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

Abstract

Photoionization of rubidium atoms cooled in a magneto-optical trap, characterized by the coexistence of the ground $5{S}_{1/2}$ and excited $5{P}_{3/2}$ states, is investigated experimentally and theoretically with the 400-nm femtosecond laser pulses at intensities of $I=(3\ifmmode\times\else\texttimes\fi{}{10}^{9})\text{--}(4.5\ifmmode\times\else\texttimes\fi{}{10}^{12})$ $\mathrm{W}/{\mathrm{cm}}^{2}$. The recoil-ion momentum distribution (RIMD) of ${\mathrm{Rb}}^{+}$ exhibits rich ringlike structures and their energies correspond to one-photon ionization of the $5{P}_{3/2}$ state and two-photon and three-photon ionizations of the $5{S}_{1/2}$ state, respectively. With increasing $I$, the dips near zero momentum (NZM) in the experimental RIMDs become shallow dramatically and their peaked ${\mathrm{Rb}}^{+}$ momenta ionized from the $5{P}_{3/2}$ state move obviously toward zero while the peaks from the $5{S}_{1/2}$ state do not shift. In addition, the ion-yield ratio of the $5{S}_{1/2}$ state to the $5{P}_{3/2}$ state varies from $I$ to ${I}^{1.5}$ as $I$ increases. These features indicate a transition from perturbative ionization to strongly perturbative ionization for the $5{P}_{3/2}$ state. Numerical simulations by solving the time-dependent Schr\"odinger 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.