Raman Spectroscopy: Decoding Molecular Composition

[This post is intended for General Audience]

Have you ever wondered how scientists can identify the chemical composition of a substance without even touching it? Or how does the curiosity rover on the Mars tells us about the composition of the sand and rocks there? The answer lies in a fascinating technique called Raman spectroscopy. Whether you’re a curious person from non-scientific background or an academic researcher, Raman spectroscopy has something to offer.

But first let me introduce you to what is Raman spectroscopy?

We’re all familiar with light as an electromagnetic wave (EM) and wave-particle(photons) duality and it interacts with matter in various ways—through absorption, scattering, reflection or transmission.

In absorption, photons’ energy needs to match the energy gap between two electronic energy levels. However, in the case of scattering, things are different. Scattering involves photons interacting with the structure of a crystal lattice or a molecule, causing a distortion in their electronic cloud. This process changes the polarization and involves what we call virtual states—these are temporary states that fade away quickly, leaving the electronic configuration in the real state of the system. The photon exits the system, while the electronic arrangement remains altered.

Now, the exciting part emerges. When the energy of the scattered photon matches that of the incident photon, we call it elastic scattering or Rayleigh scattering.
On the other hand, if the incoming photon has more energy than the outgoing one, it’s termed Stokes scattering; if the reverse is true, it’s anti-Stokes scattering.
This difference in energy is what we refer to as the Raman shift.

Figure retrieved from source.

This on a graph where the Raman shift is plotted against intensity. By comparing this plot with existing data, we can deduce the composition of the material.

Source: “Gold Nanoparticles Generated Using the Nanosecond Laser Treatment of Multilayer Films and Their Use for SERS Applications” by Evaldas Stankevičius et al., ACS Omega 2021, 6, 49, 33889–33898, Link to the article.

In the laboratory we use an equipment called Raman spectrophotometer which works on this phenomenon, scattering of light and the distant rovers are also equipped with a similar spectrophotometer, (we could say more advanced, compact).

When the laser light is forced to interact with the substance ,we get a fingerprint-like pattern unique to the substance’s molecular vibrations in the systems. By analyzing this pattern, we call it a raman spectra, it is possible to determine the composition and chemical bonds present in the substance, helping to identify its molecular structure and overall composition.

Xplora™ Plus Raman Spectrometer from HORIBA

On Mars, the NASA Mars 2020 rover utilizes its SHERLOC(Scanning Habitable Environments with Raman & Luminescence for Organics and Chemicals) instrument located at the end of the rover’s robotic arm to perform Raman spectroscopy remotely. The rover’s laser fires at rocks or soil, the resulting light emission, including Raman scattered light, is captured and analyzed. This allows scientists to gather information about the composition of Martian rocks and soil without physically touching them.

By studying the Raman spectra obtained, researchers can infer the mineral content and potential signs of past water or organic compounds on the Martian surface, providing valuable insights into the planet’s geological history and habitability.

Mars 2020’s SHERLOC Instrument , Source : NASA Mars Exploration

But is it only limited to these two applications? No.

Along with the Material identification and characterization, Raman spectroscopy is used widely across Biomedical applications,Environmental monitoring, Pharmaceutical analysis, Nanotechnology and so much more.

Beneath the amazing Raman spectroscopy, there’s a need for knowing how to work with tools, handle data, and understand physics. It’s like a puzzle needing skills, patience, and guidance to solve.

Are you interested? Stay tuned to know more about it.

Any type of feedback on this post is appreciated .