Oscillator Strengths in Complex Atoms: Applications to N IV

Abstract

N IV is responsible for some prominent features in Wolf Rayet stars. There is however a curious absence of $$\lambda 5820\,2p3p\,^3P-2p3d\,^3P^0$$. Neither has this line been found in the laboratory. This work was undertaken with a view to explaining that absence, but the methods employed and the corresponding computer program may be used for the calculation of oscillator strengths of any complex atom. It is based on a program for the calculation of atomic structure allowing for configuration interaction. It is found that in 16 out of the 34 lowest terms configuration interaction is considerable. The absence of N IV λ5820 may be explained as the effect of a very small branching ratio against the transition from $$2p3d\,^3P^0\,\text{to}\,{2p}^{2}\,^3P$$. That the spectral feature around λ5810 in WN stars cannot be due to $$2p3p\,^3P-2p3d\,^3P^0$$ is demonstrated by invoking the transition $$2s3s\,^3S-2p3d\,^3P^0$$ from the same upper level which gives a line at λ7413. This line is not observed although its transition probability is three times higher than that of $$2p3p\,^3P-2p3d\,^3P^0$$. For many atoms, effects of configuration interaction must be taken into account if one wishes to make accurate calculations of energy levels, transition probabilities and collision cross sections. A general purpose computer program for the calculation of wave functions, allowing for the interaction of many configurations, has been written by Eissner & Nussbaumer (1969). The present paper gives a first description of the use of this program in conjunction with a program for the calculation of radiative transition probabilities. Results are obtained for the four-electron system of N IV, since this system is of interest for the interpretation of the spectra of Wolf Rayet stars.