The anomalous hyperfine structure of Al II

Abstract

The hyperfine structure of a large number of transitions in Al II cannot be described using A and B hyperfine constants and the hyperfine structure is therefore said to be anomalous. In this paper, we have studied the hyperfine structure of a few transitions in Al II, 3s5s3S–3s5p3P, 3s4d3D–3s5p3P and 3s5p3P–3s5d3D, by combining theory and experiment. It is shown that the anomalous hyperfine structure is due to strong off-diagonal hyperfine interaction resulting not only in a deplacement of the energies of the hyperfine levels, but also resulting in large intensity redistribution among the individual hyperfine lines. It is shown that the hyperfine mixing in 3s4d3D and 3s5d3D is very large, whereas small but not negligible in 3s5p3P. By combining experimental spectra and theory we could obtain accurate wavefunctions for the 3s4d3D and 3s5d3D hyperfine levels which were used to calculate the gf-values of all individual hyperfine transitions not only for 3s5p3P, but also for 3s3p3P and 3s4p3P, where the off-diagonal hyperfine interaction leads to negligible intensity redistribution.