A method is developed for simplifying molecular interpretations of nonlinear optical phenomena. General sum-over-states expressions derived from perturbation theory can be written identically and self-consistently as simple products of lower-order effects. Electric dipole-allowed expressions for the nonlinear polarizability reduce to straightforward formulas directly connected to intuitive molecular properties without sacrificing mathematical rigor. This approach is sufficiently general to allow its application in treating electronic, vibrational, and vibronic interactions for both parametric (passive) processes (e.g., wave-mixing spectroscopies, sum- and difference-frequency generation, harmonic generation, etc.) and nonparametric (active) processes (e.g., hyper-Raman spectroscopy, multiphoton absorption, etc.). Explicit examples for sum-frequency generation and for four-wave mixing provide a convenient context for interpreting higher order nonlinear optical processes.
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