Knowledge Bank

In studies of surface-enhanced Raman scattering we have found evidence that the high field gradient near a silver surface may be responsible for the modification of the Raman selection rules for the $adsorbate.1$ Quadrupale allowed electronic transitions for molecules near silver surfaces are now reported which are made observable by the high field gradient. The excitation spectra for fluorescence from a thin film of dye deposited on both a freshly evaporated silver mirror and an aluminum oxide substrate were measured between 300 and 550 nm. Both spectra had a peak corresponding to a transition form the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ ground state to a $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bu$ excited state but spectrum taken on the silver mirror had an additional peak. The additional peak corresponds to a transition form the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ ground state to an $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ excited state. Variation of the source intensity showed that the peak did not result from a two photon effect and therefore must result from a quadrupole transition. The dyes studied were diphenyloctatetraene and diphenylhexatriene.