Fundamentals

Raman effect

Intensities

Selection rules

Resonance Raman

SERS

 

 

The Resonance Raman (RR) Effect

In Resonance Raman (RR) spectroscopy the wavelength of the exciting light source, typically a laser, coincides with the wavelength of an electronic transition of the sample. In this case, the Raman scattering intensities of totally symmetric vibrations of the molecular moiety involved in the electronic transition are enhanced by a factor of 10² to 10⁶.

The different types of Resonance Raman scattering

Advantages

High sensitivity:

The intensities of resonance Raman spectra may be enhanced by up to 10⁶. This means that, compared to the non-resonant Raman spectrometry, components at low concentrations can now be detected and analyzed.

Specificity:

By varying the excitation wavelength, different resonance Raman spectra of the same molecule can be obtained. If the excitation wavelength matches the absorption of a specific part of the molecule, then the Raman spectrum associated with this part of the molecule is selectively enhanced and hence separated from the rest of the molecule.

Fig. RR1. The UV/Vis absorption spectrum on the top left shows two major absorptions of the molecule. Exciting the Raman spectra at 363nm and 514nm results in resonantly enhanced Raman spectra. Moreover, they are highly associated with the metallo porphyrine ring (363nm) and the highly conjugated part (514nm) of the molecule. All parts of the molecule contribute to the non-resonant Raman spectrum excited at 785nm.

(Figure by courtesy of Dr. Andrew Dennis, Andor Technologies)

Overtones and combinations:

A common characteristic of a non-resonant Raman spectrum is the absence of overtones and combination vibrations. However, in a resonance Raman spectrum numerous overtones can be observed.

Applied Resonance Raman Scattering

• Resonance Raman experiments are carried out using standard Raman instrumentation but tend to be limited to samples that absorb in the visible range, close to the wavelengths of the more common excitation lasers.

• Biologically important molecules such as carotenoids and metalloporphyrines show resonantly enhanced Raman scattering if excited in the visible spectrum. The Raman spectra of RNA, DNA and proteins can be enhanced by using ultraviolet (UV) excitation lasers.

• It is possible to obtain anharmonicity constants from an overtone progression.

• In the gaseous phase, the excited state repulsive potential functions can be determined accurately for diatomic molecules.

• For polyatomic molecules, the excited state structure can be determined.