Frequency Spectrum and Resonance Effects in

Equations (3) (9) allow to calculate the frequency variation of third 

The dipolar transition moment, is related to the one photon allowed 

In EFISH experiments two photon resonances take place with one (because of selection rules break) and two photon levels. For tiiese reasons the two photon resonance observed in PDA s is slightly different (cf. Fig. 23) as in THG because the one and two photon levels are separated by 

In the case of non-c6ntrosymmetric molecules or thin films the calculation of y hyperpolarizability is more difficult because it depends on additional parameters like intervening in Eqs. (8)-(9). However by comparison of EFISH and THG data in solution one can get an independent determination of This was done by Zyss et al. (1990) for a series of noncentrosymmetric polyenes where E2 is close to E. From coupled EFISH and THG measurements on these polyenes in solution they were able to evaluate relative importance of m- 2 2 these molecules. In the case of a long polyene (with 8 double bonds) in both EFISH and THG data the terms depending on p 2 comparably important to those depending on the ground state dipolar moment 

The study of resonance enhancements of is very important for a better knowledge of electronic structure (e.g. position and width of two photon states normally unaccessible by usual spectroscopic techniques) and for a better comprehension of electron-electron interaction in these systems. In fact the theoretical calculation of Hayden and Mele (1985) show that with increasing electron-electron correlation strength a crossover of two and one photon states occurs. It means, that for large electron-electron interactions two photon state lies below one photon state as it was observed. 

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