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Resonance Raman Spectroscopy

Experimental and Theoretical Background


Raman spectroscopy for detection of molecular vibrations and excited state dynamics

Raman spectroscopy is a light scattering experiment. Rayleigh scattering is elastic light scattering, which does not result in a frequency shift of the scattered light. Raman scattering is an inelastic process, which means that light exchanges energy with a molecule. The result is that the molecule either gains or loses vibrational energy. The most common case is when a molecule gains vibrational energy. The Raman fundamental is the transition from the v = 0 state to the v' = 1 vibrational state.

The fundamentals of Raman spectroscopy involve understanding absorption and vibrational spectroscopy based on quantum mechanics. A complete course is available as a resource in the link below:

Spectroscopy Course

Resonance Raman spectroscopy

Resonance Raman spectroscopy is of great importance because Raman bands are enhanced by a factor of up to 106 when the incident laser is tuned into resonance with an absorption band of a molecule. Using this property of Raman scattering one can study the vibrations of specific chromophores, which are strongly enhanced relative to the background. For example, heme in hemoglobin, bacteriochlorophyll in photosynthetic reaction centers, copper ions in blue copper proteins and so on. The basics of resonance Raman spectroscopy are discussed in the link below:

Resonance Raman Spectroscopy

Surface-enhanced Raman spectroscopy

Surface-enhanced Raman scattering is a specific application that requires molecular scattering to be observed in the presence of a plasmonic material, a nanoprticle or nanostructure that consists of a noble metal or other material that has a plasmon in an appropriate spectral region. Surface-enhanced Raman scattering (SERS) was discovered in 1974 during a study of pyridine on a silver electrode. Since that time there have been thousands of studies of Au and Ag electrodes, patterned surfaces, nanoparticles and nanorods or other structures as plasmonic surfaces used to focus electric fields on molecules and thereby enhance their Raman scattering. This area is still full of controversy because it is difficult to characterize the precise structure, the nature of the bonding between molecules and the metals and appropriate control experiments that should be used to determine the enhancement factor. A treatment of these is given in the link below:

Surface-enhanced Raman Spectroscopy

Research observations in the Franzen group and collaborations

Surface-enhanced Raman Spectroscopy

Calculations of Raman cross sections related to SERS

Experimental observations that raise questions about the interaction of molecules and surfaces plasmons

Giant SERS Enhancement

A discussion of Giant SERS or Single-molecule SERS

Alternative view of SERS

Chemical mechanism of SERS as Resonance Raman of a molecule on a metal structure