Determination of molecular orientation in molecular monolayers by second-harmonic generation

A detailed comparison is given of several theoretical expressions commonly used in the determination of the molecular orientation in monolayers by reflection second-harmonic-generation measurements. The experimental data obtained from the Langmuir films of a hemicyanine dye are used to demonstrate that the deduced molecular-orientation angle depends sensitively on the linear dielectric constants of the monolayer. It is also pointed out that an unambiguous determination of the nonlinear susceptibilities of a monolayer by reflection second-harmonic generation requires measurements that use at least three different polarization combinations.