  A unitary model describing the electronic transitions in an atom subject to a
strong high frequency laser pulse is proposed. The model fully accounts for the
initial state coupling with the continuum spectrum. Continuum-continuum as well
as discrete-discrete transitions are neglected. The model leads to a single
integro-differential equation for the initial state amplitude. Exact numerical
and approximate closed semi-analytical solutions of this equation are obtained.
A comparison of present results with full time dependent Schr\"odinger equation
solution for Hydrogen atoms subject to a laser pulse is presented. The initial
state time dependent population is rather well described by the model and two
approximate solutions. The electron energy spectrum is also well reproduced by
the model and by a new improved Weiskopf-Wigner related approximation.
