  A model of particle interacting with quantum field is considered. The model
includes as particular cases the polaron model and non-relativistic quantum
electrodynamics. We compute matrix elements of the evolution operator in the
stochastic approximation and show that depending on the state of the particle
one can get the non-exponential decay with the rate t^{-3/2}. In the process of
computation a new algebra of commutational relations that can be considered as
an operator deformation of quantum Boltzmann commutation relations is used.
