Raman & IR Spectroscopy

Usually, a molecule cannot be fully characterized based solely on its Raman fingerprint. This due to the vast number of asymmetric vibrations that Raman doesn't detect. For this reason, infrared spectroscopy is often used as a complimentary tool to carry out an accurate characterization of a sample.^[5] It can be noted that not all vibrational modes are restrictively only Raman or IR active - some can be both. The nature of the spectroscopical activity arises from the symmetry of the vibrating molecule. Molecules with central symmetry (centrosymmetric) follow the so called rule of mutual exclusion, meaning that a Raman active mode is not detectable with IR, and vice versa. Examples of such molecules are benzene, acetylene, and CO₂. However, non-centrosymmetric molecules can adopt vibrational states that are detectable with both techniques. Examples of such molecules are water, ethanol, and acetic acid.^[9]

Fig. 2 and Fig. 3 show examples of different CO₂ vibrational modes that can only be detected by IR or Raman - but not both.

Figure 2: The symmetric stretching of CO₂ at wavenumber 1480 cm^-1. In this illustration, the same molecule is depicted in two distinctive moments in time.^[9] (License: CC BY-NC-SA 2.0 UK)

As Fig. 2 shows, the symmetric streching of the oxygen atoms has an effect on the polarizability (with respect to the equilibrium position) of the molecule, while the dipole moment remains constant. This makes this vibrational mode Raman active and, due to mutual exclusion, IR inactive.^[9]

Figure 3: The asymmetric bending of CO2 at wavenumber 526 cm^-1. In this illustration, the same molecule is depicted in two distinctive moments in time.^[9] (License: CC BY-NC-SA 2.0 UK)

As Fig. 3 shows, the asymmetric bending of the oxygen atoms has an effect on the dipole moment of the molecule, while the polarizability (with respect to the equilibrium position) remains constant. This makes this vibrational mode Raman inactive and, due to mutual exclusion, IR active.^[9]

Raman Instrumentation

Raman Sampling

Raman doesn't require any high-end sampling prior to the measurements, which makes it an easy and relatively quick characterization method. Also, many different types of samples can be characterized, including gases, liquids (also solutions), and solids.^[1][2][5]

Raman Spectrometer

Fig. 4 shows a schematic drawing of the working principle of a Raman spectrometer. As Fig. 4 shows, the Raman measurement begins as a monochromatic incident beam is created by a laser and guided towards the sample by mirrors. Once the incident beam hits the surface of the sample, it gets scattered and heads back for the detector. Since most of the refracted light has been Rayleigh scattered, filters are used to separate only the Raman scattered photons. These photons are then diffracted into a spectrum via a process called grating. Finally, the spectrum reaches the detector which converts it into electrical signals that can be interpreted by a computer. Throughout the process, a microscope can be used to monitor the sample and to scan the sample surface for areas of interest.^[10]

Figure 4: A schematic of the working principle of a Raman spectrometer (Picture by Tommi Rinne, inspiration from^[10]).

Examples of Raman Spectra

In Fig. 5, Fig. 6, and Fig. 7 the Raman spectra of eskolaite, diamond, and graphite are shown. Carbon allotropes are an example of minerals that are well characterized with Raman. This can be seen from the sharp characteristic peaks in their Raman spectra (graphite ca. 1580 cm^-1, diamond ca. 1330 cm^-1).

Figure 5: Raman spectrum of eskolaite - the two data sets correspond to two different frequencies of the incident beam. (License: Public Domain)^[11]

 Figure 6: The Raman spectrum of diamond  - the different data sets correspond to different incident beam polarization. (License: Public Domain)^[12]

Figure 7: The Raman spectrum of graphite - the different data sets correspond to different incident beam polarization. (License: Public Domain)^[13]

Summary: Pros and Cons

Advantages of Raman Spectroscopy

• Wide range of analyzable samples (gases, liquids, solids, solutions)
• No extensive sample preparation and short measuring time
• Not vulnerable to interference with glass containers or water (aqueous solutions can be analyzed)
• Measurements don't require a vacuum

• Non-destructive^[14]

Drawbacks of Raman Spectroscopy

• The inability to characterize metals/alloys^[1]
• Vulnerable to sample fluorescence^[4]

References