Weak coupling scheme

Energy migration in concentrated systems has been an issue of research in the last decade. Especially since lasers became easily available, the progress has been great. In Section VIII we will first consider the case that S is an ion to which the weak-coupling scheme applies. In practice this case consists of the trivalent rare earth ions. Subsequently we will deal with the case where S is an ion to which the intermediate-or strong-coupling scheme applies. 

Let us now turn to energy migration in concentrated systems for which the weak-coupling scheme is no longer valid. In compounds of... 

In conclusion, there are several types of nonradiative processes in luminescent materials. The two most important ones are energy transfer between two luminescent centers and intracenter nonradiative decay. The former are reasonably well understood and predictable. The latter are only understood and predictable in the case of ions for which the weak-coupling scheme is valid. For the others it is nevertheless possible to arrive at reasonable predictions by applying simple models to a series of luminescent compositions (see Section IV). 

It is usual to call the AR = 0 situation the weak-coupling scheme, AR > 0 the intermediate-coupling scheme, and AR > 0 the strong-coupling scheme. The word coupling relates to the coupling between the electrons and the vibrations of the center considered. The value of AR measures the strength of this interaction. 

In this limit the vibronie states of each site are coupled as if each vibronie state were independent of the others in the same electronic band. In a system consisting of only totally symmetric vibrations it is apparent that all the vibronie bands will have the same symmetry. These states could therefore mix considerably if the splitting became comparable with the vibrational intervals. Thus, in the weak coupling scheme we assume, following Simpson and Peterson (10), that the interactions between... 

A similar weak-coupling scheme may be equally useful for a description of interactions between two closely spaced higher excited states, e.g., the case of S2 and S3 states of azulene-[5,6,7-c,d]-phenalene (Holzwarth et al., 1977). 

Although anisotropic pressure coupling is the simplest approach computationally, especially when the weak coupling scheme has already been implemented in the code used, it may not be entirely correct from a. statistical mechanical point of view. Zhang and -Feller et al. identified a series of ensembles that can be obtained with simulations of liquid-liquid interfaces and derived equations of motion using an extended system Hamiltonian that includes a piston for pressure coupling. ... 

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