Dressed states and spectra in quasicontinuum excitation

We consider a system in which a ground state is coupled to a general quasicontinuum of levels and to a true continuum. In the long-time limit, the true continuum spectrum can exhibit zeros, the positions of which depend on the energies of the quasicontinuum levels. The initial conditions determine the existence of these zeros. We describe this phenomenon by studying dressed states for the complete system and in particular derive dressed states which are ground-state independent. We use these to explain the locations of the zeros and their dependence on the initial conditions.