Non-Centrosymmetric Molecules in

For non-centrosymmetric molecules in solution EFISH comes from two contributions (independently whether or not the solvent molecules are centrosymmetric) Y(-2o) (o,a),0) and first order molecular hyperpolariza- 

The separation of two terms is not always easy. In some cases the second term on RHS of Eq. (35) is much larger than the first one and consequently the first one can be neglected (cf. Barzoukas et al. (1989)). It is not true for long conjugated ir-electron system and other experiments are necessary to separate both terms. By coupling EFISH with THG and using two or three photon level one can separate both terms (cf. Barzoukas et al (1989), Zyss et al.(1990)). In fact in the case of polyenes (Zyss et al.(1990)) with 6 to 8 double bonds the first term on RHS of in (35) is important and cannot be neglected with respect to the second one. 

The last technique employed by these authors is very useful because it allows to do femtosecond time resolved experiments simply by using incoherent nanosecond laser pulses. 

Two types of phase conjugation configurations can take place. 

This is an incoherent process characterized by an enhanced complex (-a3 a),co,-(o) susceptibility. In one photon resonance the input photon energy matches with the first allowed electronic transition energy whereas in two photon resonance with the half of energy difference between fundamental and excited state of the same symmetry, respectively. One photon resonant values of e.s.u. in thin films of 

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