Spontaneous relativistic two-photon decay rate mathematical expression in helium-like systems

We derive a theoretical expression for the two-photon emission rate of two- electron systems, in a form suitable for easy implementation in numerical c alculations. Racah algebra techniques were used to extended previous work o n two-photon emission in hydrogen-like systems to more complex ones. The ob tained expression is, as far as we are aware, the first general expression that gives the spontaneous two-photon decay rates of helium-like systems fo r any combination of multipoles.