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Physical Review A

Pathway for two-color photoassociative ionization with ultrafast optical pulses in a Rb magneto-optical trap

G. Veshapidze, M. L. Trachy, Hyounguk Jang, C. W. Fehrenbach, Brett D DePaola

Published Nov 19, 2007• 9 citations
Publisher

Abstract

Ultrafast and narrow linewidth lasers are combined to study multiphoton photoassociative ionization (PAI) in a Rb magneto-optical trap. Through the use of time of flight spectroscopy and by varying the parameters of the ultrafast laser, the excitation pathway for PAI was determined. It was found that ${{\mathrm{Rb}}_{2}}^{+}$ is formed, not by direct photoionization, but by sequential excitation of ${\mathrm{Rb}}_{2}$ to an autoionizing state. Determining the excitation and ionization pathway was a necessary first step for future efforts in trying to establish coherent control techniques, first for the efficient production of translationally cold ${\mathrm{Rb}}_{2}$, and then for transferring population to a desired vibrationally cold molecular state.

DOI
10.1103/physreva.76.051401

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Ultrafast and narrow linewidth lasers are combined to study multiphoton photoassociative ionization (PAI) in a Rb magneto-optical trap. Through the use of time of flight spectroscopy and by varying the parameters of the ultrafast laser, the excitation pathway for PAI was determined. It was found that Rb2+{{\mathrm{Rb}}_{2}}^{+}Rb2​+ is formed, not by direct photoionization, but by sequential excitation of Rb2{\mathrm{Rb}}_{2}Rb2​ to an autoionizing state. Determining the excitation and ionization pathway was a necessary first step for future efforts in trying to establish coherent control techniques, first for the efficient production of translationally cold Rb2{\mathrm{Rb}}_{2}Rb2​, and then for transferring population to a desired vibrationally cold molecular state.
We report the observation of direct, two-color photoassociative ionization (PAI) in collisions between 85Rb^{85}\mathrm{Rb}85Rb atoms optically cooled and confined in a magneto-optical trap. We also measure the resulting R2+{\mathrm{R}}_{2}^{+}R2+​ ion spectrum and compare it to earlier trap-loss fluoresecence spectra. Unlike PAI in Na, which ionizes principally by collisional autoionization, the Rb2∗{\mathrm{Rb}}_{2}^{\mathrm{*}}Rb2∗​ intermediate photoionizes directly to Rb2+{\mathrm{Rb}}_{2}^{+}Rb2+​ in a fixed optical field. These observations confirm PAI as a sensitive and broadly applicable spectroscopic tool.
Many people in the science community are interested in the prospect of cold molecules for such applications as quantum computing and molecular Bose-Einstein condensates. Current methods of production fall short of the requirements for such projects. Photoassociation is a promising technique for forming cold molecules, but is currently facing significant obsta-cles. By understanding the photoassociation process and utilizing higher excited states, it is hoped that cold molecules can be formed from more easily produced cold atoms. Photoasso-ciative Ionization (PAI) is presented as a means to study excited state molecular dynamics at large internuclear separation, including photoassociation. This thesis presents a number of techniques for studying PAI in cold rubidium and a number of results obtained with the tech-niques. Excitation pathways for the process are explored in both narrow linewidth (MHz) and ultrafast (fs), large bandwidth (20 nm) domains.
We have produced ultracold polar RbCs molecules via photoassociation starting from laser-cooled 85 Rb and 133 Cs atoms in a dual-species, forced dark magneto-optical trap. The formed electronically excited RbCs* molecules correlated to the Rb(5S 1/2 ) + Cs(6P 1/2 ) dissociation limit are observed by trap loss spectroscopy. Following the decay of these excited RbCs* molecules, the formed ground state molecules are directly ionized by a two-photon single-color pulse dye laser, which is a new ionization mechanism for ground state RbCs molecules and thence detected by time-of-flight mass spectroscopy.
In this work, a reaction microscope with a magneto-optical trap for 6Li atoms, was extended by an optical dipole trap in order to be able to investigate in detail laser-induced atomic and molecular ionization dynamics in a cold quantum gas. \nThe optical dipole trap was operated at a full trap depth of 2.3 mK for 6Li atoms and about \n1% of the atoms in the MOT could be transferred into the dipole trap, with 1/e storage times \nexceeding 5 s. The optically trapped ensemble of 6Li atoms was used as a target for ionisation with intensive and broadband femtosecond laser pulses (λ = 750−820 nm, P = 10^11−10^14 W/cm2, Δt = 30 fs) and allowed to perform kinematically complete experiments, in which 6Li+ ions \nas well as photoelectrons were measured coincidentically. \nAs a first application, in this work, a series of association- and ionization mechanisms, which \nled to production of molecular 6Li2+ ions, were investigated with trapped lithium atoms. \nIn photoassociative ionization, two atoms collide, which were previously lifted into the asymptotic 2p − 3s potential energy curve by ladder excitation. During the collision the atoms autoionize into the 12Σg+(6Li2+) groundstate of the molecular ion, since these two potential energy curves exhibit an avoided crossing. This process was observed when magneto-optically trapped atoms were illuminated with the femtosecond laser. \nIn the dipole trap, using single-color photoassociation, excited state molecules were produced \nin high-lying vibrational states 11Σg+(ν = 65) and 13Σg+(v = 57) and spectroscopically investigated. A fraction of the excited state molecules decay via fluorescence into the molecular ground state. The 11Σg+(ν = 38)(6Li2) ground state molecules created via the singlet res- \nonance 11Σu+(ν = 65)(Li∗2) were detected via direct 3 photon ionisation. The momentum spectra show very low kinetic energies for the photo electrons of below 100 meV. Therefore \nin the molecular ion only vibrational states of Li2+ are getting populated, which are directly below the 3-photon transition energy. \nFinally, a stepwise ionization mechanism was identified, which leads into the continuum via \nan intermediate molecular state of 6Li2∗ after photoassociation. The starting point is a pho- \ntoassociated excited state molecule 11Σu+(ν = 65)(6Li∗2), which absorbs two photons of the dipole trap laser (λ = 1070 nm ± 2 nm). This happens via an intermediate molecular state \n31Σg+(2s + 3s), after which it leads into the 12Σg+(6Li2+) potential.
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.
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.