Charge-Transfer Effect on Surface-Enhanced Raman Spectroscopy in Ag/PTCA: Herzberg-Teller Selection Rules

The adsorption of 3,4,9,10-perylene tetracarboxylic acid (PTCA) on Ag and Au self-assembled films has been investigated through surface-enhanced Raman scattering (SERS). The spectrum of PTCA reveals a strong excitation wavelength dependence. On the basis of Herzberg-Teller selection rules, it is deduced that appearance of a new b1 mode of PTCA is selectively enhanced by charge-transfer resonance. Moreover, density functional theory (DFT) calculations were also performed to identify the expected vibrational modes and electronic transitions. The information obtained from the adsorption of PTCA on Ag/Au by SERS is helpful to understand the Herzberg-Teller selection rule. Meanwhile, it provides evidence to explain Raman vibronic modes observed when charge-transfer resonances appear in metal-dye systems.