ABSTRACT
A special type of evolution in time is decay. The initial conditions which determine decay are different from the scattering problems, and because of this it cannot be treated as the subject of the scattering theory. Since the mathematical description is similar in both cases, because unbound states are involved, the two processes are often treated together. There are two basic differences between decay and scattering, in addition to the fact that in the former only one molecule is present initially and two, in the latter. The difference between the time evolution of a decaying system and the time evolution in scattering is large. For a start, in the case of decay one partial wave is present and the system evolves symmetrically in the past and future, which is not the case in scattering. No matter how long-lived the intermediate state in collisions, it cannot be isolated and treated as a decaying state.
