Using hyperfine structure to investigate perturbations between highly-excited states: the HF C-X spectrum

It has long been known that the B $^1\Sigma^+$ ion-pair state in HF is strongly perturbed by electronic Rydberg states [1]. We present a quantitative analysis of these perturbations through their effects on the fluorine orbital magnetic hyperfine parameter obtained from XUV spectra of the C $^1\Pi$, $v$=0-X $^1\Sigma^+$, $v$=0 transition [2]. A $\Lambda$-doubling interaction between the ground vibrational level of the C-state and the nearby $v$=29 level of the B-state produces an apparent rotational state dependence in the values of this parameter derived from the R-branch lines. This work demonstrates how insight into the extent of inter-state perturbations can be obtained from the variation of hyperfine parameters.\\[4pt] [1] A. E. Douglas and F. R. Greening, Can. J. Phys. {\bf 57}, 1650 (1979).\\[0pt] [2] J. N. Philippson, R. C. Shiell, E. Reinhold and W. Ubachs, J. Chem. Phys. {\bf 129}, 174310 (2008).