Anomalous results obtained for long-path measurements of both continuum absorption and level populations in a pure recombining hydrogen plasma

The continuum absorption in a pure hydrogen plasma between 6400 and 6545 AA has been measured using laser spectroscopy over a 400 m path length. Contributions to the opacity from a number of processes have been calculated, including the bound-free absorption from high-lying neutral hydrogen levels, estimated from a direct interferometric measurement of the n=2 level population. Theoretical values for the n=2 level population, calculated using a 20-level collisional-radiative code, disagree with the measured values by up to a factor of three, and calculations of the absorption fail to explain the measured results by approximately an order of magnitude. No explanation for these discrepancies is obvious.