Two-photon resonant three-photon ionization of the ndD2states of cesium, rubidium, and sodium: Photoelectron angular distributions

Energy-resolved photoelectron angular distributions have been measured for the nd ${}^2$D states (n=11–23) in cesium and rubidium atoms for two-photon resonant, three-photon ionization under conditions where the spin-orbit fine structure was not resolved. The photoelectron angular distributions reveal strong fine-structure mixing effects when the time duration of the laser pulse is longer than the time corresponding to the inverse of the fine-structure splitting of the nd states involved. Three-photon ionization photoelectron angular distributions for sodium atoms have been studied both experimentally and theoretically for laser frequencies in which resonance enhancement occurs via the two-photon excited nd ${}^2$D states (n=5–9). The laser pulse duration is ∼6 ns which is comparable to or less than the fine-structure mixing time for the unperturbed fine-structure nd ${}^2$ $D_{5/2}$,3/2 levels. In addition, the high laser powers ∼${10}^8$ W/${\mathrm{cm}}^2$ employed result in alterations of the energy separations of the fine-structure levels (and cause corresponding changes in the mixing times) due to the ac Stark effect. A detailed theoretical analysis is presented and good agreement is obtained with the experimental results.