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arXiv:1912.03247 · Journal of Physics B Atomic Molecular and Optical Physics

Photo-ionization of polarized lithium atoms out of an all-optical atom trap: a complete experiment

F Thini, K. L. Romans, B. P. Acharya, A H N C de Silva, Katrina Compton, K Foster, Cole Rischbieter, O. Russ, Sachin Sharma, Santwana Dubey, D. Fischer

Published Feb 14, 2020• 15 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1088/1361-6455/ab7671

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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.
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.
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.
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.
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 investigate the resonance-enhanced few-photon ionization of atomic lithium by linearly polarized light whose frequency is tuned near the $2s\text{\ensuremath{-}}2p$ transition. Considering the direction of light polarization orthogonal to the quantization axis, the process can be viewed as an atomic ``double-slit experiment'' where the 2p2p2p states with magnetic quantum numbers ${m}_{\ensuremath{\ell}}=\ifmmode\pm\else\textpm\fi{}1$ act as the slits. In our experiment, we can virtually close one of the two slits by preparing lithium in one of the two circularly polarized 2p2p2p states before subjecting it to the ionizing radiation. This allows us to extract the interference term between the two pathways and obtain complex phase information on the final state. The experimental results show very good agreement with numerical solutions of the time-dependent Schr\"odinger equation. The validity of the two-slit model is also analyzed theoretically using a time-dependent perturbative approach.