Spectroscopic study of laser-produced plasmas in hydrogen

A 300-mJ, 7-ns, 1064-nm pulse from a Q-switched neodymium-doped yttrium aluminum garnet laser was tightly focused into ${\mathrm{H}}_2$ at pressures of the order of 1 atm or less. Charge-coupled-device images, time-resolved schlieren, and time-resolved spectra from the vacuum ultraviolet through the visible distinguish the prompt centrally localized emission during the initial breakdown from the subsequent blast wave and shock dissociation of the gas. Schlieren images show that the shock wave expands from the center with a velocity in excess of 13 km/s, and then slows to close to sound wave speed within 1.5 μs. Inside the shock front, starting with an initial electron density greater than 4×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and a temperature of ${10}^5$ K, the emitting atomic gas cools and leaves a low-density core surrounded by a cylindrical shell of slowly expanding excited H about 1.5 mm from the focal axis after 8 μs. The regions responsible for visible and vacuum ultraviolet emission are in local thermodynamic equilibrium. The density, temperature, and flow agree quantitatively with the predictions of Saha ionization and excitation equilibrium in a cylindrical blast wave with counterpressure. These models characterize the postplasma gas close to the region of breakdown and permit the source to be used for quantitative spectroscopic studies of atomic and molecular processes.