Two-Photon Emission of X Rays

Two-photon emission from heavy elements with a single $K$-shell vacancy (e.g., ${\mathrm{Cu}}^{+}$) is considered. Simple closed-form expressions are given for the spontaneous, singly stimulated, and doubly stimulated two-photon emission rates. The material parameters relevant to these two-photon processes are shown to be the one-photon absorption cross sections and oscillator strengths.