SPECTRA AND ANGULAR-DISTRIBUTIONS OF FLUORESCENCE EMITTED FROM ANTHRACENE ON A SILVER SURFACE

The fluorescence spectra of anthracene molecules adsorbed on a silver substrate were measured as a function of substrate temperature, T. At lower T, the spectra consist of a broad structureless excimer fluoresscence band; at higher T, a structured band due to monomer fluorescence appears and grows with T to dominate the spectra. The angular distributions of the excimer fluorescence, arising from interference at the surface, were also obtained. By analyzing the data on the basis of a classical model, the orientations of the excimers at several temperatures were determined. Moreover, the adsorption state of the molecules as a whole and its change with T were deduced from both the data of the spectra and the angular distributions: with increasing temperature, the molecules are rearranged from an amorphous state to a crystalline state, in which the molecules are oriented with their short axis parallel to the surface and their long axis almost perpendicular to it.