Birefringence molecular expression

This approach is based on the introduction of molecular effective polarizabilities, i.e. molecular properties which have been modified by the combination of the two different environment effects represented in terms of cavity and reaction fields. In terms of these properties the outcome of quantum mechanical calculations can be directly compared with the outcome of the experimental measurements of the various NLO processes. The explicit expressions reported here refer to the first-order refractometric measurements and to the third-order EFISH processes, but the PCM methodology maps all the other NLO processes such as the electro-optical Kerr effect (OKE), intensity-dependent refractive index (IDRI), and others. More recently, the approach has been extended to the case of linear birefringences such as the Cotton-Mouton [21] and the Kerr effects [22] (see also the contribution to this book specifically devoted to birefringences). 

As a first example consider birefringence in an electric field by a gas of molecules with D3h symmetry and a twofold degenerate ground electronic term belonging to representation E. Using the explicit form of the matrices of the components of the operators of the dipole moment [Eq. (20)] and polarizability and by means of Eqs. (84) and (85a)-(85c), we obtain for the molecular constant in the Kerr effect in the E term case under consideration the expression... 

In the event the pulse duration tp is longer than the molecular relaxation time, the expression in [14] corresponds to a third-order correlation function Gq - (t). Note also from equation [13] that the transient birefringence has both an instantan us and a slower response to the picosecond pump pulse, since n as the electronic component of the nonlinear susceptibility is expected to follow the laser pulse profile but the molecular component n > a combination of vibrational and orientational parts, carries with it a time-dependence characteristic of each molecular system It is through the latter that information on orientational correlations can be deduced (39). 

Birefringence is a classical method of measurement of orientation. The refractive index represents the slowing of the progress of an electromagnetic wave through a material because of interaction of the wave with polarizable molecules. This problem was first analyzed for isotropic materials (87-89) when the refractive index n was related to the molecular polarizability a through the expression... 

When the molecular orientation of the ciystallize phase is taken into consideration, the birefringence of the spimung filament can be expressed by ... 

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