The interplay of femtosecond optical excitation and retarded vibrational relaxation in a molecular system is studied using the non-Markovian version of the Quantum Master Equation. To solve non-Markovian equations with an arbitrary memory kernel an expansion with respect to Laguerre polynomials is introduced and the applicability of the method is tested. The non-Markovian effects are identified and parameter regimes are indicated where these effects become predominant. For an early time region just after the optical excited state preparation it is demonstrated that the convolutionless quantum master equation with a time-dependent Redfield-tensor may give a reasonable approximation of the correct non-Markovian dynamics. (C) 2001 American Institute of Physics.