Raman Spectroscopy – Principles

Raman spectroscopy involves molecular and crystal lattice vibrations and is therefore sensitive to the composition, bonding, chemical environment, phase, and crystalline structure of the sample material in any physical form: gases, liquids, solutions, and crystalline or amorphous solids. 

The Raman phenomenon is a consequence of sample illumination with a monochromatic photon beam (laser), most of which are absorbed, reflected, or transmitted by the sample. However, a small fraction of photons interacts with the sample. Duirng this interaction, some energy is transmitted to elementary particles of which materials are constituted (electrons, ions etc.). This causes their transition from ground energy levels to ‘virtual’ excited states. These excited states are highly unstable and particles decay instantaneously to the ground state by one of the following three different processes:
-    Rayleigh scattering: the emission of a photon of the same energy allows the molecule to relax to its ground vibrational state (elastic scattering). Rayleigh scattering, therefore, bears no information on vibrational energy levels of the sample.
-    Stokes and anti-Stokes Raman photons (inelastic scattering): emission of a photon with an energy either below or above that of Rayleigh photons, thereby generating a set of frequency-shifted ‘Raman’ photons. The energy differences of the Stokes and anti-Stokes Raman photons with respect to the excitation energy give information about molecular vibrational levels.

These potons are collected by a detector and transformed to electrical signals and finally to the corresponding Raman spectrum. Usually, Stokes bands which are more intense than anti-stokes bands are called “Raman spectrum” of the sample (Fig. 4.1.2). The Rayleigh band is filtered out before the detector. 

Raman spectroscopy, discovery by C.V. Raman in the 1920s, remained without interest for decades due to the lack of an efficient monochromatic source and appropriate detectors. Raman spectroscopy was born again with the invention and availability of lasers and CCD detectors.

More detailed treatises can be found in “Raman Microscopy, developments and applications” (G. Turrell & J. Corset, Eds., Academic Press, 1996). Reports on the most recent applications of Raman spectroscopy can be found in “Modern Raman Spectroscopy–A Practical Approach” (E. Smith & G. Dent, Eds., John Wiley & Sons, Chichester, 2005).

4.1.3       Raman Spectrometers

A typical Raman spectrometer is made up of five basic parts (Fig. 4.1.3):
-    Excitation source (generally a laser): A laser is used to produce Raman spectra because it gives a coherent beam of monochromatic light. This gives sufficient intensity to produce a useful amount of Raman scatter.
-    Sample illumination and scattered light collection system (probe): The probe is a collection device that collects the scattered photons, filters out the Rayleigh scatter and any background signal from the fibre optic cables, and sends the scattered light to the spectrograph. Many probes also focus and deliver the incident laser beam.
-    Sample holder
-    Spectrograph: When Raman-scattered photons enter the spectrograph, they are passed through a transmission grating to separate them by wavelength and are passed to a detector.
-    Detection system (optical multichannel analyser, PMT, intensified photo array, or a charged coupled device, CCD): this records the intensity of the Raman signal at each wavelength. This data is represented as a Raman spectrum.

The monochromatic light from a laser passes through focussing optics and a beamsplitter to the sample. The scattered light passes through the beamsplitter to a detector. 

The laser light illuminates the sample through a microscope objective (magnification from 100x to 1000x, typically), which is used both for the illumination (laser beam coming from the laser through mirrors and/or optical fibre) and collection of the scattered light. The scattered light goes to the CCD detector via two steps, the first one to suppress the Rayleigh scattering and the second one to split the selected spectral window on the CCD array in order to be able to see all the spectral components. A computer is used to scan, collect, and process the data creating a graph showing the intensity of light at each wavelength. The change in energy is observed as a change in frequency of the incident beam upon scattering.

The microscope offers visualization of the studied area and of the laser spot, which facilitates the choice of the appropriate region to be analyzed. 

A large variety of laser beams are in use, UV (250 nm), visible (green, 514 nm, or red, 633 nm), near-IR (780 nm or 1064 nm). The intensity of the Raman signal is proportional to 1/λ⁴, where λ is the laser wavelength. As a consequence, UV lasers produce Raman bands of a higher intensity and IR lasers produce the least intense Raman spectra. Unfortunately, selection of the ideal laser is not easy because UV lasers are expensive and spectral resolution is not optimal. On the other hand, IR lasers produce a weak Raman signal. In the middle, visible lasers give a good signal/background ratio but in some cases fluorescence spectra of a sample overlap with the Raman spectrum which prevents assignation of Raman bands. In general, green laser (514 nm) leads to more intensive fluorescence than red laser (780 nm), as shown in Fig. 4.1.4.

In general, three types of Raman spectrometer configuration are possible.

4.1.3.1       Large-Size, High-Resolution Instruments

This type of multichannel spectrometer (Fig. 4.1.5) is equipped with a double monochromator as a filter and a liquid nitrogen-cooled CCD matrix (2000 x 800 pixels) detector. Resolution and accuracy are high (<0.5 cm^-1) and the Raman spectrum can be recorded down to ~10 cm^-1. The required power of illumination on samples is typically between 0.1 and 10 mW. The corresponding recording times range between a few tens of seconds to a few hours, depending on sample composition. Laser illumination and collection of scattered light can proceed through lenses (macroconfiguration) or through a (confocal) microscope (magnification up to 1000x, resolution ~1 μm).