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Abstract:
n 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. The optical dipole trap was operated at a full trap depth of 2.3 mK for 6Li atoms and about 1% of the atoms in the MOT could be transferred into the dipole trap, with 1/e storage times exceeding 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 as well as photoelectrons were measured coincidentically. As a first application, in this work, a series of association- and ionization mechanisms, which led to production of molecular 6Li2+ ions, were investigated with trapped lithium atoms. In 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. In the dipole trap, using single-color photoassociation, excited state molecules were produced in 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- onance 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 in the molecular ion only vibrational states of Li2+ are getting populated, which are directly below the 3-photon transition energy. Finally, a stepwise ionization mechanism was identified, which leads into the continuum via an intermediate molecular state of 6Li2∗ after photoassociation. The starting point is a pho- toassociated 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 31Σg+(2s + 3s), after which it leads into the 12Σg+(6Li2+) potential.
