SFG STUDY OF 2-DIMENSIONAL ORIENTATION OF SURFACE METHYL-GROUPS ON CADMIUM ARACHIDATE LANGMUIR-BLODGETT-FILMS

The orientational order of the methyl groups on the surface of cadmium arachidate Langmuir-Blodgett (LB) films with a various number of accumulated monolayers on fused silica has been studied by infrared-visible sum frequency generation (SFG) and Fourier transform infrared absorption (FTIR) spectroscopies. An analysis has been made of the significant in-plane anisotropy of the SFG signals observed for multilayered films and of the change in peak intensities as a function of the number of layers using previously published formulas. The analysis reveals that the SFG signal observed originates from the uppermost layer where the methyl groups are oriented so that the hydrocarbon chains stand perpendicular to the surface with an orthorhombic subcell structure and with the terminal methyl groups inclined away from the direction of withdrawal when the film was prepared. The apparent increase of the SFG signal intensity with number of accumulated layers is ascribed to an increased degree of two-dimensional orientational order of the methyl groups.