High Resolution Spectroscopy of Rydberg States

High resolution spectroscopy of Rydberg states with sub-Doppler resolution has so far been performed using two-photon spectroscopy, the quantum beat and the double resonance methods. For the very highly excited states detection cannot be performed by observation of fluorescence since the lifetimes of these states scale as the principal quantum number cubed. Field or collisional ionization is used instead. As a result of the relatively long lifetimes Rydberg states have small natural widths which allow one to resolve splittings of less than 1 MHz. Therefore a very narrow laser linewidth is necessary if the two-photon absorption technique is used. In addition the Ramsey method of separated laser fields has to be applied in order to avoid transit time broadening [1]. In the case of the quantum beat and the double resonance methods there is no such requirement on the laser.