The process of simultaneous emission of two photons (of continuous probability distribution in energy) in the transition from one discrete quantum level to another is here analyzed in the correspondence principle limit in which the quantum numbers of the two states are large and the change in quantum number is small by comparison. In this limit, it proves possible to define a classical dynamical quantity [Eq. (19)], the frequency analysis of whose classical variation with time in a definite orbit determines in the given approximation the absolute transition probability and intensity distribution in the continuous spectrum in question.
JOHN A. WHEELER, "Correspondence Principle Analysis of Double-Photon Emission," J. Opt. Soc. Am. 37, 813-816 (1947)