About thirty years ago, new ways of investigating the level structure of excited states were opened by the radio-frequency experiment of Lamb and Retherford(1) and by the optical double resonance (ODR) technique(2) suggested by Brossel and Kastler.(3) Together with the level-crossing(4,5,7) and the anticrossing techniques,(6,9) these methods enabled spectroscopists to study the structure of excited states with unprecedented accuracy. Further, they allowed the extraction of information about the strength of internal couplings in atomic states, lifetimes, the influence of external perturbations, and also provided much insight into the interaction between atoms and photons. Of course, all these methods are interrelated, and Figure 1 summarizes some of their characteristics. The ODR and rf techniques make use of changes in the population differences between the two states when a resonant radio-frequency field is applied. Changes in the population are detected as intensity changes in the resonance fluorescence radiation. In a similar way, the populations of the two states tend to equalize in anticrossing experiments when the two states approach each other. In level-crossing experiments, on the other hand, interference takes place between the amplitudes describing the rates for the absorption and the emission of the resonance line at the intersection of the Zeeman levels.
KeywordsMagnetic Field External Perturbation Stark Shift Static Electric Field Electron Impact Excitation
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