Universal properties in ultracold ion-atom interactions.

We present some of the universal properties in ion-atom interaction derived from a newly formulated quantum-defect theory for the -1/r4 type of long-range interactions. For bound states, we present the universal bound spectrum, namely, the equivalent of the Rydberg formula, for ion-atom systems. For scattering, we introduce the concept of a universal resonance spectrum to give a systematic understanding of many resonances present in ion-atom scattering. The theory further provides a method for an accurate spectroscopic determination of the atomic polarizability. It also suggests the existence of atomlike molecules, in which multiple atoms orbit around a heavy ion.