  The character of the interaction between an impurity vibrational mode and a
heat bath leads to certain peculiarities in the relaxational dynamics of the
excited states. We derive a non-Markovian equation of motion for the reduced
density matrix of this system which is valid for initial, intermediate and
kinetic stages of the relaxation. The linear phase-sensitive character of the
interaction ensures the ultrafast disappearance of the quantum interference of
the initially superpositional state and the effect of classical squeezing of
the initially coherent state. On the other hand, the second power interaction
induces a partial conservation of the quantum interference.
