We investigate the energy dependence and the internal-state dependence of the charge-exchange collision cross sections in a mixture of 6 Li atoms and 40 Ca + ions. Deliberately excited ion micromotion is used to control the collision energy of atoms and ions. The energy dependence of the charge-exchange collision cross section obeys the Langevin model in the temperature range of the current experiment, and the measured magnitude of the cross section is correlated to the internal state of the 40 Ca + ions. Revealing the relationship between the charge-exchange collision cross sections and the interaction potentials is an important step toward the realization of the full quantum control of the chemical reactions at an ultralow temperature regime.

We demonstrated sympathetic cooling of a single ion in a buffer gas of ultracold atoms with small mass. Efficient collisional cooling was realized by suppressing collision-induced heating. We attempt to explain the experimental results with a simple rate equation model and provide a quantitative discussion of the cooling efficiency per collision. The knowledge we obtained in this work is an important ingredient for advancing the technique of sympathetic cooling of ions with neutral atoms.PACS numbers: 37.10. Ty,03.67.Lx Sympathetic cooling, where we thermally contact two distinct systems at different temperatures, is an effective method for cooling an object to a desired energy regime. Nowadays, this is commonly used in the field of low-temperature physics for producing a cold sample for a molecular beam or degenerate atomic gases. The elemental mechanism of this technique extracts energy from a target object through interaction (normally by collisions) with a coolant system. In cooling of translational motion, for example, an exchange of moment between the two systems can remove kinetic energy from the thermal system, and eventually they reach thermal equilibrium, resulting in cooling of the target object.To introduce an ultracold atomic gas as a coolant for trapped ions is attractive, since collisions with ultracold atoms enable efficient cooling of a number of vibrational modes simultaneously. It is beneficial for many applications, for example, continuous cooling of ion qubits in quantum information processing. In addition, this method has been proven to be effective for the cooling of atomic or molecular systems, especially when no conventional laser cooling transition is accessible, as demonstrated in rotational-vibrational cooling of molecular ions [1,2].In buffer-gas cooling of a charged particle, however, the situation is not as simple as in the usual schemes, e.g., evaporation or sympathetic cooling in a mixture of neutral gases. This is simply attributed to the dynamics of trapping it in a radiofrequency (RF) trap, where slow (secular) and rapid (micro) motion are superimposed on the motion of the ion. The main point is that an abrupt interception of coherent ion motion by an atom-ion collision complicates the kinetics of the ion. Importantly, an ion can be either cooled or even heated, depending on the instantaneous phase of micromotion at the moment of a collision, which prevents efficient collisional cooling [3]. In addition, this peculiar feature modifies the energy distribution of an ion by inducing a deviation from a normal (Maxwell-Boltzmann) distribution to * haze@ils.uec.ac.jp a super-statistical (so-called Tsallis) distribution accompanied with a power-law tail in a high-energy region [4][5][6]. These subjects have been pointed out since the early stages of ion trapping experiments [7], and they were recently revisited again in conjunction with the rapidly growing field of ultracold atom-ion hybrid systems [8].A key parameter for characterizing the kinetics of the ion in a buffer gas is t...