The present paper reports on a merged-beam experiment on mutual neutralization between
Na+ and ${\mathrm{D}}^{\ensuremath{-}}$. For this experiment, we have used the DESIREE ion-beams storage-ring facility. The reaction products are detected using a position- and time-sensitive detector, which ideally allows for determination of the population of each individual quantum state in the final atomic systems. Here, the
4s,3d, and
4p final states in Na are observed and in all cases the D atom is in its ground state
1s2S. The respective branching fractions of the states populated in Na are determined by fitting results from a Monte Carlo simulation of the experiment to the measured data. The center-of-mass collision energy is controlled using a set of biased drift tubes, and the branching fractions are measured for energies between 80 meV and 393 meV. The resulting branching fractions are found to agree qualitatively with the only available theoretical calculations for this particular system, which are based on a multichannel Landau-Zener approach using dynamic couplings determined with a linear combination of atomic orbitals model.
We have developed an experimental technique to study charge- and energy-flow processes in sub-eV collisions between oppositely charged, internally cold, ions of atoms, molecules, and clusters. Two ion beams are stored in separate rings of the cryogenic ion-beam storage facility DESIREE, and merged in a common straight section where a set of biased drift tubes is used to control the center-of-mass collision energy locally in fine steps. Here, we present measurements on mutual neutralization between
Li+ and ${\mathrm{D}}^{\ensuremath{-}}$ where a time-sensitive imaging-detector system is used to measure the three-dimensional distance between the neutral Li and D atoms as they reach the detector. This scheme allows for direct measurements of kinetic-energy releases, and here it reveals separate populations of the
3s state and the
(3p+3d) states in neutral Li while the D atom is left in its ground state
1s. The branching fraction of the
3s final state is measured to be $57.8\ifmmode\pm\else\textpm\fi{}0.7%$ at a center-of-mass collision energy of $78\ifmmode\pm\else\textpm\fi{}13$ meV. The technique paves the way for studies of charge-, energy-, and mass-transfer reactions in single collisions involving molecular and cluster ions in well-defined quantum states.
We demonstrate the control of 4He(23S1)–7Li(22S1/2) chemi-ionization reactions by all-optical electron-spin-state preparation of both atomic species prior to the collision process. Our results demonstrate that chemi-ionization is strongly suppressed (enhanced) for non-spin-conserving (spin-conserving) collisions at thermal energies. These findings are in good agreement with a model based on spin angular momentum coupling of the prepared atomic states to the quasi-molecular states. Small deviations from the model indicate the contribution of the 4Σ+ channel to the reaction rate, which is in violation of spin conservation.
A combined optical-pumping--laser-induced-fluorescence method was developed to determine the fraction of metastable-state ions in a
Ba+ beam. This allowed us to measure the ratio of the charge-exchange cross sections for collisions of
Ba+(6s) and
Ba+(5d) ions with a sodium vapor for impact energies ranging from 3 to 29 keV. For a collision energy of 5 keV, the ratio of the cross sections for
Ba+(5d
2D3/2) and
Ba+(5d
2D5/2) was measured. All results are compared to theoretical near-resonance predictions.
We report on mutual neutralization measurements between
7Li+ and $^{1}\mathrm{H}^{\ensuremath{-}}$ at effective center-of-mass collision energies in the range of 100 to 350 meV. We find that final states of lithium with principal quantum number
n=3 dominate with
3s separated from the unresolved
3p and
3d states. We measure the
3s branching fraction to be 0.665(12) at 100(16) meV and no significant dependence on collision energy is observed in the studied range. Comparing to previous results on mutual neutralization between
7Li+ and $^{2}\mathrm{H}^{\ensuremath{-}}$ [G. Eklund et al., Phys. Rev. A 102, 012823 (2020)], we find that
7Li+ collisions with $^{1}\mathrm{H}^{\ensuremath{-}}$ result in a significantly higher
3s branching fraction than collisions with $^{2}\mathrm{H}^{\ensuremath{-}}$. The difference is 0.087(14). The
3s branching fraction of $^{7}\mathrm{Li}^{+} + ^{1}\mathrm{H}^{\ensuremath{-}}$ and the determined isotope difference are in agreement with results from extended full quantum calculations based on the same input data and numerical method as in Croft et al. [H. Croft, A. S. Dickinson, and F. X. Gad\'ea, J. Phys. B 32, 81 (1999)]. These calculations reveal strong Stueckelberg oscillations of the
3s branching fraction for both isotopes.
Within this thesis two experiments will be described. In the first experiment a velocity filter was used to obtain a beam of polar molecules (ND3) with a translational temperature of less than 5K. For the first time the dynamics of individual rotational states inside a velocity filter could be investigated directly. Usually a velocity filter defines only a maximumvelocity that can pass the filter. We successfully implemented two different approaches where molecules below a certain minimal velocity are removed from the beam, turning the velocity filter into a bandpass filter. The second and more interesting experiment is the merged beamexperiment. We developed and constructed the first apparatus which successfullymerged two guided beams, one consisting of polar molecules, the other of paramagnetic particles, into an overlapping parallel movement. With this approach we could lower the temperature at which the reaction of metastable Ne* with ND3, NH3 and CH3F has been investigated by 3 orders of magnitude down to the sub-kelvin regime. Especially the stereochemistry of those reactions revealed an unexpected energy-insensitivity at temperatures below 200K. As our setup is very general the investigation of manymore neutral-neutral reactions will be possible, opening a new route into an temperature regime where only a few reactions have been studied before.