Observation of theH−shape resonance in the hydrogen-arc emission spectrum

Abstract

Theoretical calculations by Macek and others have shown that the ${\mathrm{H}}^{-}$ photoabsorption cross section should be affected by a narrow shape resonance at 1129.5 Å. This resonance is expected to cause a noticeable 1.5-Å-wide structure in the usual vacuum-ultraviolet emission spectrum of a thermal hydrogen plasma, composed of Lyman line wings, H and ${\mathrm{H}}^{-}$ continua, and molecular-${\mathrm{H}}_2$ emission lines. The present paper describes the very thorough investigation of a steady-state local-thermodynamic-equilibrium hydrogen-arc plasma in this spectral region. The arc is operated at atmospheric pressure and its axis temperature ranges from 11 300 to 13 200 K. In contrast to some earlier similar experiments, there is a clear indication of the resonance at exactly the predicted wavelength. The recorded spectra demonstrate that Macek's theory certainly overestimates the strength of the resonance. The reduced oscillator strength of Broad and Reinhardt's more recent calculations can be confirmed, but with half the peak height and twice the width predicted by this theory. This is considered to be an indication for pressure broadening of the resonance in the plasma.