J. M. Kwolek

We present measurements of the charge-exchange reaction rate between neutral sodium (Na) and ionized calcium (\ce{Ca+}) in a hybrid atom-ion trap, which is comprised of a Na magneto-optical trap concentric with a linear Paul trap. Once the Na and \ce{Ca+} are co-trapped, the reaction rate is measured by continuously quenching the reaction product \ce{Na+} from the ion trap, and then destructively measuring the decay of the remaining ion population. The reactants' electronic state and temperature are experimentally controlled, allowing us to determine the four individual reaction-rates between $\text{Na}[\text{S~or~P}]$ and $\text{Ca}^+[\text{S~or~D}]$ at different collision energies. With the exception of the largest reaction-rate channel ($\text{Na}[\text{S}]+\text{Ca}^+[\text{D}]$), our rates agree with classical Langevin rate limit. We have also found evidence of reactant collision-energy thresholds associated with two of the four entrance-channels.

We present model-independent measurements of the excited-state population of atoms in a sodium (Na) magneto-optical trap (MOT) using a hybrid ion-neutral trap composed of a MOT and a linear Paul trap (LPT). We photoionize excited Na atoms trapped in the MOT and use two independent methods to measure the resulting ions: directly by trapping them in our LPT, and indirectly by monitoring changes in MOT fluorescence. By measuring the ionization rate via these two independent methods, we have enough information to directly determine the population of MOT atoms in the excited-state. The resulting measurement reveals that there is a range of trapping-laser intensities where the excited-state population of atoms in our MOT follows the standard two-level model intensity-dependence. However, an experimentally determined effective saturation intensity must be used instead of the theoretically predicted value from the two-level model. We measured the effective saturation intensity to be $I_\mathrm{se}=22.9(3)\:\textrm{mW}/\textrm{cm}^2$ for the type-I Na MOT and $I_\mathrm{se}=48.9(7)\;\textrm{mW}/\textrm{cm}^2$ for the type-II Na MOT, approximately 1.7 and 3.6 times the theoretical estimate, respectively. Lastly, at large trapping-laser intensities, our experiment reveals a clear departure from the two-level model at a critical intensity that we believe is due to a state-mixing effect, whose critical intensity can be determined by a simple power broadening model.

We present experimental measurements of the steady-state ion number in a linear Paul trap (LPT) as a function of the ion-loading rate. These measurements, taken with (a) constant Paul trap stability parameter $q$, (b) constant radio-frequency (rf) amplitude, or (c) constant rf frequency, show nonlinear behavior. At the loading rates achieved in this experiment, a plot of the steady-state ion number as a function of loading rate has two regions: a monotonic rise (region I) followed by a plateau (region II). Also described are simulations and analytical theory which match the experimental results. Region I is caused by rf heating and is fundamentally due to the time dependence of the rf Paul-trap forces. We show that the time-independent pseudopotential, frequently used in the analytical investigation of trapping experiments, cannot explain region I, but explains the plateau in region II and can be used to predict the steady-state ion number in that region. An important feature of our experimental LPT is the existence of a radial cut-off $\hat R_{\rm cut}$ that limits the ion capacity of our LPT and features prominently in the analytical and numerical analysis of our LPT-loading results. We explain the dynamical origin of $\hat R_{\rm cut}$ and relate it to the chaos border of the fractal of non-escaping trajectories in our LPT. We also present an improved model of LPT ion-loading as a function of time.

We predict that the steady-state ion number $N_s$ for radio-frequency (rf) traps, loaded at a rate of $λ$ particles per unit time, shows universal non-monotonic behavior as a function of loading rate $λ$. The shape of $N_s(λ)$, characterized by four dynamical regions, is universal in the sense that it is predicted to manifest itself in all rf traps independently of the details of their construction. For $λ\ll$ 1 particles / rf cycle (Region I), as expected, $N_s(λ)$ increases monotonically with $λ$. However, contrary to intuition, at intermediate $λ\sim 1$ particles / rf cycle (Region II), $N_s(λ)$ reaches a maximum, followed by a minimum of $N_s(λ)$ (Region III). For $λ\gg 1$ particles / rf cycle (Region IV), $N_s(λ)$ again rises monotonically. In Region IV numerical simulations, analytical calculations, and experiments show $N_s(λ)\sim λ^{2/3}$. We confirm this prediction experimentally with MOT-loaded Na$^+$ ions stored in a hybrid ion-neutral trap.

We present measurements of the total elastic and resonant charge-exchange ion-atom collision rate coefficient $k_\mathrm{ia}$ of cold sodium (\ce{Na}) with optically-dark low energy \ce{Na+} ions in a hybrid ion-neutral trap. To determine $k_\mathrm{ia}$, we measured the trap loading and loss from both a \ce{Na} magneto-optical trap (MOT) and a linear radio frequency quadrupole Paul trap. We found the total rate coefficient to be $7.4 \pm 1.9 \times 10^{-8}$ cm$^3$/s for the type I \ce{Na} MOT immersed within an $\approx 140$ K ion cloud and $1.10 \pm 0.25 \times 10^{-7}$ cm$^3$/s for the type II \ce{Na} MOT within an $\approx 1070$ K ion cloud. Our measurements show excellent agreement with previously reported theoretical fully quantal \textit{ab initio} calculations. In the process of determining the total rate coefficient, we demonstrate that a MOT can be used to probe an optically dark ion cloud's spatial distribution within a hybrid trap.