In weak coupling

MSN. 19.1. Prigogine and R. Brout, Irreversible processes in weakly coupled systems. Proceedings International Symposium on Transport Processes in Statistical Mechanics, part 11, Brussels, 1958, Interscience Publishers, New York, 1958, p. 25-32. 

As Av/J decreases, the simple multiplets observed in weakly coupled spectra become increasingly distorted new lines can appear and others merge or disappear. Such spectra are termed second-order or strongly coupled spectra. In these cases the chemical shift does not lie in the center of the multiplet and coupling constants are not always obvious. A simple example of such a change is seen... 

Wu MW, Conwell EM (1998) Theory of photoinduced charge transfer in weakly coupled donor-acceptor conjugated polymers application to an meh-ppv cn-ppv pair. Chem Phys 227 11... 

Optical state preparation has been employed much more often. First experiments by the IBM Yorktown Heights and Miinchen groups excited NO vibrationally by an infrared laser [118,126-131]. In all cases, it was found that vibrational relaxation at surfaces is very inefficient. Perhaps this can be understood from the comparison made above of rotational and vibrational excitation. Rotational excitation occurs in the electronic ground state of the molecule-surface system. The coupling is mechanical and is fairly efficient. This has also been seen for rotational relaxation, discussed above. Vibrational relaxation may involve electronic excitation of the molecule-surface system. This is much more improbable, and results in weak coupling between vibration and the other degrees of freedom. More detailed experiments are needed to shed more light on this issue, and those will be discussed next. 

Meakin and Jesson (48) used the Bloch equations in part of their work on the computer simulation of multiple-pulse experiments. They find that this approach is efficient for the effect upon the magnetization vector of any sequence of pulses and delays in weakly coupled spin systems. However, relaxation processes and tightly coupled spin systems cannot be dealt with satisfactorily in this way and require the use of the density matrix. 

For many of the model molecules studied by the trajectory simulations, the decay of P t) was exponential with a decay constant equal to the RRKM rate constant. However, for some models with widely disparate vibrational frequencies and/or masses, decay was either nonexponential or exponential with a decay constant larger than k E) determined from the intercept of P(f). This behavior occurs when some of the molecule s vibrational states are inaccessible or only weakly coupled. Thus, a micro-canonical ensemble is not maintained during the molecule s decomposition. These studies were a harbinger for what is known now regarding inelficient intramolecular vibrational energy redistribution (IVR) in weakly coupled systems such as van der Waals molecules and mode-specific unimolecular dynamics. 

The free induction decay acquired is thus the second half of a spin echo, the amplitude of which is modulated as a function of tj by scalar couplings. In weakly coupled systems, then, the effect of the 180 pulse is to suppress the effects of chemical shifts (and indeed... 

Experimental evidence on excitation transfer derives from different sources. In strongly and weakly coupled systems most of the information has been obtained from absorption spectra and is, thus, rather on the delocalization of excitation. In weakly coupled systems the direct observation of actual excitation transfer is the only source. Depending on the nature of excited states involved, different effects are to be used here, such as fluorescence, phosphorescence, electron-spin resonance and even chemical reactivity. 

Level shifts of order co,- can usually be derived directly from a comparison between observed and expected (i.e., zeroth-order) levels. In weakly coupled systems, the shifts are very small, so that the coupling strength can be obtained more accurately from other data such as intensities. 

In strongly coupled plasmas (small M lp), the Coulomb energy dominates, whereas in weakly coupled plasmas (large M lp), the kinetic energy of the particles dominates. 

A detailed investigation of the temperatiue dependence of the ESR linewidth in weakly coupled van-Vleck paramagnets has been published by Davidov et al. (1977). They have shown that the impurity relaxation sensitively depends on the fluctuation spectan of the host lanthanide ions, due to transitions within the Zeeman levels of the excited states. The host exchange interaction has been extracted by fitting a model function to the experimentally observed thermal broadening. 

The pairwise approximation tends to be accurate in weakly coupled systems. For example, Tauer and Sherrill demonstrated that more than 98% of the interaction energy of various benzene tetramer structures can be recovered by simply adding together the pairwise interactions (or dimers ) in the sys-tem. °° Despite ignoring higher order cooperative effects (3-body and 4-body in this case), differs from Eint by no more than 2% for the benzene tet-... 

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