Spectroscopic Observation Radiative Recombination of Ground State Oxygen Atoms

Detailed knowledge of the continuous radiation processes contributing to the emission and absorp­ tion of low tem perature plasmas, as encountered in e.g. laboratory electrical discharges, high pres­ sure discharge lamps and shock-tube plasmas is still missing in m any cases. This is the reason why active research is continuing in such a classic field of plasma spectroscopy [1, 2, 3, 4]. In a recent ex­ perim ent [5] the uv, visible and near ir radiation em itted from the axis of a low current oxygen arc w ith a tem perature near 9000 K was measured. Dom inant contributions to the radiation of this plasm a are the oxygen affinity continuum, result­ ing from electron-atom attachm ent, the oxygen electronion free-bound and free-free continuum and the so-called (>2 +-association continuum, resulting from positive ion-neutral atom recombination [1]. Additionally, a continuous molecular bandlike ra ­ diation was observed near the short wavelength end of the spectral region investigated, i.e. between 200 and 300 nm, with a maximum a t A ~ 260 nm. Figure 1 shows the relative spectral emission co­ efficient in the visible and uv spectral range for three different plasma tem peratures. The features annoted in Fig. 1 belong to the oxygen ^ -reco m b in a­ tion threshold (1), the oxygen 3S-recombination threshold (2), the negative oxygen ion 3P-attachm ent threshold (3) and the continuous band (4) which will be discussed here. The purpose of this communication is to present an explanation of the