Why do we need three levels to understand the molecular optical response
2011
Traditionally, the nonlinear optical response at the molecular level has been modeled using the two-level approximation,
under the assumption that the behavior of the exact sum-over-states (SOS) expressions for the molecular polarizabilities
is well represented by the contribution of only two levels. We show how, a rigorous application of the Thomas-Kuhn
sum-rules over the SOS expression for the diagonal component of the first-hyperpolarziability proves that the two-level
approximation is unphysical. In addition, we indicate how the contributions of potentially infinite number of states to
the SOS expressions for the first-hyperpolarizability are well represented by the contributions of a generic three-level
system. This explains why the analysis of the three-level model in conjugation with the sum rules has lead to successful
paradigms for the optimization of organic chromophores.
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