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.

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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.

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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.

Raman scattering occurs when photons of monochromatic light (laser) contact with a molecule, resulting in the emission of inelastic photons.

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When a molecule is close to the surface and consequently that enhanced electric field, a large enhancement in the Raman signal can be observed, resulting in Raman signals several orders of magnitude greater than normal Raman scattering. This makes it possible to detect low concentrations without the need to add steps to the process such as labeling and then fluorescence measurements.

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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:

Surface enhanced Raman scattering is a method used to amplify weak Raman signals by use of nanostructured or roughened metal surfaces, typically of gold or silver. Read more on surface enhanced Raman scattering.

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.​

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. ​

SERS is finding increased use in applications ranging from drug discovery to analytical testing – in the lab and in the field, forensic testing, and medical diagnostics.

Coherent anti-Stokes Raman scattering (CARS) is based on a nonlinear mixing process of multiple lasers that are used to enhance the weak (spontaneous) Raman signal.  In the CARS process a pump laser beam and a Stokes laser beam interact, producing an anti-Stokes signal at a certain frequency.  When the frequency difference (beat frequency) between the pump and the Stokes lasers matches the frequency of a Raman active vibrational mode, the molecular oscillators are coherently driven. This results in an enhanced anti-Stokes (shorter-wavelength) Raman signal.

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.

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.​

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.​

When interacting with a molecule, most photons disperse elastically. A tiny percentage is inelastically dispersed at frequencies that differ from and are typically lower than the incoming photons. Rayleigh scatter refers to elastically scattered photons that have no analytical value whereas Raman scatter refers to photons that are inelastically scattered.

​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.​

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It has been found that if a sample is irradiated with a very strong laser pulse, new non-linear phenomena could be observed. The electric field generated by the pulsed lasers is about 5 orders of magnitude greater than that generated by continuous wave (CW) lasers, which transforms a much larger percent of incident light into useful Raman scattering and substantially improves the signal-to-noise ratio that leads to a measurably lower limit of detection as opposed to standard Stokes Raman spectroscopy.

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.

SERS uses nanostructured or roughened metal surfaces, typically of gold or silver. Laser excitation of these metal structures drives surface charges to create a localized plasmon field, an enhanced electrical field.

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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.​

In 1928, Sir C.V. Raman and K.S. Krishnan observed the phenomenon that is now known as the Raman Effect and is the basis for Raman spectroscopy.  The phenomenon involves the interaction of photons with a molecule followed by inelastic scattering typically at a lower energy. Generally, photons scatter elastically. These one-in-ten million lower energy, inelastic scattered photons are referred to as Stokes scattering and are specific to bonds within a molecule resulting in a unique spectral signature for a given molecular structure.  ​

Stimulated Raman scattering (SRS) is another example of non-linear Raman spectroscopy. Stimulated Raman scattering takes place when an excess number of Stokes photons are present or are deliberately added to the excitation beam. This wavelength coincides with the strongest mode in the regular Raman spectrum that subsequently is greatly amplified while all other Raman-active modes are suppressed.

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)​

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Two fields benefitting from the development of CARS technology are cell biology and tissue imaging. Traditionally, cell interrogation is performed using fluorescence spectroscopy. With CARS it is possible to gather the same chemically specific information without labeling the molecule of interest, thus providing information from the sub-micron scale.

Their experiment was done using monochromatic light, sunlight filtered to leave only a single color, and found in 1923 that a number of liquids did change the color of the light, but very weakly. Then in 1927 they found a particularly strong color change from light scattered by glycerine where the incident blue light changed to green. Finally in 1928 the first Raman spectrum was constructed and subsequently has undergone numerous engineering improvements as material science has advanced in the areas of lasers, optics and detectors.

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.

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:​

Surface-enhanced Raman scattering (SERS) is one method used to amplify weak Raman signals. Raman signals are inherently weak, a result of the statistically low number of scattered photons available for detection.

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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.

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

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.

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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.

Another type of non-linear Raman spectroscopy is stimulated Raman scattering. Stimulated Raman scattering occurs when there is an excess of Stokes photons in the excitation beam or when they are purposely introduced. This wavelength corresponds to the brightest mode in the standard Raman spectrum, which is then substantially amplified while all other Raman-active modes are muted. Read more on stimulated Raman scattering.

Raman scattering, commonly referred to as the Raman effect, is an optical phenomenon in which the interaction of incoming excitation light with a sample generates scattered light. The energy of the scattered light is reduced by the vibrational modes of the chemical bonds present in the specimen.

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The Raman effect is founded on light scattering, which involves Rayleigh scattering (elastic) at the same wavelength as the incident beam, as well as Raman scattering (inelastic) at various wavelengths caused by molecular vibrations. Rayleigh scattering is about one million times more intense than Raman scattering.​