Absorption of Soft X Rays by Insulators with a Forbidden Exciton Transition

In this note, we consider the absorption of soft x-ray radiation by an insulating crystal for which the lowest excitonic transition from the core state to the conduction band is forbidden. We argue that a proper description must go beyond simple Hartree-Fock band theory and should include substantial energy corrections due to the hole being a local phenomenon during the absorption process, as well as substantial corrections due to electron-electron interactions. Approximate calculations are made for LiF.