Observation of the far wing of Lyman α due to neutral atom and ion collisions in a laser-produced plasma

The time-resolved spectrum of a laser-produced plasma in ${\mathrm{H}}_2$ exhibits a continuum that we attribute to radiative collisions of H atoms in a $2p$ state with ground-state neutral atoms and protons. This binary collision Lyman-α line wing observed from 1150 Å to 1700 Å is emitted by the shock dissociated gas that surrounds the focal region. The observed temperature, electron density, and shock front speed confirm models of the shock that predict that Lyman α arises from a shell of atomic hydrogen in which the neutral atom and ion densities are sufficient to create the observed line broadening. The experimental far wing spectrum agrees with unified theory calculations of the Lyman-α line, which allow for the dependence of the radiative dipole moment on internuclear separation during a radiative collision. Broad neutral atom effects become evident at 1180 Å, 1260 Å, and 1600 Å when the density of neutral perturbers is of the order of ${10}^{20}{\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^3,$ while features at 1230 Å, 1240 Å, and 1400 Å appear when the plasma is highly ionized. These satellites result from free-free transitions of a H atom during collisions with other neutral atoms and protons and are correlated with potential curves and radiative dipole moments of ${\mathrm{H}}_2$ and ${\mathrm{H}}_2^{+}.$