A general theory of the intermediate state interaction in the second order optical process is developed by extending the stochastic theory of line shape. We introduce the stochastic models of the intermediate state interaction and discuss the general features of the secondary emission spectrum, focusing particular attention on how the correlation between the incident and the scattered photons are affected by the intermediate state interaction. It is clarified how the spectral features or the relative weights among the various components of the second order optical process are determined by a few physical parameters; the radiative damping rate and the modulation amplitude and rate. Further we discuss the relationships between the first and the second order optical processes.
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