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Journal of the Optical Society of America B

Measurement of the isotope selectivity of the strontium magneto-optical trap by a time-of-flight mass spectrometer

Kwang-Hoon Ko, You-Kyoung Lim, Do-Young Jeong, Hyunmin Park, Taek-Soo Kim, Gwon Lim, Hyungki Cha

Published Nov 29, 2006• 5 citations
Publisher

Abstract

The isotope selectivity of the strontium (Sr) magneto-optical trap (MOT) is measured by a time-of-flight (TOF) mass spectrometer. The thermally generated Sr atoms are slowed down by the Zeeman slowing technique and trapped by the magneto-optical method. The trapped atoms are photoionized by the nonresonant two-photon ionization, and the generated ions are analyzed by the TOF mass spectrometer. The isotope selectivity of the MOT is up to approximately 2000 for 86Sr, and the overall selectivity of the system, which consists of the MOT and the mass spectrometer is also discussed.

DOI
10.1364/josab.23.002465

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The isotope selectivity of the strontium (Sr) magneto-optical trap (MOT) is measured by a time-of-flight (TOF) mass spectrometer. The thermally generated Sr atoms are slowed down by the Zeeman slowing technique and trapped by the magneto-optical method. The trapped atoms are photoionized by the nonresonant two-photon ionization, and the generated ions are analyzed by the TOF mass spectrometer. The isotope selectivity of the MOT is up to approximately 2000 for 86Sr, and the overall selectivity of the system, which consists of the MOT and the mass spectrometer is also discussed.
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We report the experimental realization of strontium magneto-optical trap (MOT) operating on the intercombination transition 1 S 0 − 3 P 1 at 689 nm, namely red MOT. A 689 nm laser used for cooling and trapping is injection locked to a master laser, whose linewidth is narrowed to 150 Hz by locking to a high finesse optical reference cavity. 88 Sr atoms pre-cooled and trapped by the broad 1 S 0 − 1 P 1 transition at 461 nm are transferred to the red MOT with the help of a time sequence controller. The transfer ratio is about 20% and the red MOT's temperature is estimated to be less than 20 μK by the time-of-flight (TOF) image analysis.
Measurements of the multiple ionization cross section ratios of Cesium were performed with ion time-of-flight (TOF) spectroscopy with a magneto-optical trap (MOT) apparatus, updating the previous measurement which dates back almost a century. Results are presented for collisions at energies of 50 eV to 120 eV. With a MOT, experiments can be performed with trapped, cold atomic targets which allow for unparalleled accuracy and experiments with signicant excited state target fractions above those achievable in most atomic beam experiments. A basic overview of optical cooling trapping, electron collision and atomic phenomena are presented. Experimental studies of electrons with Argon and Cesium targets were performed, measuring the multiple ionization ratios with ion TOF spectroscopy. The experimental apparatus and analysis methods are described in detail. Results are compared with previous measurements of multiple ionization ratios for both targets. Agreement within experimental error is found with the results of Tate and Smith across the energy range.
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.
The design of ion optical components for a time-of-flight mass spectrometer (TOF-MS) was motivated by the recent interest in the study of chemical reaction resonances. An optimal setting to study these resonances would have reactants with very well defined kinetic energies. The system proposed to accomplish this uses a magneto-optical trap (MOT) to localize and cool atoms to temperatures below 100µK, which act as targets for a beam of cold molecules created using a hexapole Stark guide. The reaction products would then be ionized through resonance-enhanced multiphoton ionization (REMPI) and analyzed with a TOF-MS. Many obstacles were presented by the high voltage hexapole’s close proximity with the TOF-MS electrodes. However, they were overcome by a careful selection of dimensions and active shielding. Adding the MOT brought about other constraints: off-axis ion detection and open aperature extraction plates. The issues that these generated were solved by employing a set of parallel deflection plates and an Einzel lens. Engineering solutions were tested by simulating ion trajectories in SIMION 3D, a commonly used commerical program in mass spectrometer design. To keep solutions financially feasible, a budget constraint of $100,000 was put in place.