We present optimal control, using femtosecond laser pulses in a closed-loop experiment, of the ionization process of ultracold rubidium molecules formed in a magneto-optical trap. The resulting spectrum of the optimally shaped pulse exhibits two frequency bands, which drive a resonantly enhanced three-photon ionization. The obtained data reveals the ionization process where only the electronic states corresponding to the
5s5d-asymptote are utilized for the second excitation step.
We report on the analysis of the line shapes in a two-color, three-photon ionization process of rubidium atoms in a magneto-optical trap. A cw and a pulsed laser are tuned almost at resonance with the ${5}^{2}{S}_{1/2}{\ensuremath{-}5}^{2}{P}_{3/2}$ and the ${5}^{2}{P}_{3/2}{\ensuremath{-}6}^{2}{D}_{5/2}$ transitions, respectively. The line shapes, recorded as functions of the pulsed laser wavelength, are strongly asymmetric with respect to the near resonance peak positions: in fact, they present long tails only on one side, according to the sign of the detuning of the cw laser. Moreover, the signal is overall stronger for negative than for positive detunings. Theoretical considerations based on the Autler-Townes scheme and computer simulations of the excitation-ionization dynamics show that the main source of asymmetry is the coupling of the two ionization pathways going through the
62D3/2 and
62D5/2 levels. In order to reproduce the experimental line shapes one must take into account the variability of the pulsed laser intensity and its finite coherence time.
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 $I=(3\ifmmode\times\else\texttimes\fi{}{10}^{9})\text{--}(4.5\ifmmode\times\else\texttimes\fi{}{10}^{12})$
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
I to
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 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.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
We report on coherent control of excitation processes of translationally ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond laser pulses. Evolution strategies are applied in a feedback loop in order to optimize the photoexcitation of the
Rb2 molecules, which subsequently undergo ionization or fragmentation. A superior performance of the resulting pulses compared to unshaped pulses of the same pulse energy is obtained by distributing the energy among specific spectral components. The demonstration of coherent control to ultracold ensembles opens a path to actively influence fundamental photoinduced processes in molecular quantum gases.