Experimental Absorption Cross Sections Research Articles

Ultraviolet Absorption of Atomic Nitrogen in its Ionization Continuum*

A method has been developed to measure absolute absorption coefficients of atomic nitrogen in the region of its ionization continuum below 800 A. This was achieved by passing radiation from an appropriate light source through the plasma of a Philips Ionization Gauge (P.I.G.) type discharge into a normal incidence vacuum spectrograph. Light intensities at various wavelengths between 400 A and 800 A were recorded on film and measured photometrically for the cases of the absorbing plasma either on or off. By taking into account information on the concentration of ions in such discharges, and by considering the relative probabilities of formation of other plasma constituents from their appearance potentials by electron impact methods, it was possible to estimate the concentration of atomic nitrogen in the plasma and to show that aside from neutral N2 atomic nitrogen was the sole cause of the attenuation of light. The experimental absorption cross sections were found to agree well with the theoretical values of Bates and Seaton.In order to support some of the assumptions used, separate experiments were performed in which the light source pulse was passed through the plasma region after the P.I.G. discharge had been extinguished. It could be shown that the ions had been swept out and that excited and metastable states had also disappeared. Thus, the only remaining plasma component was atomic nitrogen, and its absorption cross sections were within the error limits the same as those of the first investigation.