Establishment of population inversion by photoionization of inner electrons in atoms

It is shown that the photoionization of the inner-shell electrons can yield metastable ions. Therefore, the accumulation of excited particles becomes possible at practically realizable optical pumping powers. This method of population inversion may be effective because a large part of the emission spectrum of an optically transparent plasma in a radiation source is in the form of "hard" photons, which are emitted when electrons are captured and atoms or ions go over to their ground state.