H−shape-resonance studies in an arc plasma

Calculations by Macek have shown that the ${\mathrm{H}}^{-}$ photoabsorption cross section should be affected by a shape resonance at 1129.5 Å. This experiment is an attempt to observe the resonance in emission according to the reaction $e+\mathrm{H}\to {\left[{\mathrm{H}}^{-}\right]}^{*}\to {\mathrm{H}}^{-}+h\nu$. The plasma is in a condition of local thermodynamic equilibrium and is generated by a stationary wall-stabilized hydrogen arc. With an axis temperature of 14000 K and 1 atm pressure, the ${\mathrm{H}}^{-}$ free-bound continuum contributes only about 2% of the total radiation which is dominated in the 1130-Å region by the Ly-$\alpha$ wing. However, the peak of the shape resonance, according to Macek, should have a cross section about 25 times greater than the continuous free-bound cross section; therefore, it should appear as a very noticeable 50% structure superimposed on the Ly-$\alpha$ wing. Except for some small features which are attributed to weak molecular emission, there is no obvious indication of the shape resonance in either deuterium or hydrogen spectra between 1105 and 1135 Å. It is estimated that the minimum feature which could have been detected at 1129 Å was about 2% of the total signal.