Résumé:
In a recent work we have reported an investigation of the applicability of ground state electron density inferred from X-ray diffraction to
the determination of the linear polarizability of an ‘in situ’ conjugated molecule labelled POM and corresponding to a nonlinear optical
organic compound (F. Hamzaoui, A. Zanoun, G. Vergoten, J. Mol. Struct. 397 (1–3) (2004) 17–22). In this work and using the Unso¨ld
approximation one can connect the susceptibility tensor of rank n to the multipolar charge distribution (in the fundamental state) of order 2n.
Thus, the linear polarizability is associated to the quadrupole moment, whereas the first order hyperpolarizability is linked to the octupolar
moment of the charge distribution. Using an approach similar to the one used for the determination of the linear polarizability, we present
here the estimated values of the tensor b derived from the X-ray experiment. However more surprising is the good agreement between the
calculation of the lowest order macroscopic optical nonlinearity as a tensor sum of molecular polarizabilities of the free molecule and the
macroscopic non linear measurements, the generalization of this remark means that the crystalline effects are negligible in the estimation of
nonlinear optical properties.
However, it seems difficult to infer from the Unso¨ld approximation a faithful description of the molecular optical properties, whereas an
estimation of the first order hyperpolarizability components of b seems out of reach.
