Few-photon single ionization of cold rubidium in the over-the-barrier regime

Photoionization of rubidium atoms cooled in a magneto-optical trap, characterized by the coexistence of the ground 5S1/2 and excited 5P3/2 states, is investigated experimentally and theoretically with the 400-nm femtosecond laser pulses at intensities of I = (3 x 109)-(4.5 x 1012) W/cm2. The recoil-ion momentum distribution (RIMD) of Rb+ exhibits rich ringlike structures and their energies correspond to one-photon ionization of the 5P3/2 state and two-photon and three-photon ionizations of the 5S1/2 state, respectively. With increasing I, the dips near zero momentum (NZM) in the experimental RIMDs become shallow dramatically and their peaked Rb+ momenta ionized from the 5P3/2 state move obviously toward zero while the peaks from the 5S1/2 state do not shift. In addition, the ion-yield ratio of the 5S1/2 state to the 5P3/2 state varies from Ito I1.5 as I increases. These features indicate a transition from perturbative ionization to strongly perturbative ionization for the 5P3/2 state. Numerical simulations by solving the time-dependent Schrodinger equation (TDSE) can qualitatively explain the measurements of the RIMD, photoion angular distributions, and ion-yield ratio. However, some discrepancies still exist, especially for the NZM dip, which could stem from the electron-electron correlation that is neglected in the present TDSE simulations since we have adopted the single-active-electron approximation.

Automated summary

The photoionization process of rubidium atoms cooled in a magneto-optical trap was investigated experimentally and theoretically. The results showed that as the laser intensity increased, the momentum of the Rb+ ions from the 5P3/2 state approached zero, while the peaks from the 5S1/2 state remained unchanged. The ion-yield ratio between the 5S1/2 and 5P3/2 states also varied with laser intensity, indicating a transition from perturbative to strongly perturbative ionization for the 5P3/2 state.