Transport of O+ through argon gas

New experimental and theoretical results are presented that address the movement of O+ ions through argon gas. On the experimental front, improved ion mobility results are presented. These results confirm the presence of the oft-cited mobility minimum as a function of electrostatic field strength at room temperature. On the theoretical side, high-level ab initio potential energy curves are calculated for the Ar-O+ system and, from these, transport properties are calculated and compared to experiment. A crossing between the lowest 2Pi curve and the ground state 4Sigma(-) curve near the minimum of each potential becomes an avoided crossing on the inclusion of spin-orbit coupling. It is shown that the more appropriate potential for the description of the motion of O+(4S degree) through Ar at the energies of interest is the diabatic potential, neglecting fine structure. By using an improved 4Sigma(-) potential, agreement with the mobility measurements is obtained for low and intermediate electrostatic field strengths, although small discrepancies remain for high field strengths. The appropriate choice of diabatic or adiabatic potentials is also considered for related systems of interest: He-O+, Ne-O+, and Rg-O(-) (Rg=He,Ne,Ar).