Molecular orientation of submonolayer rhodamine-6G on quartz substrates: A comparative study using reflection and transmission UV-Vis spectroscopy

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Abstract
Reflection and transmission UV-Vis spectroscopy have been applied to study the molecular orientation and surface density of rhodamine-6G molecules physisorbed on optically flat quartz (SiO2) substrates. Our results have shown that for the s-polarized excitation, the submonolayer of physisorbed rhodamine-6G dye molecules causes enhanced reflection in the wavelength region from 400 nm to 600 nm where the electronic transition takes place. For the p-polarized excitation, the reflection is enhanced when the angle of incidence is smaller than Brewster’s angle of quartz at 55.6°, and the reflection is reduced when the angle of incidence is larger than Brewster’s angle of quartz. An independent method has been established in this paper by which the molecular orientation can be determined accurately by carrying out optical measurements in both the reflection and transmission directions.