Hyperpolarizabilities permanent moment measurement

Because a strong electric field is required to align the molecules, further restrictions are imposed on the molecules they should have a permanent dipole moment. For instance, EFISHG can not be applied to measure the second-order nonlinear susceptibilities of octopolar molecules, even though at the molecular level, then-molecular hyperpolarizability, is non-zero. Also, EFISHG can not be used with ionic molecules or with a polar solvent. 

The first term in this series is obviously just the permanent dipole moment of the molecule. The coefficient of the second term is the polarizability of the molecule, which measures the change in dipole moment induced by the external field. Higher terms in the series will yield higher hyperpolarizabilities. Analogous expressions may be derived for the quadrupole moment. 

In the case of the first hyperpolarizability, the measured quantity is the vector component of li in the direction of the permanent dipole moment ft, defining the molecular z axis. The relevant averages are given by ... 

The calculation of Sefish requires knowledge of various physical constants for the molecule and the solutions, such as the dielectric constant, the permanent dipole moment of the solute, and the intrinsic second hyperpolarizability of the solute (found from a separate experiment or sometimes ignored). Often results of EFISH are given as /i Sefish rather than jS itself, due to the difficulties in measuring dipole moments. 

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