Two-photon ionization of cold rubidium atoms with a near resonant intermediate state

By means of a simple theoretical model and experimental results, we analyse the two-photon ionization of cold rubidium atoms. We consider a two-step process in which ground state atoms are ionized by two blue (421 nm) photons, chosen to be quasiresonant with the 5S --> 6P transition. We show that for a good description of the process we have to take into account not only the three states coupled by the laser radiation, but also all the atomic states involved in the spontaneous emission cascade from the 6P to the 5S state, since optical pumping from 5S(1/2) (F = 2) to the 5S(1/2) (F = 1) states modifies the ionization efficiency when ionizing near resonance. The experimental and theoretical determination of ionization cross sections for excitations near resonance with the 6P(1/2) and the 6P(3/2) intermediate levels is presented.