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A Laser Cooled Lithium Beam Source for the Loading of a Magneto-Optical Trap in a Reaction Microscope

Dominik Globig

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

A new experiment is being built at the Max-Planck-Institut für Kernphysik (MPIK) within the PRIOC project (Precision studies on Ion Collisions) where a reaction microscope (Remi) and a magneto-optical trap (MOT) as target are combined to a so called MOTRemi. This setup will be implemented into the ion storage ring 'Testspeicherring' (TSR) of the MPIK. The world-wide unique combination of these three state-of-the-art technologies shall enable to perform kinematically complete studies of ion-atom collision with unprecedented detail and resolution. In this thesis, a two-dimensional magneto-optical trap (2D-MOT) was developed and tested as a source of slow lithium atoms for the loading of the target MOT. This method allows a very compact setup and oers a collimated beam and a ux of cold atoms comparable to the highest uxes achieved by using laser cooling. The temperature of the atomic beam was determined to be at the scale of a few Kelvin and, thus, it is already an adequate stand-alone target for momentum resolved experiments.

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Reaction microscopes enable kinematically complete measurements of atomic and molecular fragmentation. An ultracold atomic target is usually provided by a supersonic gas jet. The apparatus developed in the course of this thesis for the first time combines the principle of the reaction microscope with a magneto-optical trap. This allows for the preparation of lithium atoms in the sub-mK range. Being a three-electron system, its simple atomic structure makes lithium a model system of great topical interest for all kinds of ionization reactions. In order not to deteriorate the determination of the momenta of the charged fragments by the magnetic field of the trap, a pulsed mode of operation is adopted, creating field-free conditions during data acquisition and making efficient use of the stored target. The novel apparatus provides a versatile target for investigations on fragmentation of lithium by electron, ion and photon impact. For the first time, momentum-resolved measurements on multiphoton ionization in intense laser fields with pulse durations of 25~fs and peak intensities in the range between 10$^{11}$~W/cm$^2$ and 10$^{16}$~W/cm$^2$ were performed. The acquired photoelectron spectra exhibit unexpected structures such as a preferred emission in the direction perpendicular to the laser polarization axis, which are not fully understood yet. Presently, corresponding calculations are being done in several theory groups.

•Jan 1, 2007•3 citations
A new experiment is being built at the Max-Planck-Institut für Kernphysik (MPIK) within the PRIOC project (Precision studies on Ion Collisions) where a reaction microscope (Remi) and a magneto-optical trap (MOT) as target are combined to a so called MOTRemi. This setup will be implemented into the ion storage ring 'Testspeicherring' (TSR) of the MPIK. The world-wide unique combination of these three state-of-the-art technologies shall enable to perform kinematically complete studies of ion-atom collision with unprecedented detail and resolution. In this thesis, a two-dimensional magneto-optical trap (2D-MOT) was developed and tested as a source of slow lithium atoms for the loading of the target MOT. This method allows a very compact setup and oers a collimated beam and a ux of cold atoms comparable to the highest uxes achieved by using laser cooling. The temperature of the atomic beam was determined to be at the scale of a few Kelvin and, thus, it is already an adequate stand-alone target for momentum resolved experiments.
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.
In the course of this work dynamics in ionization processes of lithium by photon and electron impact were studied. Due to its electronic structure, lithium is an ideal candidate to examine electronic correlation effects and collision dynamics of simple few-body systems. The lithium target is cooled in a magneto-optical trap (MOT) down to the sub-millikelvin regime, which in combination with a reaction microscope (ReMi) allows for the determination of the momentum information of all collision products with unprecedented resolution. An electron gun serving as a new projectile source was implemented in the MOTReMi experiment. From first measurements, which showed that the developed setup is principally operational, time-of-flight and position spectra were analyzed, determining the ratio of Li2+=Li+ ion creation. In a second series of experiments, photoionization of laser-excited lithium has been investigated and fully differential cross sections (FDCS) were examined. As the target and also the laser beam were polarized, orientational effects could be studied. In some cases the electron emission characteristics could directly be deduced from dipole selection rules.
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.
Within the MOTREMI experiment at the Max-Plank-Institut für Kernphysik a new optical trap was constructed. A already installed magneto-optical trap cools 6Li atoms down to temperatures on the order of 100 μK. From the corresponding atoms an ultracold lithium gas is trapped inside the center of the main vacuum chamber. In the further course of the experiment, ionization measurements with a degenerate quantum gas will be carried out. In order to create theses gases, one needs to surpass the temperature limit of the magneto-optical trap. Traps using a high intensity, far red-detuned focused laser beam are able to achieve temperatures on the order of 100 nK. An intensity stabilization which is able to extend the lifetime of trapped gas was constructed in the scope of this thesis. With the use of two different photodiodes, every intensity level of the laser is covered. The high laser powers which are needed for the trap, can lead to thermal effects in the entire setup. Consequently, the properties of the laser and the optical elements the laser is passing through can change. Therefore, a quantitative analysis of these thermal effects has been carried out. Besides the shift and reduction of the focused laser beam, the thermal effect on the trap size was obtained.
This thesis deals with the reconstruction of a MOTRe- Mi apparatus, which represents the combination of a magneto-optical trap (MOT) and a Reaction Microscope (ReMi). With this setup an ultracold degenerate Fermi gas consisting of 6Li atoms will be generated and studied in future. Primarily the Li atoms will be vaporized, precooled in a Zeeman Slower, trapped and stored with the MOT at typical temperatures of a few 100 μK. Within the scope of this thesis initially the parameters of the Zeeman Slower were optimized. For this purpose the magnetic field profiles of the single coils were measured and the ideal coil currents determined. The efficiency and other properties of the Zeeman Slower were calculated with a simulation programme. The focus of this thesis was on planning and reconstructing the laser system and the optical setup for the magneto-optical trap. This included among others the adaptation of the laser frequencies from 7Li to 6Li and the integration of an additional amplifier in order to achieve a higher output power. Using the frequency stabilization scheme of a frequency offset lock ([SEG+99]) it is now possible to tune the laser frequency over a large range at a constant laser power. This is required on the one hand for the later transfer of the atoms to a dipole trap and on the other hand for the imaging of the atom cloud by light absorption at high magnetic fields. Finally the vacuum chamber and the Reaction microscope was baked out and activated at a vacuum of about 7 · 10-10 mbar.
Reaction microscopes enable kinematically complete measurements of atomic and molecular fragmentation. An ultracold atomic target is usually provided by a supersonic gas jet. The apparatus developed in the course of this thesis for the first time combines the principle of the reaction microscope with a magneto-optical trap. This allows for the preparation of lithium atoms in the sub-mK range. Being a three-electron system, its simple atomic structure makes lithium a model system of great topical interest for all kinds of ionization reactions. In order not to deteriorate the determination of the momenta of the charged fragments by the magnetic field of the trap, a pulsed mode of operation is adopted, creating field-free conditions during data acquisition and making efficient use of the stored target. The novel apparatus provides a versatile target for investigations on fragmentation of lithium by electron, ion and photon impact. For the first time, momentum-resolved measurements on multiphoton ionization in intense laser fields with pulse durations of 25~fs and peak intensities in the range between 1011^{11}11~W/cm2^22 and 1016^{16}16~W/cm2^22 were performed. The acquired photoelectron spectra exhibit unexpected structures such as a preferred emission in the direction perpendicular to the laser polarization axis, which are not fully understood yet. Presently, corresponding calculations are being done in several theory groups.