Analytical wave function of an atom in the presence of a laser pulse

We study a simple model atom that has two bound states and a continuum of free states, interacting with a strong electromagnetic field. In our analysis we assume that only the continuum-continuum transitions occur- ring between degenerate free states are important for the dynamics of the atomic system; adopting this sim- plifying hypothesis, we show that it is possible to describe the time evolution of the atom by means of an infinite but discrete set of first-order differential equations describing a formal model atom that has two bound states and a degenerate quasicontinuum of states. Moreover, these equations depend on a small number of parameters of the bare atom and of the external laser pulse. The role of these parameters in the laser-atom dynamics is investigated.