For a long time the interaction between light and atoms has only been studied in the perturbative regime. The reason for this is that the forces that keep an atom together are enormous compared to our every-day experience. For instance, the electric field that an electron in a hydrogen atom feels at its most probable distance from the nucleus is 5.1011V/m. This field defines the atomic unit of electric field strength. It is orders of magnitude larger than any dc field that can be made in the laboratory, but rather typical for the force that binds a valence electron in an atom.

D. Liesen

Muller, H. G. (1995). Photoionization with super-intense lasers. In D. Liesen (Ed.), Physics with Multiply Charged Ions : Proceedings of a NATO Advanced Study Institute on Physics with Multiply Charged Ions, held July 18-30, 1994, in Cargèse, France (pp. 239–261).