Chemical Effects and the SERS Spectrum

Abstract

After the local electromagnetic field enhancement (EM mechanism) of the Raman scattering intensity has been discussed, the effect of the electronic interactions between the metal and the adsorbate should be considered in order to tackle the interpretation of the observed Raman spectra. Recently, Otto wrote [1], "For sure, without the EM mechanism there would be no signal. But the chemical mechanism determines what is observed." This comment captures the fundamental fact that the observed SERS spectrum contains the information about the adsorbate and its environment, in particular its interaction with the enhancing nanoparticle, its spatial orientation and the polarization properties of the local electric field.In this chapter, we examine the Raman vibrational spectrum of admolecules, molecules adsorbed on an active SERS nanostructure. The factors affecting the rate of adsorption are not discussed here (see Chapter 6), nor are the effects induced by the photon flux (photodesorption and photoreactions), so we examine the Raman spectrum of an adsorbate that does not degenerate during illumination. The electronic cloud of the adsorbate represented by the volume α(Q, ω) can be distorted by the interaction with the solid surface of the nanostructure. When the energy of this interaction (enthalpy of adsorption) is more positive than ∼ −25 kJ mol −1 , the interaction is classified as physisorption, and the Surface-Enhanced Vibrational SpectroscopyR. Aroca