Quasi-molecular satellites in the Lyman series of atomic hydrogen

This work is an experimental study of the emission of Lyman β by a laser-produced hydrogen plasma. Observations of the profiles of the Lyman series lines of atomic hydrogen in a class of cool white dwarf stars reveal temperature-sensitive features that are due to radiative collisions between the absorbing hydrogen atom and perturbing protons. The recent theoretical models of spectral line formation in ionized hydrogen plasmas appear to account for the features well, although some disagreement remains. The models also account for previous observations of Lyman alpha in laser plasmas. Here we extend the laboratory observations into the windowless vacuum ultraviolet. The goal of the experimental work is to obtain spectra that may be compared with the same fundamental theory used to explain white dwarf spectra. The line shape theory depends in turn on recent calculations of the potentials of $H+2$ and H₂, and on calculations of the dipole transition moments as a function of interatomic separation which are in progress. The fundamental problem addressed by the experiments described here is to obtain photon flux through the vacuum spectrometer from the plasma sufficient to observe the continuum emission in the far wings of the lines, while maintaining low pressure in the spectrometer and high hydrogen pressure in the light source. The effectiveness of small apertures and differential pumping is described, and a system that allows the laser plasma to operate at 400 Torr H₂ while the vacuum chamber remains at 10−4 Torr is studied. Preliminary spectra show Lyman β with a resolution of 0.1 Å and reveal continuum between Lyman α and Lyman β associated with radiative collisions in the plasma.