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Physical Review A

Optimal control of multiphoton ionization of Rb2 molecules in a magneto-optical trap

Fabian Weise, Sascha Birkner, Andrea Merli, Stefan M. Weber, Franziska Sauer, Ludger H. Wöste, Albrecht Lindinger, Wenzel Salzmann, Terry Mullins, J. Eng, Magnus Albert, Roland Wester, Matthias Weidemüller

Published Dec 12, 2007• 8 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.76.063404

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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 5s5d5s5d5s5d-asymptote are utilized for the second excitation step.
We have investigated multiphoton-ionization passages of rubidium atoms initiated from the excited 5P3/25{P}_{3/2}5P3/2​ energy state. For this we used coherent control schemes based on femtosecond laser pulse shaping applied to cold atoms spatially isolated in a magneto-optical trap. With programed laser pulses of spectral $\ensuremath{\pi}$-phase step, of which location was varied within the laser spectrum, the sequential two-photon-ionization passage along the 5P3/25{P}_{3/2}5P3/2​-5D5D5D continuum was probed in terms of trap-loss spectroscopy. The coherent control two-photon-ionization method unveiled not only the resonantly enhanced two-photon ionization but also the asymmetric nature of the ionization profile structure given as a function of the spectral phase step location. Experimental results show good agreement with the second-order perturbation calculation of the constituent possible ionization passages.
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{\mathrm{Rb}}_{2}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.
Calculations relating to two experiments that demonstrate coherent control of preformed rubidium-85 molecules in a magneto-optical trap using ultrafast laser pulses are presented. The two experiments are a step toward the stabilization of ultracold rubidium dimers using ultrafast lasers. In the first experiment, it is shown that preassociated molecules in an incoherent mixture of states can be made to oscillate coherently using a single ultrafast pulse. A mechanism that can transfer molecular population to more deeply bound vibrational levels is used in the second. Optimal parameters of the control pulse are presented for the application of the mechanism to molecules in a magneto-optical trap. The calculations make use of an experimental determination of the initial state of molecules photoassociated by the trapping lasers in the magneto-optical trap and use shaped pulses consistent with a standard ultrafast laser system. The experiment's purpose is to demonstrate and evaluate the use of ultrafast shaped pulses to manipulate ultracold rubidium dimers with a view to the eventual stabilization of the molecules.
In this paper, we report the measurement of Rb2 molecule formation rate constant due to a two body process in a magneto-optical trap as a function of the sample temperature. The ground state molecules are detected by two-photon ionization, through the intermediate a 3Σ + → 23Π g molecular band. Our results show that the Rb2 molecules formed in the MOT could be due to a wave shape resonance, which enhances the molecule formation rate. This effect may be used to enhance the molecule production; and therefore it maybe important to future experiments involving production and trapping of cold ground state molecules.
We report on the observation of ultracold ground electric-state cesium molecules produced directly in a magneto—optical trap with a good signal-to-noise ratio. These molecules arise from the photoassociation of magneto—optical trap lasers and they are detected by resonantly enhanced multiphoton ionization technology. The production rate of ultracold cesium molecules is up to 4×10 4 s −1 . We measure the characteristic time of the ground electric-state cesium molecules generated in the experiment and investigate the Cs 2 + molecular ion intensity as a function of the trapping laser intensity and the ionization pulse laser energy. We conclude that the production of cold cesium molecules may be enhanced by using appropriate experimental parameters, which is useful for future experiments involving the production and trapping of ultracold ground electric-state molecules.
In this paper, we report the measurement of molecular formation rate constant of Rb2{\mathrm{Rb}}_{2}Rb2​ directly in a magneto-optical trap (MOT). The ground state molecules are detected by two-photon ionization, resonantly enhanced through the intermediate ${a}^{3}{\ensuremath{\Sigma}}_{u}^{+}\ensuremath{\rightarrow}{2}^{3}{\ensuremath{\Pi}}_{g}$ molecular band. We have measured the rate constant as a function of atomic density to conclude that the molecules Rb2{\mathrm{Rb}}_{2}Rb2​ are formed in the MOT by a photoassociation process caused by the trapping laser beams. We also measured the rate constant as a function of the trapping laser intensity. The results here presented are of importance to future experiments involving trapping of cold molecules.