Perturbed Rydberg series

One of the most intensively studied manifestations of channel interaction in the bound states is the perturbation of the regularity of the Rydberg series, which is evident if one simply measures the energies of the atomic states. Although measurements of Rydberg energy levels by classical absorption spectroscopy show the perturbations in the series which are optically accessible from the ground state, the tunable laser has made it possible to study series which are not connected to the ground state by electric dipole transitions as well. One of the approaches which has been used widely is that used by Armstrong et al. As shown in Fig. 22.1, a heat pipe oven contains Ba vapor at a pressure of ∼1 Torr. Three pulsed tunable dye laser beams pass through the oven. Two are fixed in frequency, to excite the Ba atoms from the ground 6s21So state to the 3P1 state and then to the 6s7s 3S1 state. The third laser is scanned in frequency over the 6s7s 3S1 → 6snp transitions. The Ba atoms excited to the 6snp states are ionized either by collisional ionization or by the absorption of another photon. The ions produced migrate towards a negatively biased electrode inside the heat pipe. The electrode has a space charge cloud of electrons near it which limits the emission current.