Quantum-mechanical elastic and transport cross sections for Mg+-He collisions: from ultracold sympathetic cooling to room-temperature ion mobility.

Abstract

Although Mg+-He is a benchmark system for sympathetic cooling of trapped ions and for ionic mobility measurements, its collisional dynamics has never been characterized: previous studies addressed only the underlying electronic structure. We close this gap with a fully quantum-mechanical, partial-wave study of elastic and momentum-transfer collisions between ground-state Mg+(3s 2S) ions and He(1 1S) atoms, covering twelve orders of magnitude in collision energy, 10-12-10-1 a.u., for the four low-lying molecular states X 2Σ+, A 2Π, B 2Σ+ and C 2Σ+. We calculated the potential energy surfaces of this system in a previous study using a highly accurate ab initio method, and, employing them here as dynamical input while supplementing the exact numerical solution of the radial Schrödinger equation with the JWKB semiclassical approximation at high angular momentum, we obtain the elastic, diffusion and viscosity cross sections and identify a common three-regime pattern: an energy-independent Wigner plateau below 10-9 a.u. (interrupted, for the C 2Σ+ state, by a pronounced virtual-state threshold resonance), a resonance-dominated intermediate regime governed by the depth of each potential well, and a high-energy regime in which the diffusion and viscosity cross sections collapse by several orders of magnitude relative to the elastic cross section, signalling increasingly forward-peaked scattering. The resulting state-resolved cross sections, tabulated here for direct use in transport modelling, provide the first quantitative basis for evaluating sympathetic-cooling rates of trapped Mg+ ions in an ultracold helium buffer gas and the ionic mobility of Mg+ in helium gas.