Intense quadrupole transitions in the spectra of molecules near metal surfaces

Abstract

The fluorescence, Raman, and fluorescence excitation spectra of diphenyloctatetraene and diphenylhexatriene condensed on rough silver are reported. The fluorescence excitation spectra reveal that on silver the normally dipole‐forbidden ¹A_g←X ¹A_g absorption takes place in addition to the allowed ¹B_u←X ¹A_g process. This is attributed to the large electric field gradient which is present near an illuminated metal surface causing the intensity of quadrupole absorptions to become unusually great. The possible contribution of this field gradient to two‐photon processes in centrosymmetric molecules and to the intensity of bands in infrared and EELS spectra is also noted. In addition, the relative fluorescence intensities of these molecules adsorbed on rough and smooth silver are discussed in terms of the effectiveness of nonradiative decay into metal channels.