Theoretical calculation of electron transfer between calcium ground-state atoms and Rydberg atoms

We calculated the electronic interaction associated with the exchange of an electron between an atom of calcium excited to a Rydberg state ($n\sim 10-15$) and another, neighbouring calcium atom in its ground state. In this range the Rydberg states have an energy that is comparable to the electron affinity of Ca, enabling resonant or near resonant charge transfer at large internuclear separations (200-700 $a_0$). We calculated the interaction strength while systematically and critically assessing the approximations made, and found it to be large, ranging from $10^{-5}$ $E_h$ (70 GHz) to $10^{-8}$ $E_h$. Charge transfer is thus expected to be efficient and to significantly affect the molecular dynamics at a range of internuclear distances where ultralong range Rydberg molecules also exist.