Calculated cross sections and measured rate coefficients for electron-impact excitation of neutral and singly ionized nitrogen

Abstract

R-matrix calculations for neutral (N I) and singly ionized nitrogen (N II) have been carried out in the energy region below 50 eV for N I and 180 eV for N II, respectively. The collision strengths were calculated for optical allowed and forbidden transitions as well as for exchange transitions involving energy levels with principal quantum numbers n⩽3. In both atomic systems, spectroscopic measurements in the visible and vacuum-ultraviolet spectral range have been performed in order to investigate several rate coefficients in the theoretically uncertain near-threshold regions. For this purpose seven N I- and sixteen N II-rate coefficients were determined by means of a 7 mm diameter stationary cascade arc discharge operated at a current of 8 A and different pressures in the range from 1 to 30 hPa. The spectroscopic system for the vacuum-ultraviolet wavelength range was intensity calibrated by means of the continuum radiation of a normal pressure pure helium arc discharge and the calibrated line emission of a hollow cathode lamp. Plasma diagnostics yielded electron densities from 1.8 to 7.0×1013 cm−3. By applying a collisional-radiative model the rate coefficients were obtained from absolute level densities. This collisional-radiative model also yielded the ground and metastable state number densities on the basis of calculated rate coefficients for optical allowed transitions. Variation of the arc pressure resulted in electron temperatures ranging from 4.5 to 8.4 eV. Over this range the calculated rate coefficients have been partly confirmed by experimental ones.