Symmetry and Second-Order Susceptibility of Hemicyanine Monolayer Studied by Surface Second-Harmonic Generation

Theoretical expressions for the surface second-harmonic generation are given for some particular combinations of polarizers. Simulated results are displayed and are compared with the experiments made for the hemicyanine monolayer on the water surface and on a substrate. The main results are as follows. (1) The formation of the H-aggregates on water, which strongly depends on the compression process, breaks C∞v symmetry, and the deposition onto a substrate gives rise to a symmetry change of the H-aggregates. (2) The average molecular tilt angles of pure and mixed hemicyanine monolayers with arachidic acid scarcely change throughout the compression process, unless the H-aggregates are formed. (3) The frequency dispersion of the dielectric constant in the monolayer should not be neglected to obtain correct susceptibility tensor elements.