Broadening by neutral collisions following recombination in laser-produced plasmas

When the 532 nm second harmonic of a pulsed YAG laser is focussed into molecular hydrogen, a compact plasma is created close to the focal spot. For a 350 mJ laser pulse with a duration of 7 nsec, electron densities in excess of 1017 cm-3 are observed if the molecular gas density is of the order 1019 cm-3 or greater. Since the emission spectrum in the vacuum ultraviolet is free of electrode impurities, the profile of Lyman α and contributions from H2, H3, and H3+ can be identified without interference. The reversal of the atomic resonance line that is seen as these spectra evolve during and following the excitation pulse offers a simple way to investigate the broadening in the neutral atomic gas that develops