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 ground $5{S}_{1/2}$ and excited $5{P}_{3/2}$ states, is investigated experimentally theoretically with 400-nm femtosecond laser pulses at intensities $I=(3\ifmmode\times\else\texttimes\fi{}{10}^{9})\text{--}(4.5\ifmmode\times\else\texttimes\fi{}{10}^{12})$ $\mathrm{W}/{\mathrm{cm}}^{2}$. The recoil-ion momentum distribution (RIMD) ${\mathrm{Rb}}^{+}$ exhibits rich ringlike structures their energies correspond to one-photon ionization state two-photon three-photon ionizations state, respectively. With increasing $I$, dips near zero (NZM) experimental RIMDs become shallow dramatically peaked momenta ionized from move obviously toward while peaks do not shift. In addition, ion-yield ratio varies $I$ ${I}^{1.5}$ as increases. These features indicate transition perturbative strongly for state. Numerical simulations solving time-dependent Schr\"odinger equation (TDSE) can qualitatively explain measurements RIMD, photoion angular distributions, ratio. However, some discrepancies still exist, especially NZM dip, which could stem electron-electron correlation that neglected present TDSE since we have adopted single-active-electron approximation.