Experimental Measurements of Quadratic Molecular Hyperpolarizabilities

Two major experimental techniques exist to probe the molecular first hyperpolarizability of solutions the EFISHG [41, 42] and the HRS [43]. The EFISHG technique consists in measuring the coherent light intensity at a frequency equal to twice the fundamental frequency of an incident laser pulse, generated by a solution submitted to a static electric field. This method is applicable only to polar and neutral molecules, and gives information on the projection of the vector part of on the dipole moment vector, so that only a combination of tensorial components of P can be obtained  

On the other hand, the HRS method allows measurements on nonpolar and/or ionic molecules. In HRS experiments, the first hyperpolarizability is derived from the intensity of the scattered light at optical frequency 2a on incidence of a laser pulse at co. Contrary to EFISHG, the HRS signal originates from incoherent fluctuations of individual molecular scatterers that do not exhibit specific phase relations with respect to the others. 

Linearly polarized lights are described by i// = 0 (horizontal, H) or i/ = ttl2 (vertical, V) giving the standard HV and VV scattering geometries, respectively. 

For a two-component chromophore/solvent system in the W scattering geometry, the scattering intensity is given as a simple sum of contributions of both species  

For low-symmetry structures, it is convenient to consider (i) the extracted from the total HRS intensity for both VY and HV geometries, which also 

---