In 1978 it was discovered, largely through the work of Fleischmann, Van Duyne, Creighton, and their coworkers that molecules adsorbed on specially prepared silver surfaces produce a Raman spectrum that is at times a millionfold more intense than expected. This effect was dubbed surface-enhanced Raman scattering (SERS). Since then the effect has been demonstrated with many molecules and with a number of metals, including Cu, Ag, Au, Li, Na, K, In, Pt, and Rh. In addition, related phenomena such as surface-enhanced second-harmonic generation, four-wave mixing, absorption, and fluorescence have been observed. Although not all fine points of the enhancement mechanism have been clarified, the majority view is that the largest contributor to the intensity amplification results from the electric field enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure. Small in this context is gauged in relation to the wavelength of light. The special preparations required to produce the effect, which include among other techniques electrochemical oxidation-reduction cycling, deposition of metal on very cold substrates, and the generation of metal-island films and colloids, is now understood to be necessary as a means of producing surfaces with appropriate electromagnetic resonances that may couple to electromagnetic fields either by generating rough films (as in the case of the former two examples) or by placing small metal particles in close proximity to one another (as in the case of the latter two). For molecules chemisorbed on SERS-active surface there exists a "chemical enhancement" in addition to the electromagnetic effect. Although difficult to measure accurately, the magnitude of this effect rarely exceeds a factor of 10 and is best thought to arise from the modification of the Raman polarizability tensor of the adsorbate resulting from the formation of a complex between the adsorbate and the metal. Rather than an enhancement mechanism, the chemical effect is more logically to be regarded as a change in the nature and identity of the adsorbate.

Surface-enhanced spectroscopy

On the basis of different types of experiments, the authors develop implicitly the model of surface-enhanced Raman scattering (SERS) of adsorbates on metal surfaces. The long-range enhancement by resonances of the macroscopic laser and Stokes field is separated quantitatively from the metal electron-mediated resonance Raman effect. The latter mechanism proceeds by increased electron-photon coupling at an atomically rough surface and by temporary charge transfer to orbitals of the adsorbates. This model can account for the chemical specificity and vibrational selectivity of SERS and (partly) for the SERS specificity of the various metals.

https://iopscience.iop.org/article/10.1088/0953-8984/4/5/001/pdf

Surface-enhanced Raman scattering

A theory for surface enhanced Raman scattering (SERS) is developed Effects due to realistic surface geometry and dielectric properties are included Three sources of enhanced Raman scattering are noted: the image dipole enhancement effect, the increase of local field (’’lightning rod’’ effect), and the resonant excitation of surface plasmons The surface is modeled as a hemispheroid protruding from a conducting plane, although other models are considered The spherical limit is discussed in some detail and molecular orientation effects are considered Cross sections for Mie, Rayleigh, and Raman scattering are derived

Electromagnetic theory of enhanced Raman scattering by molecules adsorbed on rough surfaces

Electromagnetic interactions of molecules with metal surfaces

Molecular surface enhanced Raman scattering (SERS) in compact clusters of 30−70 nm Ag nanocrystals has shown single molecule Raman scattering cross sections that are orders of magnitude larger than free space single molecule luminescence cross sections. We analyze certain aspects of this phenomenon with new numerical electromagnetic calculations, and we also present new spectral depolarization data for single molecule rhodamine 6G scattering. We stress the central role of the Ag femtosecond radiative lifetime, and the spatial distribution of the excited Ag electrons, in the near field and far field optical properties. The fundamental nature of the Ag plasmon excited-electronic-state changes from a volume excitation to a surface junction excitation as two particles approach each other within 1 nm. Adsorbed molecules in the junction interact directly with the metallic excited-state wave function, showing electron-transfer-initiated photochemistry as well as enhanced Raman scattering. Depolarization studies ...

Y.J. Chen

Papers

“Giant” Raman scattering by adsorbed molecules on metal surfaces