Reflection and transmission of light by dye monolayers

Using a point-dipole model, we discuss short- and long-range interactions in a 2D array representing a monolayer. Long-range forces may be treated exactly if the film is homogeneous at the wavelength scale and if the effective polarizability including short-range forces is known. Reflection and transmission amplitudes for polarized light are calculated for two monolayer models taking the subphase into account for arbitrary incidence angle and reflection strength. Experimental results are then compared to the theory. The good agreement shows that oblique incidence reflection and transmission spectra are valuable tools for investigating orientation and organization of chromophores in monolayers. Although in most of the studied cases—including J aggregates—the dye chromophore lies flat on the water surface, two instances are presented of molecular aggregates whose transition moments are vertical.