Surface‐enhanced resonance Raman spectroscopy of water‐insoluble Ni(II) porphine adsorbed on aqueous silver sol: suppression of antisymmetric scattering, antiresonance effect and higher order scattering

With the intention of examining how adsorption on a SERS‐active surface affects the electronic resonance enhancement in surface‐enhanced resonance Raman scattering (SERRS) from Ni(II) porphine (NiP), we acquired high‐quality SERR spectra from very low concentrations of NiP (∼10 −8 M ) in aqueous Ag sol from 50 to 3500 cm −1 with excitations across both the Soret and Q absorption bands. Water‐insoluble NiP was transferred to aqueous silver sol surface by using pyridine as carrying solvent, taking advantage of the fact that it both dissolves NiP and is partially miscible with water at the same time. It was found that SERS boosts the overall sensitivity of resonance Raman scattering by providing extra enhancement (the surface enhancement) to a 1 g , b 1 g and b 2 g modes, but not to a 2 g modes. As a consequence, a relative ‘suppression’ effect of antisymmetric scattering by a 2 g modes was observed when excitation was tuned into the Q absorption band region in SERR spectra. With excitations in the valley between the Soret and Q absorption bands, an antiresonance effect was observed in the SERR spectra for the totally symmetric ν 6 mode at 995 cm −1 which was attributed to the destructive interference between Franck–Condon and Herzberg–Teller mechanisms. Strong second‐order and weak third‐order Raman scattering were observed in the SERR spectra of NiP when excitation was tuned close to and across the Q 1 absorption band. A vibronic theory based on the extension of Albrecht's formalism was developed to account for the activation of high‐order Raman scattering and for the antiresonance effect of the ν 6 band. More than 100 Raman wavenumbers, including both fundamentals and overtones/combinations, both in‐plane and out‐of‐plane modes, were observed for NiP in its SERRS spectra. The wavenumbers from the SERR spectra agree very well with those obtained from RR spectra from a homogeneous NiP solution and FTIR data from KBr (CsI) pellets, suggesting that adsorption of NiP on a SERS‐active surface is physisorption in nature, and that such adsorption has a negligibly small effect to the vibrational wavenumbers. The observed Raman wavenumbers from SERR spectra are also in excellent agreement with those predicted from ab initio /DFT calculations for both in‐plane and out‐of‐plane modes. Many out‐of‐plane modes and IR‐active in‐plane modes were observed in the SERR spectra of NiP, suggesting that the effective symmetry of the adsorbed NiP is lower than the ideal planar D 4 h . Copyright © 2001 John Wiley & Sons, Ltd.