Laser measurement of intensity ratio anomalies in principal series doublets of caesium Rydberg states: Does the D₁ line vanish?

The authors have measured the anomalous ratio of the oscillator strengths in the principal series doublet lines of caesium for quantum numbers n=18 to 30. The light of a frequency-doubled tunable pulsed laser was absorbed by an atomic beam of caesium and the resonances were detected by the very sensitive field ionisation method. The results show a regular increase of the ratio rho of the D₂ over D₁ line strengths as a function of n. rho values in excess of 10³ are obtained for n around 30. However, the analysis of the data rules out the possibility of a pole for rho (i.e. a disappearance of the D₁ line) in the discrete spectrum. This experiment is in very good agreement with recent theoretical calculations.