Physical Review A
American Physical Society
A formula for photoabsorption cross sections of hydrogen and alkali-metal atoms in a static electric field is derived, based on the closed-orbit theory previously used to study hydrogen in a magnetic field. Electric fields are simpler than magnetic fields, because the classical motion is regular and closed orbits can be enumerated. In alkali metals the core modifies the relevant dipole matrix elements, and it produces additional phase shifts.
Gao, J. and Delos, John B., Closed-Orbit Theory of Oscillations in Atomic Photoabsorption Cross Sections in a Strong Electric Field. II. Derivation of Formulas (1992). Physical Review A, 46(3), 1455-1467.