Star-unitary transformations, of the type developed by I. Prigogine and his group, are used to study the pulse excitation and subsequent decay of a single atom coupled to the electromagnetic field. A systematic perturbation scheme is proposed for atomic properties, valid without limitation of time scale or strength of pulse irradiation, provided that the total duration of the pulses is shorter than the radiative lifetimes. In the absence of irradiation, Bloch-like rate equations are obtained as an exact result (in the "thermodynamic limit" of a large box enclosing the field).