  We demonstrate the application of the recently introduced parametric
projector operator technique to a calculation of the second order non-linear
optical response of a multilevel molecular system. We derive a parametric
quantum master equation (QME) for the time evolution of the excited state of an
excitonic system after excitation by the first two pulses in the usual
spectroscopic four-wave-mixing scheme. This master equation differs from the
usual QME by a correction term which depends on the delay \tau between the
pulses. In the presence of environmental degrees of freedom with finite bath
correlation time and in the presence of intramolecular vibrations we find
distinct dynamics of both the excite state populations and the electronic
coherence for different delays \tau.
