PHOTOCHEMICAL BLEACHING OF ADSORBED RHODAMINE 6G AS A PROBE OF BINDING GEOMETRIES ON A FUSED-SILICA SURFACE

Photochemical bleaching and polarization-dependent fluorescence measurements were used to probe the adsorption characteristics of submonolayers of rhodamine 6G on a fused silica surface. The presence of three unique binding geometries of adsorbed dye molecules is inferred from the kinetics of the photochemical bleaching reaction, obtained by monitoring the fluorescence yield as a function of irradiation time over a period of 1500-2000 s. Fluorescence from molecules in each distinctive binding geometry is characterized by a unique ratio between its s- and p-polarized components. A theoretical model relates the polarization ratio of the fluorescence to the angle of orientation of the transition dipole with respect to the plane of the surface. The transition dipole angles for the three inferred geometries are 0-degrees, 8-degrees and 16-degrees. The photochemical bleaching cross section of rhodamine 6G is strongly dependent on the orientation of the transition dipole. Possible ways in which the molecules may be physically oriented are discussed.