DFT, SERS, and Single-Molecule SERS of Crystal Violet

Applying the method of density functional theory calculations, we examine the Raman and surface-enhanced Raman spectra (SERS) of crystal violet. The resulting optimized structure is of point symmetry D-3, and the calculated Raman spectrum provides an excellent match with the observed normal Raman spectrum. This provides a reliable assignment of the symmetry and normal modes of the observed spectrum, which consists of bands assigned to modes of either a(1), or a symmetry. The a modes are not split, showing that D-3 symmetry remains, even on the surface. The SERS spectra, both normal and single-molecule, are dominated by the nontotally symmetric a vibrations, which are preferentially enhanced in accord with the Herzberg-Teller-surface selection rules. The mechanism involves intensity borrowing through vibronic coupling between a charge-transfer state and the lowest-lying pi ->pi* transition. A quantitative measure of the degree of charge transfer is obtained by analyzing the potential dependence of SERS intensities. This indicates a considerable contribution of charge-transfer intensity to the overall SERS enhancement.