TRANSITION-PROBABILITY SATURATION THEORY FOR 2-PHOTON IONIZATION OF CS(2) AROUND 540 NM - POLARIZATION SPLITTING OF THE SATURATION CURVE

Saturation phenomena in two-photon step-by-step transitions, analyzed by monitoring the intermediate state population, provide a valuable to ol to get absolute cross-sections, free of uncertainties associated wi th the densities of the molecular or atomic species. At first sight, h owever, there is an ambiguity concerning which cross-section must be a scribed to each one of the two steps. One way to resolve this ambiguit y is to look at the dependence of the saturation of the intermediate s tate population upon the polarization of the incident beam. This is il lustrated here in the case of the two-photon ionization Of Cs2 molecul es involving a bound-free transition with production of 5D5/2 Cs atoms in the intermediate state. The dominant polarization-splitting mechan ism proceeds from the strong dependence upon the 5D5/2 magnetic quantu m number of the photoionization cross-section, relative to circularly polarized photons, while a similar feature is much less striking with plane polarized light. For the time scale involved in the experiments analyzed in this paper, the polarization splitting is strongly affecte d by the hyperfine interaction which mixes the 5D5/2 magnetic numbers. We have extended our previous simplified analysis to situations where the hf frequency is not assumed to be large compared to the inverse o f the light pulse duration. Our theoretical treatment is based upon th e solution of a quantum Liouville equation with a non-hermitian Hamilt onian and an external source term. In view of the rather large atomic state multiplicity, we have developed a method which bypasses a direct numerical solution of the Liouville equation: we solve first Schrodin ger-like equations, the final answer is then obtained from a quadratur e. We also discuss other mechanisms contributing to the polarization s plitting. Although they are of small importance in the present experim ental context, they could be of relevance for shorter pulses of light or when the transition probability of the second step process is no,lo nger the dominant one.