A simple and analytical model of quantum recurrences in wave packet dynamics of nonlinear vibrational systems is presented. It is shown that in addition to "normal’ rephasing time tau(R) the width of the packet experiences very strong recurrences at tau(R)/2. In a quasiclassical limit, which can be studied explicitly in this model, the recurrences disappear and the decay becomes irreversible. Recent femtosecond experimental results of Zewail and co-workers are discussed in the framework of this analytical model.