What is resonance Raman spectroscopy?

Resonance Raman spectroscopy is a variant of ‘normal’ Raman spectroscopy.  ‘Normal’ Raman spectroscopy uses laser excitation at any wavelength in order to measure the Raman scattering of this laser light.  Notwithstanding the many practical issues caused by the use of different laser wavelengths, the end result will be very similar whatever wavelength is used.

In resonance Raman the excitation wavelength is carefully chosen to overlap with (or be very close to) an electronic transition – this typically means in an area of UV-visible absorption.  Such overlap can result in scattering intensities which are increased by factors of 102-106 – thus, detection limits and measurement times can be significantly decreased.  However, since the excitation coincides with UV-visible absorption fluorescence backgrounds can be significant and more problematic than with ‘normal’ Raman scattering. 

An alternative approach is Surface Enhanced Raman Scattering (SERS), which offers similar order of magnitude increases in intensity – the advantage of SERS over resonance Raman is that fluorescence is suppressed whilst the Raman is enhanced, thus removing the fluorescence background problem of resonance Raman.

For certain specific applications the benefits of resonance Raman can be powerful – one such example is the use of resonance Raman for the analysis of environmental pollutants, where concentrations in the parts per billion (ppb) and parts per million (ppm) range can be detected.

Practically, resonance Raman can be explored on any Raman system, and the actual measurement is made in the standard way.  The obvious requirement is to have suitable laser excitation in order to meet resonance conditions.