On the perturbation in the f values of the principal series of Ba I

The authors report new measurements of f values for the principal series of Ba I for n values extending up to n=42. The measurements, performed by Faraday rotation spectroscopy using a tunable dye laser, are believed to extend to higher values of n than any previous quantitative determinations. They cover a prominent series perturbation due to the 5d8p 1 P 1 doubly excited autoionising line, and a comparison is made between the authors' data and the predictions of multichannel quantum defect theory extrapolated from lower values of n.