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 aff ects the electronic resonance enhancement in surface-enhanced resonance Ram an scattering (SERRS) from Ni(II) porphine (NiP), we acquired high-quality SERR spectra from very low concentrations of NiP (similar to 10(-8) m) in a queous 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 sil ver 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 re sonance Raman scattering by providing extra enhancement (the surface enhanc ement) to a(1g), b(1g) and b(2g) modes, but not to a(2g) modes. As a conseq uence, a relative,suppression' effect of antisymmetric scattering by a(2g) modes was observed when excitation was tuned into the Q absorption band reg ion 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 nu (6) mode at 995 cm(-1) which was attributed t o the destructive interference between Franck-Condon and Herzberg-Teller me chanisms. Strong second-order and weak third-order Raman scattering were ob served in the SERR spectra of NiP when excitation was tuned close to and ac ross the Q, absorption band. A vibronic theory based on the extension of Al brecht's formalism was developed to account for the activation of high-orde r Raman scattering and for the antiresonance effect of the nu (6) band. Mor e than 100 Raman wavenumbers, including both fundamentals and overtones/com binations, both in-plane and out-of-plane modes, were observed for NiP in i ts SERRS spectra. The wavenumbers from the SERR spectra agree very well wit h those obtained from RR spectra from a homogeneous NiP solution and FTIR d ata from KBr (CsI) pellets, suggesting that adsorption of NiP on a SERS-act ive surface is physisorption in nature, and that such adsorption has a negl igibly small effect to the vibrational wavenumbers. The observed Raman wave numbers from SERR spectra are also in excellent agreement with those predic ted from ab initio/DFT calculations for both in-plane and out-of-plane mode s. Many out-of-plane modes and IR-active in-plane modes were observed in th e SERR spectra of Nip, suggesting that the effective symmetry of the adsorb ed NiP is lower than the ideal planar D-4h. Copyright (C) 2001 John Wiley & Sons, Ltd.