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Journal of Physics Conference Series

Coincident detection of recoil ions and photoelectrons after multiphoton-ionisation of cold 6Li atoms trapped in a dipole trap

Nathan Kurz, Deepthy Maria Mootheril, Silva Mezinska, Thomas Pfeifer, Alexander Dorn

Published Jan 1, 2020• 0 citations
Publisher

Abstract

Synopsis We report on performing high resolution recoil ion momentum spectroscopy in a reaction microscope (ReMi) on a target of dipole-force trapped cold 6Li atoms in a non-interacting |1〉-|2〉 spin mixture and coincident detection of recoil ions and photoelectrons. Photo-ionization occurs from the electronic ground state of the 6Li atoms and IR photons significantly contribute to the ion yield via 3-photon resonances. High resolution time-of-flight spectra are obtained by modulating the trap depth synced with the arrival of the fs laser pulses.

DOI
10.1088/1742-6596/1412/12/122022

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Synopsis We report on performing high resolution recoil ion momentum spectroscopy in a reaction microscope (ReMi) on a target of dipole-force trapped cold 6Li atoms in a non-interacting |1〉-|2〉 spin mixture and coincident detection of recoil ions and photoelectrons. Photo-ionization occurs from the electronic ground state of the 6Li atoms and IR photons significantly contribute to the ion yield via 3-photon resonances. High resolution time-of-flight spectra are obtained by modulating the trap depth synced with the arrival of the fs laser pulses.
We report on the implementation of a far-off-resonant, optical dipole force trap in a reaction microscope combined with a magneto-optical trap. Kinematically complete multi-photon ionization experiments were performed on optically trapped 6Li atoms and photo-associated 6Li2 molecules in their highest vibrational state. The apparatus allows us to distinguish different ionization mechanisms related to the presence of the IR field of the optical dipole trap that can occur during ionization of 6Li and 6Li2 in strong fields. In a series of proof-of-principle experiments, we detect weakly bound dimers via three-photon ionization with femtosecond pulses (τ = 30 fs) at a central wavelength of 780 nm and measure directly the momenta of the photoelectrons in coincidence with recoil ions.
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 developed a magneto-optical trap reaction microscope (MOTREMI) for strontium atoms by combining multiparticle coincident detection with the laser cooling technique. The present compact injection system enables the production of cold Sr atoms in three modes of two-dimensional (2D) magneto-optical trap (MOT) beam, molasses, and three-dimensional (3D) MOT, providing adjustable densities and ratios of the ground state 5s25{s}^{2}5s2 (1S0^{1}S_{0}1S0​) and the excited states 5s5p1P15s5p{\phantom{\rule{4pt}{0ex}}}^{1}{P}_{1}5s5p1P1​ and 3PJ^{3}P_{J}3PJ​. The target profiles including temperature, density, and size of the 3D MOT as well as the 2D MOT beam cold atomic flux were characterized in detail. Using the present setup, we investigate the single photoionization of Sr atoms with molasses through few-photons absorption in 800 nm, where Sr+{\mathrm{Sr}}^{+}Sr+ and ${e}^{\ensuremath{-}}$ are detected in coincidence and most of the ionization channels are identified. The momentum resolution for coincident Sr+{\mathrm{Sr}}^{+}Sr+ and ${e}^{\ensuremath{-}}$ along time-of-flight direction are achieved up to 0.12 a.u. and 0.02 a.u., respectively. The spectra of photoelectron ionized from the ground state as well as several excited states show distinct structures manifesting prominent contributions from multiphoton absorption. Detecting complete vector momenta of electrons and recoil ions in coincidence paves a way for further investigation of two-electron correlation dynamics and multielectron effects with alkaline-earth atoms.
A reaction microscope (ReMi) has been combined with a magneto-optical trap (MOT) for the kinematically complete investigation of atomic break-up processes. With the novel MOTReMi apparatus, the momentum vectors of the fragments of laser-cooled and state-prepared lithium atoms are measured in coincidence and over the full solid angle. The first successful implementation of a MOTReMi could be realized due to an optimized design of the present setup, a nonstandard operation of the MOT, and by employing a switching cycle with alternating measuring and trapping periods. The very low target temperature in the MOT (∼2 mK) allows for an excellent momentum resolution. Optical preparation of the target atoms in the excited Li 2(2)P3/2 state was demonstrated providing an atomic polarization of close to 100%. While first experimental results were reported earlier, in this work, we focus on the technical description of the setup and its performance in commissioning experiments involving target ionization in 266 nm laser pulses and in collisions with projectile ions.
We report an experiment on recoil-ion momentum spectroscopy of Rb+{\mathrm{Rb}}^{+}Rb+ ions formed by two-photon ionization of Rb2P3/2{\mathrm{Rb}}^{2}{P}_{3/2}Rb2P3/2​ at 790 nm. Laser-cooled atoms in a magneto-optic trap are used as the target in time-of-flight analysis of the recoil momentum. We demonstrate a resolution of 1 \ensuremath{\mu}eV with a limiting momentum resolution below $\ensuremath{\delta}p=0.05$ a.u. We also show the use of the photoionization technique for the characterization of cold atomic samples.
Rydberg atoms are in the focus of intense research due to the peculiar properties which make them interesting candidates for quantum optics and quantum information applications. In this work we study the ionization of Rydberg atoms due to their interaction with a trapping laser field, and a reaction microscope is used to measure photoelectron angular and energy distributions. Reaction microscopes are excellent tools when brandished against atomic photoionization processes involving pulsed lasers; the timing tied to each pulse is crucial in solving the subsequent equations of motion for the atomic fragments in the spectrometer field. However, when used in pump-probe schemes, which rely on continuous wave probe lasers, vital information linked to the time of flight is lost. This study reports on a method in which the standard ReMi technique is extended in time through coincidence measurements. This is then applied to the photoionization of 6^66Li atoms initially prepared in optically pumped 22S1/22^{2}S_{1/2}22S1/2​ and 22P3/22^{2}P_{3/2}22P3/2​ states. Multi-photon excitation from a tunable femtosecond laser is exploited to produce Rydberg atoms inside an infrared optical dipole trap; the structure and dynamics of the subsequent cascade back towards ground is evaluated.