Sum-over-essential states models

Sum-Over-Essential States Models for Molecular Materials. 

Two-Photon Resonant Enhancement. The frequency dependence of the essential states models is also important as indicated by the potential for resonant enhancement in the denominators D of the sum-over-states (with appropriate damping effects not indicated in eq. 23). Generally, it is undesirable to operate near a resonance with either the fundamental input frequency > or the desired harmonic output. When 2co approaches com, however, an important intermediate two-photon resonance is evident from the second Dm term. In this case, resonant enhancement appears without imdesirable absorption effects. The (other types of frequency conversion and even the nonlinear retractive index. Intermediate resonant enhancement applies not only to the two-level model. Absorption and/or resonant enhancement... 

Therefore, prediction of the p values for a given chemical structure is essential in the second-order NLO field in order to prepare appropriate systems for a specific application and constitutes the basis for the optimization of the microscopic NLO performance. The most widely used theoretical model to discuss the NLO performance of organic molecules consists of a summation over the electronic states of the system (SOS sum over states). The general SOS expressions for the components of the p tensors are well-known [30]. In many cases, only a few electronic levels contribute significantly to the NLO response and the general expressions simplify. For ID charge-transfer molecules (such as p-nitroaniline) a two-level model has yielded satisfactory results for the quadratic hyperpolarizability. For planar (2D) systems, as most phthalocyanines and analogous compounds, three... 

Another procedure is to average over initial H2O states of different parity. Fig.l4 shows relative populations of A-doublet states for different initial rotational states of H2O, averaged over two initial rotational states of H2O. For J=6, for example, the contributions from the states 6oe and 610 are summed, modelling an in-plane rotation (upper panel). If the contributions from 6go and 6gi are summed an out-of-plane rotation is modelled (lower panel). The words "inplane and "out-of-plane have to be used with care, because H2O is an asymmetric rotor. Nevertheless, the essential part of the rotation will be in-plane for the upper and out-of-plane for the lower part of Fig.l4. 

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