MEASUREMENT OF THE 3RD-ORDER NONRESONANT SUSCEPTIBILITY OF PHTALATE ION IN SOLUTION AND IN A C6H4COOH.COOK CRYSTAL

The third-order non-linear susceptibility of the phtalate ion has been measured in solution of N,N-dimethylformamide and in the crystal of potassium hydrogen phtalate at room temperature by CARS technique. The C-N symmetric stretching of the solvent molecule has been chosen as an internal standard to compare the contributions to the total non-resonant background of the pure solvent and of 1 M solutions of phatalate ion, benzaldehyde and benzene. An increase of the total non-resonant susceptibility is observed on going from the parent molecule (benzene) to benzaldehyde and phtalic acid. The analysis of the dispersive shape of the band relative to the ring breathing mode at 1038 cm-1 of the phtalate ion in solution and in the crystal allowed us to estimate the value of the non-resonant susceptibility of C6H4COOH.COOK. The measured increase of the molecular hyperpolarizability due to the presence of two polar groups is compared with the results obtained by semi-empirical calculations.