Relaxation mechanism, general features

The relaxation properties of semi-IPNs are characterized by the same general features as for full IPNs [199-202], The investigation of the temperature dependence of the shear modulus and of the mechanical losses for semi-IPNs based on linear PMMA or polyacrylonitrile and cross-linked PU has shown that the transition temperatures inherent to PU are shifted to higher... 

A relaxation process will occur when a compound state of the system with large amplitude of a sparse subsystem component evolves so that the continuum component grows with time. We then say that the dynamic component of this state s wave function decays with time. Familiar examples of such relaxation processes are the a decay of nuclei, the radiative decay of atoms, atomic and molecular autoionization processes, and molecular predissociation. In all these cases a compound state of the physical system decays into a true continuum or into a quasicontinuum, the choice of the description of the dissipative subsystem depending solely on what boundary conditions are applied at large distances from the atom or molecule. The general theory of quantum mechanics leads to the conclusion that there is a set of features common to all compound states of a wide class of systems. For example, the shapes of many resonances are nearly the same, and the rates of decay of many different kinds of metastable states are of the same functional form. 

It thus appears that, in general, the rates of electronic relaxation processes in dense media are of the same order of magnitude as those found for isolated molecules in the gas phase, assuming that the molecule has a sufficiently large number of vibrational degrees of freedom. Therefore it may be concluded that the mechanism which operates in isolated molecules must be responsible for the gross features of the phenomena observed in dense media. 

The model described is in agreement with the major features of the observations and is consistent with the data available for the predissociation of Hel van der Waals complexes. It remains to be seen if detailed close coupling calculations will provide quantitative verification of all of its features. Although there are not now data to support a generalization, it seems plausible that the zero energy orbiting resonance mechanism for efficient collision-induced vibrational relaxation will occur in all systems. It will be particularly interesting to see what selectivity of vibrational pathway exists in the case of relaxation of a polyatomic molecule. 

---