Light scattering spectroscopy

Light scattering spectroscopy exploits the scattering ability of electromagnetic radiations as they strike a target medium. Scattering occurs due to the interaction between light photons and matter. How do different energy changes facilitate these interactions? What role the scattering phenomenon actually plays in light scattering spectroscopy? What are the different scattering principles and why is this analytical spectroscopic technique valuable to a scientist?

 You will find answers to all these questions in this particular article.

What is light scattering spectroscopy

Light scattering spectroscopy is a molecular vibrational spectroscopic technique. When electromagnetic radiation (or simply light) from an Infrared or a visible light source strikes matter, the electrons present in the matter interact with this light energy. These electrons undergo vibrational energy changes. A part of the radiation is absorbed by the electrons while the remaining are scattered. The scattered radiation beam has a different wavelength, frequency, and direction of motion which is different from the incident beam. The nature and extent of light scattering are thus used to analyze the chemical and molecular structure of the target matter.

What are the 2 main types of scattering in light scattering spectroscopy

Light scattering always involves the vibrational excitation of molecular electrons on absorbing photon energy. The two main scattering principles can be differentiated based on the de-excitation or relaxation of electrons post-excitation.

Rayleigh scattering

De-excitation takes place from the higher energy state to the ground state. All the energy absorbed for electronic excitation is released by de-excitation. Thus, following the law of energy conservation, the photon is scattered elastically. The frequency of scattered and incident photons stays equal in Rayleigh scattering. The blue color of the sky is based on the Rayleigh light scattering principle.

Raman scattering

Raman scattering involves excitation followed by de-excitation of molecular electrons from a higher energy level to a lower level other than the ground state. The frequency of scattered photons is different from that of the incident.

Raman scattering can be sub-classified into:

• Stokes scattering: The molecular electrons are de-excited to a vibrational energy state higher than their ground state. Inelastic scattering occurs.

The energy of scattered photon = hv- ∆E

h=Planck’s constant, v= frequency of incident radiation, hv= energy of the incident photon, ∆E= difference between two energy levels within which the excitation and de-excitation occurred.

• Anti-Stokes scattering: The molecular electrons are originally present at a vibrationally excited state called a virtual state. Absorption of incident photons leads to further excitation. However, the de-excitation occurs to a vibrational energy state lower than the vibrational state occupied by the electrons in the first place. Super elastic scattering takes place.

The energy of scattered photon = hv +∆E

In light scattering spectroscopy, most of the incident light undergoes elastic scattering (Rayleigh scattering) while a small portion of incident light (1 in 10⁸ photons) is Raman scattered. The scattering principle depends on the nature of target molecules and their polarizability. Raman scattering is largely based on the Stoke scattering phenomenon, as anti-stoke scattering is not very common. Based on this scattering principle, light scattering spectroscopy is also popularly known as Raman spectroscopy.

Raman spectroscopy was discovered back in 1928 by an Indian scientist, Chandrasekhara Venkata Raman. He was awarded the Nobel Prize in Physics for this contribution in 1930.  

The importance of molecular polarizability in light scattering spectroscopy

 Molecular polarizability is an essential requirement for light scattering spectroscopy. Polarizability refers to the distortion in a molecule’s electronic cloud under the influence of an applied electric field. A large concentration of weakly held electrons in a bond or molecule leads to high polarizability. Oppositely charged poles are induced in the molecule when it is subjected to an external electric field.

The polarizability should change as the molecule vibrates. Molecular polarizability is an important factor in light scattering spectroscopy because the molecule must squeeze and stretch to undergo vibrational energy changes as a result of photon-molecule collisions. An intense signal can thus be recorded for this molecule. 

Basic components and the working principle of light scattering spectroscopy

Step I: Light irradiation

• High-power solid-state laser diodes are typically used as a radiation source in light scattering spectroscopy.
• These lasers supply monochromatic light beams of wavelengths 532 nm, 785 nm,  1064 nm, etc.
• The laser energy is transmitted to the sample and collected by fiber optics.

Step II: Light scattering

• Radiant light interacts with the sample molecules. Molecular electrons absorb packets of radiant energy called photons and undergo vibrational transitions.
• In addition to vibrational energy changes, molecular electrons also change their rotational state of motion.
• The light travels as a straight path as long as the sample is homogenous.
• In the place of inhomogeneities, the light gets scattered. Both elastic (Rayleigh scattering) and inelastic (Raman scattering) scattering of light occurs.

Step III: Collection of scattered light

• An edge or a notch filter eliminates Rayleigh and anti-stokes scattered light from the scattered light beams.
• In contrast to that, the stokes scattered light is passed onto a dispersion element such as a holographic grating.
• The detector finally collects stokes scattered light and plots the Raman spectrum.
• The commonly used detectors in light scattering spectroscopy are photomultiplier tubes and charge-coupled device (CCD).

Step IV: Raman spectrum

• The Raman intensity is plotted against the Raman shift in a Raman spectrum.
• The Raman shift refers to the frequency difference between incident and scattered light beams.
•  The Raman spectrum displays a series of peaks, each at a different optical wavelength.
• These peaks signify the chemical constituents present in the sample. The intensity of each peak is directly proportional to the concentration of that specific chemical constituent.

Advantages of light scattering spectroscopy

• Highly sensitive technique.
• Non-destructive, non-invasive spectroscopic technique.
• No extensive sample preparation is required.
• Direct physical contact with the sample is not required.
• Offers a diversity of applications.

What are the uses of light scattering spectroscopy

• Light scattering spectroscopy such as Raman spectroscopy helps in studying metallic coordination complexes. The composition (metal-ligand bonds), structure, and,  stability of the complex can be determined.
• The functional groups present in an organic molecule can be analyzed.
• Newly synthesized chemical compounds in the lab and in the industry such as nanomaterials can be studied using Raman spectroscopy.
• Light scattering spectroscopy is extremely useful in polymer structural analysis. It provides a detailed probe on the polymer’s crystallinity, phase, and polymorphs.
• Any impurities or contaminants present in the target sample can also be readily identified using Raman spectroscopy. It provides both qualitative as well as quantitative data.
• Biological molecules can be immobilized on nanoparticles and studied through surface-enhanced Raman spectroscopy.
• Point-to-point scanning of different peaks obtained in a Raman spectrum can differentiate the characteristic regions present in a biological molecule such as its nucleus and its mitochondria.
• In the biomedical field, light scattering spectroscopy can detect malignancy in tumor tissues. 

You may consult this source for studying further the applications of Raman spectroscopy.

What is dynamic light scattering

A different type of light scattering technique is dynamic light scattering. It is also called photon correlation spectroscopy. This technique is based on the scattering of a monochromatic light beam as it passes through a liquid medium in which are suspended colloidal particles.  The Brownian motion of the colloidal particles leads to the scattering of light in different directions.

The wavelength of the scattered light is then measured and the wavelength difference between incident and scattered light beams is used to find the colloidal size. Thus, the dynamic light scattering technique is particularly useful for finding the average particle sizes of nanomaterials.

Some of the other interesting light scattering techniques are Mei scattering and the Tyndall effect.

The difference between light scattering spectroscopy and IR spectroscopy

As a last thought, we would like to tell you that you may not confuse light scattering spectroscopy with IR spectroscopy. Although IR radiations (2.5 to 16 μm) are used as the incident light source in both techniques, both involve vibrational energy changes. But they are two different spectroscopic techniques:

• IR spectroscopy measures the frequency at which vibrational transitions occur. Contrarily, light scattering spectroscopy is based on determining the shift of the incident beam from its original frequency, as we already discussed in this article.
• Change in molecular dipole moment is a necessary pre-requisite for IR spectroscopy while light scattering is predominantly based on molecular polarizability.
• IR inactive vibrations may give a strong Raman signal.
• The symmetric stretch of CO₂ (a linear molecule) does not change its dipole moment, so it is IR inactive, but it does change the polarizability of the molecule, so it is Raman active.
• Similarly, hydroxyl (OH) functional groups which give a characteristic broad peak on an FTIR spectrum, are not particularly Raman active.
• IR spectroscopy help in structural identification by analyzing vibrational transitions in specific groups of the molecule. Conversely, light scattering spectroscopy depends on the vibrations of the entire molecule.   

Thus, both the techniques are quite different but can function complementarily for complete molecular structure determination.

Also check out an advanced version of light scattering Raman spectroscopy, i.e., surface-enhanced Raman spectroscopy.

You may also like to read about fluorescence spectroscopy and luminescence spectroscopy.

Finally, here is for you an introduction to spectroscopy to revise all your basic concepts.

References

1. Jones, R. R., D. C. Hooper, L. Zhang, D. Wolverson, and V. K. Valev (2019). “Raman Techniques: Fundamentals and Frontiers.” Nanoscale Res Lett 14(1): 231.

2. Kuzmany, H. (1998). Light-Scattering Spectroscopy. Solid-State Spectroscopy: An Introduction. Berlin, Heidelberg, Springer Berlin Heidelberg: 169-198.

3. Wolverson, D. (2008). 8 – Raman spectroscopy. Characterization of Semiconductor Heterostructures and Nanostructures. C. Lamberti. Amsterdam, Elsevier: 249-288.