Hyper-Rayleigh and hyper-Raman scatterings with intermediate and two-photon resonances

The normally weak process of hyper-Raman scattering can be greatly enhanced when the excitation is two photon resonant with an electronic transition that is both one and two photon allowed. It might be expected to be further enhanced when a one-photon allowed transition provides an intermediate state resonance in the two-photon excitation step. The theory of this triply resonant process is developed for organic nonlinear chromophores. Experimental results are obtained for one donor-acceptor substituted push-pull chromophore in which the energy of the strongly allowed lowest-lying one-photon state may be tuned by varying the solvent without substantially affecting the two-photon resonant transition. Surprisingly, bringing the one-photon allowed state into resonance does not significantly increase the hyper-Rayleigh or hyper-Raman hyperpolarizabilities. Analysis of the resonance Raman, hyper-Rayleigh, and hyper-Raman profiles suggests that the triply resonant path does not make the dominant contribution to the hyperpolarizability in this system.