  We investigate time evolution of prepared vibrational state (system) coupled
to a reservoir with dense spectrum of its vibrational states. We assume that
the reservoir has an equidistant spectrum, and the system - reservoir coupling
matrix elements are independent of the reservoir states. The analytical
solution manifests three regimes of the evolution for the system: (I) weakly
damped oscillations; (II) multicomponent Loschmidt echo in recurrence cycles;
(III) overlapping recurrence cycles. We find the characteristic critical values
of the system - reservoir coupling constant for the transitions between these
regimes. Stochastic dynamics occurs in the regime (III) due to inevoidably in
any real system coarse graining of time or energy measurements, or initial
condition uncertainty. Even though a specific toy model is investigated here,
when properly interpreted it yields quite reasonable description for a variety
of physically relevant phenomena.
