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

摘要

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 D3,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 e symmetry.The e modes are not split,showing that D3 symmetry remains,even on the surface.The SERS spectra,both normal and single-molecule,are dominated by the nontotally symmetric e 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.