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arXiv:physics/0607248 · Physical Review Letters

Photodetachment of Cold OH− in a Multipole Ion Trap

Sebastian H. Trippel, Jochen Mikosch, R. Berhane, Rico Otto, Matthias Weidemüller, Roland Wester

Published Nov 10, 2006• 67 citations
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Abstract

The absolute photodetachment cross section of OH- anions at a rotational and translational temperature of 170 K is determined by measuring the detachment-induced decay rate of the anions in a multipole radio-frequency ion trap. In comparison with previous results, the obtained cross section shows the importance of the initial rotational-state distribution. Using a tomography scan of the photodetachment laser through the trapped ion cloud, the derived cross section is model-independent and thus features a small systematic uncertainty. The tomography also yields the column density of the OH- anions in the 22-pole ion trap in good agreement with the expected trapping potential of a large field free region bound by steep potential walls.

DOI
10.1103/physrevlett.97.193003

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The absolute photodetachment cross section of OH- anions at a rotational and translational temperature of 170 K is determined by measuring the detachment-induced decay rate of the anions in a multipole radio-frequency ion trap. In comparison with previous results, the obtained cross section shows the importance of the initial rotational-state distribution. Using a tomography scan of the photodetachment laser through the trapped ion cloud, the derived cross section is model-independent and thus features a small systematic uncertainty. The tomography also yields the column density of the OH- anions in the 22-pole ion trap in good agreement with the expected trapping potential of a large field free region bound by steep potential walls.
Photodetachment spectroscopy of OH(-) and H(3)O(2)(-) anions has been performed in a cryogenic 22-pole radiofrequency multipole trap. Measurements of the detachment cross section as a function of laser frequency near threshold have been analysed. Using this bound-free spectroscopy approach we could demonstrate rotational and vibrational cooling of the trapped anions by the buffer gas in the multipole trap. Below 50 K the OH(-) rotational temperature shows deviations from the buffer gas temperature, and possible causes for this are discussed. For H(3)O(2)(-) vibrational cooling of the lowest vibrational quantum states into the vibrational ground state is observed. Its photodetachment cross section near threshold is modelled with a Franck-Condon model, with a detachment threshold that is lower, but still in agreement with the expected threshold for this system.
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 describe the setup and characterization of a cryogenic multipole radio-frequency (RF) ion trap that enables the accumulation and cooling of mass-selected ions before injection into the SAPHIRA storage ring. To characterize the RF trap setup, we use ${\mathrm{OH}}^{\ensuremath{-}}$ anions and explore the threshold photodetachment cross section measured after storage in SAPHIRA as a probe of the rotational temperature. Beyond the temperature of the ion trap assembly, cooled to 6 K, the final rotational temperature of the ${\mathrm{OH}}^{\ensuremath{-}}$ ions is strongly influenced by the density of cooled He and the actual number of trapped ions while much less affected (possibly unaffected) by the time-varying field of the trap. To obtain rotationally cold ${\mathrm{OH}}^{\ensuremath{-}}$ ions, the RF trap must be operated with low He density and a low number of ions. High He densities, corresponding to a strong coupling of the trapped ions and He gas, lead to a significant rotational heating of the trapped ion ensemble, and the He density seems to limit the actual reachable rotational temperature. We demonstrate that cold ions in the RF trap remain cold for at least 30 ms in the SAPHIRA (300 K) storage ring at a base pressure of $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{mbar}$.
We present a scheme to embed molecular anions in a gas of ultracold rubidium atoms as a route towards the preparation of cold molecular ions by collisional cooling with ultracold atoms. Associative detachment as an important loss process in collisions between OH${}^{\ensuremath{-}}$ molecules and rubidium atoms is studied. The density distribution of trapped negative ions in the multipole radio frequency trap is measured by photodetachment tomography, which allows us to derive absolute rate coefficients for the process. We define a regime where translational and internal cooling of molecular ions embedded into the ultracold atomic cloud can be achieved.
Using accurate ab initio calculations of the interaction forces, we employ a quantum mechanical description of the collisional state-changing processes that occur in a cold ion trap with He as a buffer gas. We generate the corresponding inelastic rates for rotational transitions involving three simple molecular anions OH−(1Σ), MgH−(1Σ), and C2H−(1Σ) colliding with the helium atoms of the trap. We show that the rotational constants of these molecular anions are such that within the low-temperature regimes of a cold ion trap (up to about 50 K), a different proportion of molecular states are significantly populated when loading helium as a buffer gas in the trap. By varying the trap operating conditions, population equilibrium at the relevant range of temperatures is reached within different time scales. In the modeling of the photodetachment experiments, we analyze the effects of varying the chosen values for photodetachment rates as well as the laser photon fluxes. Additionally, the changing of the collision dynamics under different buffer gas densities is examined and the best operating conditions, for the different anions, for yielding higher populations of specific rotational states within the ion traps are extracted. The present modeling thus illustrates possible preparation of the trap conditions for carrying out more efficiently state-selected experiments with the trapped anions.
The column density distribution of trapped OH − ions in a 22-pole ion trap is measured for different trap parameters. The density is obtained from position-dependent photodetachment rate measurements. Overall, agreement is found with the effective potential of an ideal 22-pole. However, in addition, we observe ten distinct minima in the trapping potential, which indicate a breaking of the 22-fold symmetry. Numerical simulations show that a displacement of a subset of the radiofrequency electrodes can serve as an explanation for this symmetry breaking.