Raman Scattering

Raman scattering,commonly referred to as the Raman effect, is a two-photon event involving a change in the polarizability of the molecule with respect to its vibrational motion in the form of scattered energy. 

When light from a laser (single frequency) contacts a sample, it changes the polarization of the molecule's electron cloud, leaving the molecule in a temporary, higher virtual energy state. This virtual state is short-lived, and the re-emitted energy is released as scattered light.

The scattered light can be:

• Elastic (Rayleigh Scattering), the energy released is at the same frequency as that of the incident radiation; or 
• Inelastic (Raman Scattering), the energy release is at a higher or lower frequency than that of the incident radiation

The Raman scattering process, as described by quantum mechanics, is when photons interact with a molecule, and the molecule may be advanced to a higher energy, virtual state. From this higher energy state, there may be a few different outcomes. One such outcome would be that the molecule relaxes to a vibrational energy level that is different than that of its beginning state producing a photon of different energy. The difference between the energy of the incident photon and the energy of the scattered photon is called the Raman shift.

When the change in energy of the scattered photon is less than the incident photon, the scattering is called Stokes scatter. Some molecules may begin in a vibrationally excited state and when they are advanced to the higher energy virtual state, they may relax to a final energy state that is lower than that of the initial excited state. This scattering is called anti-Stokes. 

Depending on the application, there are different Raman scattering methods to choose from. Each method has specific advantages and disadvantages over others. Additionally, not all of these Raman scattering methods can be performed on a single Raman spectrometer. These methods require a specific setup that ranges from a relatively simple instrument configuration at a moderate price to quite complicated and expensive equipment. However, to obtain real-time, in-situ reaction understanding and process optimization one of the following Raman scattering methodologies may be the only means to do so:

• Stimulated Raman scattering (SRS)
• Surface enhanced Raman scattering (SERS)
• Coherent anti-Stokes Raman scattering (CARS)

Most photons scatter elastically when interacting with a molecule. A small fraction (approximately 1 in 10 million photons) are inelastically scattered with frequencies that differ from and are usually at a lower frequency than that of the incident photons. The elastically scattered photons are referred to as Rayleigh scatter and have no analytical value. The inelastically scattered photons are referred to as Raman scatter.

C.V. Raman showed that the energy of photons that are scattered inelastically serves as a ‘fingerprint’ for the substance that scatters the light. As a result, Raman spectroscopy is now commonly used in chemical laboratories and processes to identify virtually any material.

An energy-level diagram given in the figure illustrates Raman scattering. The initial state is typically the ground vibrational level state (v0) and the final state (v1). Raman scattering requires two steps for a molecule to Raman scatter:​

1. Photon energies that excite the molecule into a virtual energy state ​
2. The molecule relaxes (Raman scatters) to an elevated ground state (e.g., v1)

Rayleigh scattering, also called elastic scattering, occurs if the excitation frequency equals the scattered radiation as shown in Fig. 1 (E1=E2). Rayleigh scattering does not provide any information about a molecule's chemical composition.

Stokes scattering, also known as inelastic scattering, occurs if the molecule has a net change in vibrational energy as shown on the right side of the figure (E1>E2). As opposed to Rayleigh scattering, Stokes scattering provides information about the chemical composition of a molecule.​

Crystal polymorphism: Polymorphism occurs when a molecule is able to exist in more than one crystalline state. Many crystalline materials can form different polymorphs to minimize their crystal lattice energy under specific thermodynamic conditions. While the chemical nature remains the same, physical properties (solubility, dissolution, nucleation and growth kinetics, bioavailability, morphology, and isolation properties) can vary between polymorphs. Raman spectroscopy is ideal for recording the differences in forms and in measuring the forms while optimizing the crystallization process. ​

​Polymerization:  Raman spectroscopy tends to provide a stronger signal (than IR) from the molecular backbone, especially double and triple carbon bonds. For this reason, Raman can be a better choice for identifying polymers and monitoring polymerization reactions. Extrusion chemistry, microstructure analysis during polymerization, and polyethylene density (LDPE/HDPE) calculations are just a few practical applications where Raman spectroscopy is used.​

Chemical ​Synthesis: In-situ Raman spectroscopy is a useful technique to monitor key reaction variables of chemical syntheses where infrared spectroscopy may not be as sensitive (e.g., silicone, thiol, disulfide, etc.). Key reaction variables such as initiation, endpoint, kinetics, transient intermediate(s), and mechanistic information are vital aspects to know and fully characterize to ensure a safe and robust process development method.

Raman spectroscopy is a scattering form of molecular spectroscopy and is often compared with IR spectroscopy because both provide information about the structure and properties of molecules from their vibrational transitions. In contrast to Raman, IR spectroscopy is an absorption technique that occurs when the frequency of incoming light equals the vibrational frequency of a particular vibrational mode of the molecule which allows the photon to be absorbed (not scattered). This is a single photon event with respect to the molecule's dipole moment.​

These molecular-specific transitions, for both the IR and Raman, when plotted as a spectrum provide a unique pattern or fingerprint for the compound being investigated. Because of the symmetry properties of a molecule, vibrations that are seen in the Raman spectrum, may not be seen (or weakly observed) in the IR spectrum and vice versa when interrogating asymmetric molecules. This behavior is summarized in the selection rules that govern these types of interactions. Based on the similar, but unique, molecular information gained by these techniques Raman and IR are considered to be complementary technologies.

A laser acts as the excitation source to cause the Raman scattering in a modern Raman spectrometer, which has several fundamental parts. Fiber optic cables are used to both send and receive the laser energy from the sample. Rayleigh and anti-Stokes scattering are removed with a notch or edge filter, and the light that is still scattered by Stokes is then sent to a dispersion element, usually a holographic grating. The light is then captured by a CCD detector, which produces the Raman spectrum. It is crucial that high-quality, optically well-matched components are utilized in the Raman spectrometer since Raman scattering produces a faint signal.

The success of Raman spectroscopy comes from the following:​

• The use of very compact and stable diode lasers for the excitation​

• Miniaturized, low stray light spectrometers​

• Advances in the design and manufacture of efficient and precise optical filters to select and isolate the Raman scatter from the laser wavelength (Rayleigh scattering)​

• High-performance CCD arrays to detect and process the weak Raman scattered signal.  ​

These features contribute to the instrumentation that has brought Raman into wide acceptance in analytical and process development laboratories.​

A Raman spectrometer can be combined with an automated lab reactor to provide a unique and automated workstation for crystallization and polymorph investigations, saving valuable time and resources. Data sharing between the systems yields a comprehensive overview and report of significant events (such as dosing, thermal changes, the onset of polymorph transition, end of the transition, etc.) all in a single experiment.​

From data collection to analysis, ReactRaman™ with iC Raman™ brings compositional analysis to every lab. Automated parameter selection provides accurate data collection, enabling scientists to get confident results. Right first time, every time, in every process with every user.​

Compact performance​
Class-leading performance with excellent stability and sensitivity in a compact, stackable package. Deployment can be anywhere in the lab for batch or flow. A single robust connector provides inherent safety and ensures alignment for worry-free measurements.​

Information-rich experimentation​
Data acquisition and analysis is quick and easy with the industry standard iC Software for reaction analysis. iC Software seamlessly incorporates multiple orthogonal data streams to link process variables that drive a comprehensive process understanding.​

Shared expertise​
Thousands of PAT installations around the world and four decades of experience are built into ReactRaman 802L. Our global expert support team is committed to ensuring users’ success through training and application development, in-person or virtual, whenever needed.