The problem of the interaction of a beam of radiation in a coherent state and an atomic system is investigated. A complete set of states of the field is introduced, called displaced stationary states, which are found to be particularly convenient in dealing with first- and second-order processes such as dispersion, Raman scattering, single and double emission, and absorption. In this formalism the resonant conditions for the occurrence of processes which have their counterpart in ordinary first- and second-order perturbation theory (where the state of the field is assumed to be a stationary state) are found in a natural way. In addition, a description is indicated of physical situations which are peculiar to coherent field dynamics and thus have no such counterpart. The coherence properties of the displaced stationary states are analyzed. It is shown under which experimental conditions (such as appropriate photon correlation methods) it would appear to be possible to verify whether the displaced stationary states indeed provide a good description of the underlying dynamics. The formalism can readily be extended to higher-order processes.

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