  Analytical expressions for the semiclassical dressed states and corresponding
quasienergies are obtained for a two-level quantum system driven by a
nonresonant and/or strong laser field in a coherent state. These expressions
are of first order in a proper perturbative expansion, and already contain all
the relevant physical information on the dynamical and spectroscopic properties
displayed by these systems under such particular conditions. The influence of
the laser field parameters on transition frequencies, selection rules, and line
intensities can be easily understood in terms of the quasienergy-level diagram
and the allowed transitions between the different semiclassical dressed states.
