O--ion radiation in the spectrum of an oxygen arc plasma

The radiation emitted from the axis of a wall-stabilized, low-current cylindrical arc in pure oxygen at temperatures around 9000 K was measured quantitatively in the wavelength range from 2000 to 9000 A. In the near i.r. region, the two first thresholds of the affinity continuum appear at 8615 and 8465 A, respectively, yielding an electron affinity (EA) of 1.465 eV and a ground-state splitting Δ = 26 meV for the negative oxygen ion. Another threshold occurs in the u.v. region at 3630 A and involves a transition to O^- from the first exited state of atomic oxygen. The continuous spectrum can be shown (Fig. 17) to be dominated in the u.v. and visible regions by the affinity continuum due to the negative oxygen ion. Additional contributions, which become increasingly important towards longer wave-lengths, are the Kramers-Unsöld continuum and O₂⁺-association radiation. In the u.v., the recombination of two neutral oxygen atoms forms a small contribution. A broad peak structure centered around 7000 A cannot be definitely explained. Applying Kirchhoff's law, the photodetachment cross section of O^- is obtained as a function of photon energy. Deviations from local thermodynamic equilibrium (LTE) are accounted for.