Spin and electronic-excitation exchange in ultracold ion-atom collisions

We experimentally study the dynamics of single and many inelastic collisions between ultracold ${ }^{87}Rb$ atoms and a single ${ }^{88}Sr{ }^{+}$ ion. A single ion is trapped in a linear Paul trap, laser cooled to 1 mK, and initially optically pumped to the higher excited metastable D-state. Then the ion is immersed in an ultracold ${ }^{87}Rb$ cloud. We investigated relaxation rates of the ion from the D-state due to collisions with atoms. We measured relaxation to the S-state after two Langevin collisions on average, followed by an energy release of 1500 K. This can be explained by a non-adiabatic excitation-exchange process: $Sr^{+}(D)+Rb(S)\to Sr^{+}(S)+Rb(P)$ . We further studied the dependence of this process on the mutual spin orientation of the ion and atoms and on initializing the ion in the different spin-orbit split $D_{5/2} $ and $D_{3/2} $ levels. We also initially spin polarized the ion and atoms in their electronic ground state and investigated the spin dynamics of the ion after one to several collisions. We observed that after 7 Langevin collisions on average, the spin of the ion aligned with the spin direction of the cloud, indicating that the dominant interaction between the ion and atoms spins during a collision is that of spin-exchange. Since the steady-state spin population of the ion reached only 90{\%}, we conclude that a spin-relaxation mechanism is involved as well.