Reactive collisions of trapped anions with ultracold atoms

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${}^{-}$ 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.

Figure 1

Schematic depiction of the hybrid atom-ion trap. (A) Anions are produced by a plasma-discharge in a gas jet emerging from a pulsed valve, mass-selected by time-of-flight, and loaded into the trap through a hollow end cap. (B) The ion trap consists of eight parallel 100-$\mu$m thin wires forming an octupole to which sinusoidal voltages with a frequency of 7.7 MHz and amplitudes around 150 V are applied. Longitudinal confinement is provided by two end caps separated by 34 mm to which voltages around 20 V are applied. A cloud of 10${}^7$ rubidium atoms is laser-cooled and trapped in the center of the ion cloud. (C) Ions are extracted through the hollow end cap and detected in a time-of-flight mass-spectrometer.

Figure 2

Density profiles of OH${}^{-}$ ions in the trap after three interaction times with rubidium. Shown are the photodetachment rates at different trap positions, which are proportional to the local column density along the laser beam. Each point is determined from fitting a first-order decay process to the trapped ion signal during 2-s interaction time with the detachment laser. Given error bars represent the standard deviation of the fit procedure. Note that the MOT is only fully loaded after roughly 2 s.

Figure 3

Detected OH${}^{-}$ ions after simultaneous trapping of ions and laser cooling of rubidium atoms (red squares). Also shown is the reference measurement (black circles) without trapped rubidium atoms (see text for details).

Figure 4

Measured loss rate for different MOT detunings (3.4$\Gamma$, 2.7$\Gamma$, and 2.0$\Gamma$). The three loss rates are plotted once against the total number of atoms (red squares, determined from the amount of fluorescence light at a fixed detuning) and once against the number of atoms in the excited state (black circles, determined from the amount of fluorescence light at the given MOT detuning). The dotted lines connect each data set, and the dashed lines are fits to a linear model.