Résumé
Polarization resolved Hyper Rayleigh Scattering (HRS) is used to determine in the volume of the liquid phase the organization of aggregates of 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide (DiA), an amphiphilic probe molecule with a strong quadratic hyperpolarizability. A model is developed to describe the HRS response of a suspension of molecular aggregates. For pure methanol solutions of the probe molecule where the solute is well dispersed in the liquid phase, the data are in agreement with the expected response for a pure dipolar compound. In particular, the harmonic light scattered by the solution is adequately described by a simple model where the DiA molecules behave as well dispersed point-like dipoles locally excited by the fundamental field and radiating at the harmonic frequency. However, in a water - methanol solvent mixture, the data clearly indicate that the DiA molecules are no longer well dispersed but rather form molecular aggregates. The quadrupolar contribution to the HRS intensity of the molecular aggregate is observed through the emergence of a four lobe pattern in the polar plot of the HRS intensity as a function of the polarization angle of the incoming fundamental wave. The loss of the dipolar two lobe pattern and the rise of a four lobe pattern is attributed to the change of the molecular aggregate structure from a non centrosymetrical to a centrosymmetrical one. These results clearly underline that polarization resolved HRS can be used as a probe tool of the molecular organization in nano-sized aggregates.