Photochemical processes and chromophore aggregation in the surface and interface layers of molecular materials

Two time-resolved spectroscopic methods that make use of total internal reflection conditions, in which light penetrates into materials with a lower refractive index from a glass plate with a higher one, are described. The penetrating light is called an evanescent wave and can be used as excitation or probe beams for surface and interface spectroscopic studies. One of the methods, fluorescence spectroscopy, measures selectively the fluorescence from the interface layer; the other, absorption spectroscopy, uses the evanescent light as a probe beam. Time-correlated single-photon counting is used for measuring fluorescence emitted from the interface layer. This spectroscopy has submicrometer depth resolution, 10-ps time resolution, and a few nanometers of spectral resolution. If this measurement is conducted under a fluorescence microscope, 2-D resolution is added. This method has been used for various systems such as layered vacuum-deposited films, polymer films, and dyed fabrics. The second method has been used to obtain triplet-triplet absorption spectra of aromatic molecules in the submicrometer depth region of polymer films.