles floating in the liquids, possibly knocked off the cell walls by the strong shock waves generated by the laser focusing.