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Photon and Electron Impact Ionization of Lithium studied with a MOTReMi

Elisabeth Brühl, D. Fischer

Published Jan 1, 2013• 0 citations
OpenAlex

Abstract

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.

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Abstract An all-optical, near-resonant laser atom trap is used to prepare an electronically excited and polarized gas target at mK-temperature for complete photo-ionization studies. As a proof-of-principal experiment, lithium atoms in the 2 2 P 3/2 ( m l = +1) state are ionized by a 266 nm laser source, and emitted electrons and Li + ions are momentum analyzed in a COLTRIMS spectrometer. The excellent resolution achieved in the present experiment allows not only to extract the relative phase and amplitude of all partial waves contributing to the final state, it also enables to characterize the experiment regarding target and spectrometer properties. Photo-electron angular distributions are measured for five different laser polarizations and described in a one-electron approximation with excellent agreement.

arXiv:1912.03247•Journal of Physics B Atomic Molecular and Optical Physics•Feb 14, 2020•15 citations
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.
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.
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.
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.
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.
Photoionization induced by trapping and auxiliary laser fields is an inherent feature of many laser-cooling and optical trapping experiments, yet its microscopic dynamics are rarely investigated directly. In this work, we employ a reaction microscope implementing an event-by-event photoionization time retrieval technique to extend momentum-resolved photoelectron spectroscopy to continuous-wave laser--atom interactions. We investigate low-intensity multicolor photoionization of laser-cooled lithium atoms confined in an all-optical trap. Complete three-dimensional electron momentum distributions and kinetic-energy spectra recorded for different laser wavelengths and polarization configurations identify resonant excitation of the 5p5p5p and 5f5f5f states and reveal an additional ionization channel following spontaneous decay from the 5f5f5f to the 4d4d4d state. A pronounced polarization dependence of the photoionization yield is explained by magnetic-sublevel selection rules and the coherent interference of different virtual excitation pathways. These results demonstrate that reaction microscopy combined with event-by-event time reconstruction provides a powerful approach for investigating microscopic electronic dynamics in laser-cooled atomic systems and offers new insight into photoionization processes occurring during optical trapping.
Abstract An all-optical, near-resonant laser atom trap is used to prepare an electronically excited and polarized gas target at mK-temperature for complete photo-ionization studies. As a proof-of-principal experiment, lithium atoms in the 2 2 P 3/2 ( m l = +1) state are ionized by a 266 nm laser source, and emitted electrons and Li + ions are momentum analyzed in a COLTRIMS spectrometer. The excellent resolution achieved in the present experiment allows not only to extract the relative phase and amplitude of all partial waves contributing to the final state, it also enables to characterize the experiment regarding target and spectrometer properties. Photo-electron angular distributions are measured for five different laser polarizations and described in a one-electron approximation with excellent agreement.