Resonant Raman Scattering by Crystal-Violet Molecules Adsorbed on a Smooth Gold Surface: Evidence for a Charge-Transfer Excitation

We report here measurements of excitation-profile spectra for Raman scattering from the central-carbon breathing mode of crystal violet adsorbed on a smooth gold surface. For polarization configurations that involve the $xx(or yy)$ component of the Raman tensor, the spectra exhibit a resonance peak corresponding to the first singlet $\pi -{\pi }^{*}$ electronic excitation of adsorbed crystal-violet ions. The spectrum for the $\mathrm{zz}$ component does not exhibit this feature, but rather exhibits a steadily increasing scattering intensity with wavelength, that is attributed to an "intermolecular" charge-transfer electronic excitation.