Isolated-core excitations in strong electric fields. I. Theory

A basic theory is presented for the photoexcitation of a core state of a Rydberg atom in any type of static field; in this situation, the core state is excited by the photon while the Rydberg electron is essentially a spectator. This simple picture is made interesting through the interaction of the Rydberg electron with the core which can cause a change in the Rydberg electron's state and can cause the Rydberg electron to autoionize. The method is computationally efficient and has been used for alkaline-earth atoms in a static electric field. An approximation to the formalism is presented that illustrates a mechanism controlling the isolated core excitation in a static electric field; this approximation may serve as a paradigm for extending the interpretation of isolated core spectra to other types of fields.