Field ionisation study of high lying Rydberg states of Na

Abstract

The authors describe the details of an experiment using a sodium atomic beam and two dye lasers. By stepwise excitation they have selectively populated highly excited ns, nd and, using Stark mixing, np states fron n=20-50. The excited atoms were detected by field ionisation. The results obtained for the ionisation threshold fields for these states are presented and compared with the theoretical classical law Fc=(2n*)-4. A suitable field shape and detection technique allows the observation of multiple thresholds with high sensitivity. Multiple thresholds for nd states have been observed, as expected. Some ns singlet states also show multiple thresholds. Using suitably polarised laser beams the mod mL mod dependence of the ionisation threshold fields for the 20d state has been observed. The approach proposed recently by Gallagher for predicting the threshold fields is generally confirmed.