Stepwise Excitation and Spectroscopy of Rydberg States

When the excited state 2) has been selectively populated by optical pumping with laser LI, transitions to still higher levels m) can be induced by a second tunable laser L2 (Fig. 10.14b). This two-step excitation may be regarded 

With two visible lasers, levels m) with excitation energies up to 6eV can be reached. Optical frequency doubling of both lasers allows even the population of levels up to 12 eV. This makes the Rydberg levels of most atoms and molecules accessible to detailed investigations. The population of Rydberg levels of species M can be monitored either by their fluorescence or by detecting the ions M+ or the electrons e that are produced by photoionization, field ionization, collisional ionization, or autoionization of the Rydberg levels. 

Rydberg states have some remarkable characteristics (Table 10.1). Their spectroscopic investigation allows one to study fundamental problems of quantum optics (Sect. 14.5), nonlinear dynamics, and chaotic behavior of quantum systems (see below). Therefore detailed studies of atomic and molecular Rydberg states have found increasing interest [10.45-10.57]. 

The term value Tn of a Rydberg level with principal quantum number n is given by the Rydberg formula 

From (10.11) one can calculate that the term energy of Na ( = 40) is only about 0.005 eV below the ionization limit, and Fig. 10.22 shows that an external field with E = 100 V/cm is sufficient to ionize this state. 

When the excited state 2) has been selectively populated by optical pumping with laser LI, transitions to still higher levels m) can be induced by a second tunable laser L2 (Fig. 10.14b). This two-step excitation may be regarded as the special resonance case of the more general two-photon excitation with two different photons hu)2 (Sect. 7.5). Because the upper levels m) must have the same parity as the initial level l), they cannot be reached by an allowed one-photon transition. The one-step excitation by a frequency doubled laser with the photon energy 2hu) = ft(wi+a 2) therefore excites, in the same energy range, levels of opposite parity compared to those levels reached by the two step excitation. 

The external field E [V/m] decreases the ionization threshold of a Rydberg state down to the appearance potential AP of the ions (Fig. 5.24)  

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