A uniform theory of time-resolved resonance fluorescence and resonance Raman with pulse excitation is developed and an approach describing the evolution of a wave packet during vibrational relaxation is given. Then, the theory is applied to the spontaneous emission of a model system with two harmonic electronic states. The evolution of the wave packet during vibrational relaxation obtained leads to the changing of fluorescence signal with time. (C) 2001 American Institute of Physics.