Characterization of Laser Produced Underwater Plasma

Optical breakdown in water created by 10 ns pulsed Nd:YAG laser operating at $\lambda =$1064 nm was studied. Spatial and temporal information was obtained with two intensified CCD cameras while spectral data were recorded using a time-integrating spectrometer. We have studied three water samples with different impurity content (ultra-pure, distilled, and tap water) and followed the plasma evolution over a timespan of a few hundred nanoseconds. Images taken by the two synchronized cameras, systematically delayed relative to each other, show that the ``center of emission intensity'' in single plasma events moves toward the incoming laser beam. The emission is dominated by a broad, blackbody-like spectral feature with corresponding temperature of ca. 20000 K. Superimposed is a weak hydrogen Balmer-alpha line with a full width at half maximum exceeding 50 nm in some cases. Interpreted as purely Stark broadened, this width corresponds to electron densities well above 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$.