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.

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.​

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.

Maybe dispersion could be reversed. Newton tried a second prism as a part of an "error correction" experiment. Disperse the light with one prism then un-disperse it with a second to see if there were any distortions caused by impurities or irregularities in the glass.

Aberrations dredge

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.

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.​

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

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.

This made me take Reflections into consideration, and finding them regular, so that the Angle of Reflection of all sorts of Rays was equal to their Angle of Incidence; I understood, that by their mediation Optick instruments might be brought to any degree of perfection imaginable, provided a Reflecting substance could be found, which would polish as finely as Glass, and reflect as much light, as glass transmits, and the art of communicating to it a Parabolick figure be also attained.

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)​

The images below show a simulation of axial chromatic aberration applied to a simple object, a black grid on a white background. This is what a camera would see if it only experienced axial chromatic aberration and the camera lens was adjusted so that the three primary colors are in focus one at a time.

Then he did something really dumb (as if sticking a needle into your eye socket wasn't dumb enough). He stared at the Sun — maybe. He was hopefully more sensible and stared at a bright patch of the Sun's light projected onto a wall. Staring at a bright light source overstimulates the photoreceptor cells in the retina. This reduces their sensitivity, which is a response that allows our visual system to adapt to surroundings with different brightnesses. When the bright source is removed, the overstimulated photoreceptors are now under-sensitive (a word I just made up). The human visual system is complicated, so there's a bit more to it than that. Let's just say that staring at a bright light screws up your eyesight for a while.

Aberrations 5e

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.

Aberrations examples

Lateral chromatic aberration does not occur in the human eye or, more accurately, if it does we don't notice it. I know from firsthand experience that it does occur in eyeglasses, however, but the effect varies with the material used. Price seems to be a determining factor. Since contact lenses rest directly on the cornea, any chromatic aberration is hard to detect.

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.

Chromatic aberration is a kind of defect commonly found in simple lens systems caused by a variation in the index of refraction with wavelength. Different frequencies (or wavelengths or colors) originating from the same object point follow different paths after passing through a lens. The result is an out of focus image that cannot be corrected by merely changing the placement of the lens (focusing).

The way around this is to eliminate at least one of the lenses from the telescope (the bigger lens, the one that faces the stars, the objective lens) and replace it with a mirror.

There are therefore two sorts of colours. The one original and simple, the other compounded of these. The Original or primary colours are, Red, Yellow, Green, blew, and a Violet-purple, together with Orange, Indico, and an indefinite variety of Intermediate gradations.

58 I tooke a bodkine gh & put it betwixt my eye & ye bone as neare to ye backside of my eye as I could: & pressing my eye with ye end of it (soe as to make ye curvature a,bcdef in my eye) there appeared severall white darke & coloured circles r,s,t, &c. Which circles were plainest when I continued to rub my eye with ye point of ye bodkine, but if I held my eye & ye bodkin still, though I continued to presse my eye with it yet ye circles would grow faint & often disappeare untill I removed ym by moving my eye or ye bodkin.

Aberrations in optics

Some notes on chromatic aberration and vision I picked up along the way, mostly about the duochrome vision test — to be organized later.

63 Looking on a very light object as ye Sun or his image reflected; for a while after there would remaine an impression of colours in my eye: viz: white objects looked red & soe did all objects in the light but if I went into a dark roome ye Phantasma was blew.

But, to determine more absolutely, what Light is, after what manner refracted, and by what modes or actions it produceth in our minds the Phantasms of Colours, is not so easie. And I shall not mingle conjectures with certainties.

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.

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.

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.

The reason Newton did these experiments on himself wasn't because he was some thick headed frat boy. Rather, he was fascinated by the difference between objective reality and illusion (or even delusion). One of the ways we can be fooled is in the perception of color. Newton showed through a series of now famous experiments using glass prisms that white light, which up to that point was thought to be the purest form of light, is actually a blended form of light with different colors.

The apparent three dimensional appearance of the album cover reproduced below reveals an optical illusion caused by axial chromatic aberration in the human eye. The extreme juxtaposition of the hot pink background against the neon green cutout of the band name forces the visual system to make a decision. If the brain decides the eye muscles should focus on the pink background, then the green cutout is out of focus. If the brain decides the eye muscles should focus on the green cutout, then the pink background is out of focus. That gives the artwork an apparent 3D appearance. For some people that makes the green appear to pop out, for others it makes the pink appear to pop out. If your brain can't decide which of the two colors to focus on, the eyes will then shift focus between them and the artwork will appear to shimmer with a frequency on the order of ten times a second (~10 Hz). The frequency of the shimmer gives you a sense of how long it takes the brain make to make a decision — the reciprocal of ten times a second, or one tenth of a second (~0.1 s = ~100 ms).

Lateral chromatic aberration (or transverse chromatic aberration) occurs when image points formed by off axis rays are spread out away from the center of the image plane (in the lateral or transverse direction). This spreading is least apparent for rays parallel to the principal axis and increases with increasing angle. The result is a magnification that varies with color or, in technical language, a chromatic difference of magnification. Again, using the primary colors as examples: the blue frequencies produce the largest image, the green frequencies the mediumest, and the red frequencies the smallest.

To reduce chromatic aberration, a higher quality optical device would use a combination of lenses. The simplest such system consists of two lenses made of two different kinds of glass called an achromatic lens or an achromat. The most common acromat consists of a converging lens made of crown glass (the kind commonly used for drinking glasses and food jars) and a diverging lens made of flint glass (the slightly fancier kind of glass used in chandeliers and crystal decanters). The converging lens disperses the focal lengths one way and the diverging lens disperses them the other way canceling out some of the chromatic aberration. Flint glass has twice the dispersion of crown glass, so a converging crown glass lens of power +P paired with a diverging flint glass lens of power −½P will result in a reasonably good achromat with power +½P.

Aberrations Physics

Amidst these thoughts I was forced from Cambridge by the Intervening Plague, and it was more then two years, before I proceeded further. But then having thought on a tender way of polishing, proper for metall, whereby, as I imagined, the figure also would be corrected to the last; I began to try, what might be effected in this kind, and by degrees so far perfected an Instrument (in the essential parts of it like that I sent to London,) by which I could discern Jupiters 4 Concomitants, and shewed them divers times to two others of my acquaintance. I could also discern the Moon-like phase of Venus, but not very distinctly, nor without some niceness in disposing the Instrument.

7 Taking a Prisme, (whose angle fbd was about 60gr) into a Darke roome into wch ye sun shone only at one little round hole k, and laying it close to ye hole k in such manner yt ye rays, being equally refracted at (n & h) their going in & out of it, cast colours rstv on ye opposite wall. The colours should have beene in a round circle were all ye rays alike refracted, but their forme was oblong terminated at theire sides r & s wth straight lines; theire breadth rs being 2⅓inches, theire length to about 7 or eight inches, & ye centers of ye red & blew, (q & p) being distant about 2¾ or 3 inches. The distance of ye wall trsv from ye Prisme being 260inches.

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.

Aberrations in lenses

Pressing the side of the needle against his eyeball made colored circles appear in his field of vision at a point opposite that of the needle. These circles, which can be colored solid or take on animated geometric patterns, are an example of a visual phenomenon known as a phosphene — the sensation of light when there is no light — a mechanical phosphene in this case. Under normal circumstances, when the eye is being used for its intended purpose, light falls on the photoreceptor cells in the retina which causes them to become excited (formally) or fire (colloquially). In Newton's bodkin experiment, the photoreceptor cells were firing because they were being squeezed from behind. (Newton really wedged that thing deep into his eye socket according to his account.)

When I understood this, I left off my aforesaid Glass-works; for I saw, that the perfection of Telescopes was hitherto limited, not so much for want of glasses truly figured according to the prescriptions of Optick Authors, (which all men have hitherto imagined,) as because that Light it self is a Heterogeneous mixture of differently refrangible Rays. So that, were a glass so exactly figured, as to collect any one sort of rays into one point, it could not collect those also into the same point, which having the same Incidence upon the same Medium are apt to suffer a different refraction.

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.

Entry 58 from Newton's lab notebook described the one of these experiments. Spelling, capitalization, and punctuation rules were not well established in the 17th century, so some of this may look a bit odd to contemporary readers. Pen, ink, and paper were all difficult to come by (Newton had his own recipe for ink), so abbreviations were common as well. The letter "y" was often substituted for "th" so that "the" is written ye, "that" is written yt, and "them" is written ym.

The 17th century English scientist, mathematician, and theologian Isaac Newton was interested in the history of optical illusions. Is what we see there really there? To this end, he experimented on himself in a way that should never be repeated. When he was 24 years old, he inserted a bodkin (a blunt needle used to thread ribbon through lace) deep into the socket between his nose and eyeball.

​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.​

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. ​

In optics, the deviation from perfection is called aberration. More precisely, an aberration is a deviation of a ray from the behavior predicted by the simplified rules of geometric optics. The primary rule referred to here is the one that states that rays of light parallel to the principal axis of a lens or curved mirror meet at a point called the focus. If your only options for a statement are that it is either true or false, then this statement is definitely false — as are many physical laws. If you can think beyond the law of the excluded middle (which itself isn't a law, it's a logical fallacy) then you can appreciate a real answer with more nuance.

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.

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.

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.

Because Newton was a bit of a mystic and seven is a number with mystical connotations, he divided the spectrum up into seven named segments giving primary school children everywhere something to memorize. He identified these as the "primary colors" but later experiments have shown this notion to be wrong. (Sorry primary school children.) The preferred term now is spectral colors or prismatic colors for the things Newton was naming. (The primary colors of red, green, and blue are discussed elsewhere in this book.) There are also many more than seven distinguishable colors of light in the visible spectrum — a point Newton makes clear near the end of this quotation.

Axial chromatic aberration (or longitudinal chromatic aberration) occurs when a lens cannot focus different wavelengths of light in the same focal plane. The foci of the different colors lie at different points along the principal axis in the longitudinal direction. The result is a blurring of the out of focus colors that is pretty much equally annoying all across an image. For a single converging lens made out of a typical transparent material, the focal lengths are shorter for shorter wavelengths and longer for longer wavelengths. Moving away from the lens: the blues focus toward the front, the greens focus in the middle, and the reds focus toward the back. In the human eye, the spread in focal lengths is about 0.7 mm or more than twice the thickness of the retina.

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.

This was Newton at 30 reflecting back on thoughts he had when he was 24. It took that long for the reflecting telescope to go from concept to working prototype. (The bubonic plague didn't help things much.)

We would call this thing that Newton saw an afterimage, but at the time that word did not exist and Newton was not the one to invent it. Instead he used the word phantasma (φαντασμα in Greek) which is a variation on the word phantasm or phantom — in other words, a ghost or at least something ghost-like. It's ingenious and imaginative, but also a bit otherwordly.

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:​

Six years later, when he described the prism experiment in a public letter to the Royal Society, Newton had begun the transition from the Greek loanword "phantasm" to the Latin loanword "spectrum". This is the first written example of the word spectrum with its current meaning.

All rays of light obey the law of reflection in the same way, regardless of their color. Problem solved. Newton even understood that the mirror needed to be ground and then polished with a parabolic curvature to eliminate spherical aberration — the inability of a spherical surface to bring rays far from its axis into proper focus. He most certainly didn't do this, however, as the method of grinding a parabola is much more complicated that that of grinding a sphere. (Optical devices with curved surfaces are usually ground into the desired shape instead of being cast or molded.)

Chromatic aberration can also be corrected for in digital cameras by computation. In color film cameras (and in the human eye) the locations of each of the three color images are fixed relative to one another. If the green image is in focus while the red and blue images are shifted and blurred in a film camera, well, too bad. You're stuck with it that way after you open the shutter and expose the film. If the same thing happens in a digital camera, well, it's all just numbers. Compute your way out of it. Not an easy problem to solve, but not an intractable one.

The image below shows a simulation of lateral chromatic aberration applied to the same simple object as before, a black grid on a white background. This is what a camera would see if it only experienced lateral chromatic aberration. The result is fringes on high contrast areas — blue fringes blending into cyan on the inside edges and red fringes blending into yellow on the outside edges. The effect increases with distance from the center.

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.

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.​

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.​

"Nothing is perfect" is a content-free statement. It's an excuse used over and over again to explain why things don't work out as intended. It's an explanation that explains nothing. There's no room in science for palliative blanket statements like this. Science is not the pursuit of perfection. Perfection is a dumb concept to begin with.

Then I suspected, whether by any unevenness in the glass, or other contingent irregularity, these colours might be thus dilated. And to try this, I took another Prisme like the former, and so placed it, that the light, passing through them both, might be refracted contrary ways, and so by the latter returned into that course, from which the former had diverted it. For, by this means I thought, the regular effects of the first Prisme would be destroyed by the second Prisme, but the irregular ones more augmented, by the multiplicity of refractions. The event was, that the light, which by the first Prisme was diffused into an oblong form, was by the second reduced into an orbicular one with as much regularity, as when it did not at all pass through them. So that, what ever was the cause of that length, 'twas not any contingent irregularity.

Both words had similar meanings in the 17th century — something ghostly or not of this world. Much like spelling and punctuation, scientific terminology wasn't systematized in the 17th century. It may well have been seen as a mark of proficiency to mix up spellings, punctuation placements, and word choices. (This was about the time when the thesaurus was invented after all.) In the 21st century, however, scientific terminology is reasonably well organized and consistent and, for unrelated reasons, the word spectrum has lost all its supernatural connotations.

Technically speaking, an achromatic lens only aligns the focal points of the red and the blue source light waves to one another. It does not align the focal points of the red and the blue and the green to one another. To do that a third lens is needed. Such a system is called an apochromatic lens or an apochromat for short or, if you're in a real hurry, an APO. The lenses tend to be made of specialty materials. No more melted down jam jars and chandeliers. This one reason why expensive cameras are expensive.

The reflecting telescope was a success. Not only did Newton exhibit great theoretical insight when it came to optics, but he also demonstrated that he could apply his theoretical knowledge to practical applications. He was accepted as a Fellow of the Royal Society that year. The prototype telescope he sent them is still in their archives. It is the telescope more than anything else that ushered Isaac Newton on to the public stage of 17th century science — more than his work on gravity, the laws of motion, or the invention of calculus.

Aberrations bg3

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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.

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Comparing the length of this coloured Spectrum with its breadth, I found it about five times greater; a disproportion so extravagant, that it excited me to a more then ordinary curiosity of examining, from whence it might proceed….

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.

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.​

And looking on it through the Prisme, it appeared broken in two twixt the colours, the blew parte being nearer the Prisme than the red parte. Soe that blew rays suffer a greater refraction than red ones. I call those blew or red rays &c, which make the Phantome of such colours.

Dispersion is a one way street. This realization caused Newton to rethink his work in optics. No optical device would ever be able to produce a "true" (for lack of a better word) image if it relied on refraction. It would suffer from what we now call chromatic aberration — initially collinear rays of light would follow different paths depending on their color. There would be no way for all the colored rays of an image to be in focus together. Newton was interested in astronomical telescopes at the time.

59 If ye experiment were done in a light roome so yt though my eyes were shut some light would get through their lidds There appeared a greate broade blewish darke circle outmost (as ts), & wthin that another light spot srs whose colour was much like yt in ye rest of ye eye as at k. Within wch spot appeared still another blew spot r espetially if I pressed my eye hard & wth a small pointed bodkin. & outmost at vt appeared a verge of light.

Newton produced his spectrum by refraction (the change in direction of a wave through a medium associated with changes in the wave's speed) or more precisely dispersion (the variation of a wave's speed in a medium with frequency). All transparent media are dispersive to some degree. Therefore any optical system that uses refraction to do what it needs to do will also experience dispersion. If the goal of your optical system is to produce a spectrum, then dispersion is a fine thing. If the goal of your optical system is to produce a reliable image, to "see" something for what it really is, then dispersion is a problem.

Spherical aberration

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 spectrum that Newton first saw and then named is a colored band of light produced when a source of mixed light has been decomposed or broken up into components and sorted into a characteristic sequence — sorted by frequency, it was later determined. It is a real thing and is not an optical illusion or mental delusion.

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:

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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.  ​

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

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.

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.

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.