Shape of the two-photon-continuum emission from the 1s2s$^1$S$_0$ state in He-like krypton. R. Ali, I. Ahmad, R. W. Dunford, D. S. Gemmell, M. Jung, E. P. Kanter, P. H. Mokler, H. G. Berry, A. E. Livingston, S. Cheng and L. J. Curtis in Phys. Rev. A 55:994 (1997).  What the paper says?

This page is still in progress.

They note that, since in 2nd perturbation, all intermediate states are summed over, the process allows for a test of the full structure of the emitter.

There is a nice review of the many experiments in various systems.

I am particularly interested in their Fig. 10 which features a counterpart of the leapfrog emission: