OPTICAL LASER SPECTROSCOPY AND HYPERFINE-STRUCTURE INVESTIGATION OF THE 102S, 112S, 82D AND 92D EXCITED-LEVELS IN FRANCIUM

The authors present an extension to low-lying states of francium (10s, 11s, 8d, 9d) of a previously published high-resolution experiment performed on the ns and nd Rydberg levels. These four levels were investigated at CERN with the on-line mass separator ISOLDE using stepwise laser excitation in collinear geometry. Accurate energy measurements lead to a more reliable determination of the quantum defects of the whole ns and nd series, together with the ionization potential for 212Fr (nuclear spin I=5) which is found to be 32848.872 (9) cm-1. The measurement of the fine structure splittings for the nd 2D3/2, 5/2 doublets was completed at the same time. The main result of the study is the first observation and measurement of hyperfine structures in 2S and 2D states. Dipolar hyperfine constants deduced from hyperfine splittings are as follows: A(10 2S1/2) = 401 (5) MHz; A(112S1/2) =225 (3) MHz; A (82D3/2) = 13.0 (6) MHz; A(9 2D3/2)= 7.1 (7) MHz; A(82D5/2) = -7.2 (6) MHz and A(92DS5/2) = -3.6 (4) MHz. The negative sign of the A(n2Dslz)/A(n2D5,?) ratios is a clear indication of the large core polarization effects as already pointed out in the case of other heavy alkali elements. A survey of all the available energy terms is also given to summarize the current knowledge of the francium optical spectrum. © 1990 IOP Publishing Ltd.