• electric field;
  • charge transfer;
  • SERS;
  • DFT

The influence of a static external electric field on surface-enhanced Raman scattering is investigated by calculating the Raman spectra and excited state properties of pyridine–Au20 complex with the density functional theory and time-dependent density functional theory method. The external electric field with orientation parallel (positive) or antiparallel (negative) to the permanent dipole moment is respectively applied on the complex. This field slightly changes the equilibrium geometry and polarizabilities, which results in shifted vibration frequencies and selectively enhanced Raman intensities. The changes of charge transfer (CT) excited states in response to the electric field are visualized by employing the charge difference densities. Further, the energy of charge transfer transition is tuned by electric field to be resonant or not with the incident light, leading to the Raman intensities are enhanced or not enhanced. At the same time, the intensities of vibration modes are sensitive to the orientation of the field. The positive electric field enhances the totally symmetric ring breathing mode (~1009 cm−1) but suppresses the trigonal ring breathing mode (~1051 cm−1). On the contrary, the mode at 1051 cm−1 is more enhanced than the mode at 1009 cm−1 when the negative electric field is applied on the complex. The Raman spectra could be modulated by tuning the strength and direction of the electric field. Copyright © 2013 John Wiley & Sons, Ltd.