%0 Generic
%A Kurz, Niels
%C Heidelberg
%D 2021
%F heidok:29072
%R 10.11588/heidok.00029072
%T Kinematically complete multiphoton ionization studies on optically trapped 6Li and 6Li_2 created by single-color photoassociation
%U https://archiv.ub.uni-heidelberg.de/volltextserver/29072/
%X 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.  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.