Definitive evidence for linked resonances in surface-enhanced Raman scattering: Excitation profile of Cu phthalocyanine

We have characterized the surface-enhanced Raman spectroscopy (SERS) spectrum of copper phthalocyanine as a function of excitation wavelength in the 435–635 nm region using a tunable excitation Raman system. We show that the Raman excitation spectrum within the surface plasmon resonance is dominated by totally symmetric modes in the blue and non-totally symmetric modes in the red. The findings are compared to our recent theoretical work on the nature of SERS enhancement. The experimental results are interpreted to be a consequence of vibronically coupled intensity borrowing by charge transfer resonances from allowed molecular transitions, enhancing non-totally symmetric modes.