Photodetachment of Cold ${\mathrm{OH}}^{-}$ in a Multipole Ion Trap

The absolute photodetachment cross section of ${\mathrm{OH}}^{-}$ 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 ${\mathrm{OH}}^{-}$ 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.

Figure 1

Schematic view of the 22-pole ion trap with the rf electrodes for radial and the cylindrical dc end-cap electrodes for axial confinement. The position of the photodetachment laser is scanned along the vertical axis.

Figure 2

Measured ${\mathrm{OH}}^{-}$ ion signal as a function of storage time in the trap. The upper set of points shows the signal in the absence of the photodetachment laser. The lower points show that, when the laser is switched on at 10 s, a fast additional loss channel due to photodetachment appears. The decay rates are obtained from exponential fits (solid lines).

Figure 3

Measured photodetachment rate (left axis) and derived ion column density (right axis) as a function of the laser beam position (open symbols; their size represents the obtained accuracy). The solid (dashed) line shows the calculated column density taking into account (neglecting) the dependence of the trapping potential on the $z$ direction.