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Journal of Physics Conference Series

Photodetachment Studies of Ir− Ions at DESIREE

Moa K. Kristiansson, J Sundberg, D. Hanstorp, Gustav Eklund, H. Cederquist, Henning Zettergren, Henning T. Schmidt

Published Jan 1, 2020• 0 citations
Publisher

Abstract

Synopsis Anions of iridium were stored in the cryogenic storage ring, DESIREE [1, 2, 3] and investigated using laser photodetachment. The ions were stored for up to 2000 seconds and the relaxation of several metastable states were studied by selecting different photon energies. The threshold of the previously not observed 3P2 level is located and the lifetime of this sate is found to be longer than the storage time of the ion beam. We also find that a state exists higher in energy than the previously known level with a lifetime in the range of hundreds of milliseconds.

DOI
10.1088/1742-6596/1412/13/132022

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Synopsis Anions of iridium were stored in the cryogenic storage ring, DESIREE [1, 2, 3] and investigated using laser photodetachment. The ions were stored for up to 2000 seconds and the relaxation of several metastable states were studied by selecting different photon energies. The threshold of the previously not observed 3P2 level is located and the lifetime of this sate is found to be longer than the storage time of the ion beam. We also find that a state exists higher in energy than the previously known level with a lifetime in the range of hundreds of milliseconds.
We apply near-threshold laser photodetachment to characterize the rotational quantum level distribution of OH^{-} ions stored in the cryogenic ion-beam storage ring DESIREE at Stockholm University. We find that the stored ions relax to a rotational temperature of 13.4±0.2 K with 94.9±0.3% of the ions in the rotational ground state. This is consistent with the storage ring temperature of 13.5±0.5 K as measured with eight silicon diodes but in contrast to all earlier studies in cryogenic traps and rings where the rotational temperatures were always much higher than those of the storage devices at their lowest temperatures. Furthermore, we actively modify the rotational distribution through selective photodetachment to produce an OH^{-} beam where 99.1±0.1% of approximately one million stored ions are in the J=0 rotational ground state. We measure the intrinsic lifetime of the J=1 rotational level to be 145±28 s.
Photodetachment thermometry on a beam of OH^{-} in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional frequency- and time-dependent photodetachment spectroscopy over 20 min of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the J=1 natural lifetime (about 193 s) and determine the OH^{-} rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.
We store a 10 keV OH − ion-beam at 13.5 ± 0.5 K in one of the DESIREE storage rings. Using photodetachment thermometry we measure the effective relative photodetachment cross section at different storage times and determine the rotational temperature of the ions to be 13.4 ± 0.2 K in agreement with the macroscopic temperature. A model cross section in the threshold range taking into account the formation of excited neutral OH molecules is calculated as a function of rotational temperature in order to justify the use of the rotational thermometry method developed earlier by the group of Roland Wester at Innsbruck University in the present case. In addition, we apply a selective photodetachment technique to produce an ion beam with more than 99% of the ions in the rotational ground state. The intrinsic lifetime of the J = 1 rotational level is measured to be 145 ± 28 s.
The negative ion of lanthanum has one of the richest bound-state spectra observed for an atomic negative ion and it has been proposed as a promising candidate for laser cooling, which has not yet been achieved for any negatively charged system. In the present study, radiative decays of low-lying excited states of ${\mathrm{La}}^{\ensuremath{-}}$ were experimentally investigated at the DESIREE cryogenic electrostatic ion-beam storage ring facility using state-selective photodetachment over storage times of up to 800 s. Independently, the transition rates for the relevant states were calculated using a high-precision hybrid theoretical approach. The measured lifetime of the lowest excited state of ${\mathrm{La}}^{\ensuremath{-}}$ (3F3e)({}^{3}{F}_{3}^{e})(3F3e​) of 132(20) s agrees very well with the theoretical calculations and previous predictions. However, a puzzling discrepancy exists for the next-higher state 3F4e{}^{3}{F}_{4}^{e}3F4e​, with the experiments yielding a lifetime $\ensuremath{\sim}4$ times shorter than calculated, highlighting the challenges for this complex ion. The results provide valuable insights into the prospects for laser cooling ${\mathrm{La}}^{\ensuremath{-}}$.
The lifetime of the 3P0^{3}P_{0}3P0​ state of ${\mathrm{Bi}}^{\ensuremath{-}}$ has been measured by selective photodetachment in a cryogenic ion-beam storage ring. By measuring the lifetime as a function of applied laser powers and extrapolating to zero laser power, a lifetime of $16.0\ifmmode\pm\else\textpm\fi{}0.5$ s is deduced for electric quadrupole decay of the excited state to the ground sate. The result provides a stringent test of recent state-of-the art theoretical calculations.
A hyperfine state selective laser probing technique is presented to be used for lifetime determination of metastable states in an ion storage ring. Evidence for a drastic difference between the decay rates of the hyperfine states of the metastable level 5d4D7/25d{}^{4}{D}_{7/2}5d4D7/2​ in 129Xe+{}^{129}{\mathrm{Xe}}^{+}129Xe+ has been obtained by this method.