Anisotropic interaction potential between a Rydberg electron and an open-shell ion

We develop a theoretical description of the anisotropic interaction between a Rydberg electron and an ionic core. An adiabatic formulation expresses the anisotropic nature of the interaction through angular-momentum operators of the core and the Rydberg electron. Terms with odd tensorial structure, such as a vector hyperpolarization proportional to (L) over right arrow(c) .(l) over right arrow, emerge in the long-range potential. Computed energies of neon n=10 Rydberg states are compared with recent experimental measurements. The vector term improves agreement between theory and experiment. The vector hyperpolarizability is calculated explicitly, along with some standard polarizabilities of Ne+.