Experiments with an ion-neutral hybrid trap: Cold charge-exchange collisions

Due to their large trap depths (∼1 eV or 10,000 K), versatility, and ease of construction, Paul traps have important uses in high-resolution spectroscopy, plasma physics, and precision measurements of fundamental constants. An ion-neutral hybrid trap consisting of two separate but spatially concentric traps [a magneto-optic trap (MOT) for the neutral species and a mass-selective linear Paul trap for the ionic species] is an ideal apparatus for sympathetic cooling. However, over the past few years, hybrid traps have proven most useful in measuring elastic and charge-exchange rate constants of ion-neutral collisions over a wide temperature range from kilo-Kelvin to nano-Kelvin. We report some initially surprising results from a hybrid trap system in our laboratory where we have loaded the Paul trap with Ca⁺ ions in the presence of a Na MOT (localized dense gas of cold Na atoms). We find a strong loss of Ca⁺ ions with MOT exposure, attributed to an exothermic, non-resonant ion-neutral charge-exchange process with an activation barrier, which leads to the formation of Na⁺ ions. We propose a detailed mechanism for this process. We obtain an estimated measure of the rate constant for this charge exchange of ∼2 × 10^-11 cm ³/s, much less than the Langevin rate, which suggests that the Langevin assumption of unit efficiency in the reaction region is not correct in this case.