Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic scattering, or Raman scattering of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system. Infrared spectroscopy yields similar, but complementary information. Extremely high resolution spectrogram of the Sun showing thousands of elemental absorption lines (fraunhofer lines) Spectroscopy is the study of the interaction between radiation (electromagnetic radiation, or light, as well as particle radiation) and matter. ... Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter. ... It has been suggested that the central science be merged into this article or section. ... In scattering theory and in particular in particle physics, elastic scattering is one of the specific forms of scattering. ... Raman scattering or the Raman effect is the inelastic scattering of a photon. ... Experiment with a laser (likely an argon type) (US Military) In physics, a laser is a device that emits light through a specific mechanism for which the term laser is an acronym: light amplification by stimulated emission of radiation. ... The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ... Image of two girls in mid-infrared (thermal) light (false-color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. ... UV redirects here. ... Normals modes of vibration progression through a crystal. ... Infrared spectroscopy (IR Spectroscopy) is the subset of spectroscopy that deals with the IR region of the EM spectrum. ...

Typically, a sample is illuminated with a laser beam. Light from the illuminated spot is collected with a lens and sent through a monochromator. Wavelengths close to the laser line (due to elastic Rayleigh scattering) are filtered out and those in a certain spectral window away from the laser line are dispersed onto a detector. A lens. ... Rayleigh scattering causing the blue hue of the sky and the reddening at sunset Rayleigh scattering (named after Lord Rayleigh) is the scattering of light, or other electromagnetic radiation, by particles much smaller than the wavelength of the light. ...

Spontaneous Raman scattering is typically very weak, and as a result the main difficulty of Raman spectroscopy is separating the weak inelastically scattered light from the intense Rayleigh scattered laser light. Raman spectrometers typically use holographic diffraction gratings and multiple dispersion stages to achieve a high degree of laser rejection. A photon-counting photomultiplier tube (PMT) or, more commonly, a CCD camera is used to detect the Raman scattered light. In the past, PMTs were the detectors of choice for dispersive Raman setups, which resulted in long acquisition times. However, the recent uses of CCD detectors have made dispersive Raman spectral acquisition much more rapid. Raman scattering or the Raman effect is the inelastic scattering of a photon. ... A spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Photomultipliers, or photomultiplier tubes (PMT) are extremely sensitive detectors of light in the ultraviolet, visible and near infrared. ... A specially developed CCD used for ultraviolet imaging in a wire bonded package. ...

Raman spectroscopy has a stimulated version, analogous to stimulated emission, called stimulated Raman scattering. In optics, stimulated emission is the process by which, when perturbed by a photon, matter may lose energy resulting in the creation of another photon. ...

Basic theory

Energy level diagram showing the states involved in Raman signal. The line thickness is roughly proportional to the signal strength from the different transitions.

The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud of the bonds of that molecule. A molecular polarizability change, or amount of deformation of the electron cloud, with respect to the vibrational coordinate is required for the molecule to exhibit the Raman effect. The amount of the polarizability change will determine the intensity, whereas the Raman shift is equal to the vibrational level that is involved. The incident photon (light quantum), excites one of the electrons into a virtual state. For the spontaneous Raman effect, the molecule will be excited from the ground state to a virtual energy state, and relax into a vibrational excited state, and which generates Stokes Raman scattering. If the molecule was already in an elevated vibrational energy state, the Raman scattering is then called anti-Stokes Raman scattering. Image File history File links Download high-resolution version (830x512, 34 KB) Pavlina I made the image myself. ... Image File history File links Download high-resolution version (830x512, 34 KB) Pavlina I made the image myself. ... In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ... A molecular vibration occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant translational and rotational motion. ... The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, infrared radiation, microwaves, radio waves, and visible light are all forms of light. ...

Applications

Raman spectroscopy is commonly used in chemistry, since vibrational information is very specific for the chemical bonds in molecules. It therefore provides a fingerprint by which the molecule can be identified. The fingerprint region of organic molecules is in the range 500-2000 cm^-1. Another way that the technique is used is to study changes in chemical bonding, e.g. when a substrate is added to an enzyme.

Raman gas analyzers have many practical applications, for instance they are used in medicine for real-time monitoring of anaesthetic and respiratory gas mixtures during surgery.

In solid state physics, spontaneous Raman spectroscopy is used to, among other things, characterize materials, measure temperature, and find the crystallographic orientation of a sample. Fig. ...

As with single molecules, a given solid material has characteristic phonon modes that can help an experimenter identify it. In addition, Raman spectroscopy can be used to observe other low frequency excitations of the solid, such as plasmons, magnons, and superconducting gap excitations. Normals modes of vibration progression through a crystal. ... In physics, the plasmon is the quasiparticle resulting from the quantization of plasma oscillations just as photons and phonons are quantizations of light and sound waves. ... BCS theory (named for its creators, Bardeen, Cooper, and Schrieffer) successfully explains conventional superconductivity, the ability of certain metals at low temperatures to conduct electricity without resistance. ...

The spontaneous Raman signal gives information on the population of a given phonon mode in the ratio between the Stokes (downshifted) intensity and anti-Stokes (upshifted) intensity.

Raman scattering by an anisotropic crystal gives information on the crystal orientation. The polarization of the Raman scattered light with respect to the crystal and the polarization of the laser light can be used to find the orientation of the crystal, if the crystal structure (specifically, its point group) is known. Quartz crystal Synthetic bismuth crystal Insulin crystals Gallium, a metal that easily forms large single crystals A huge monocrystal of potassium dihydrogen phosphate grown from solution by Saint-Gobain for the megajoule laser of CEA. In chemistry and mineralogy, a crystal is a solid in which the constituent atoms, molecules... In electrodynamics, polarization (also spelled polarisation) is the property of electromagnetic waves, such as light, that describes the direction of their transverse electric field. ... Enargite crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... In crystallography, a crystallographic point group or crystal class is a set of symmetry operations that leave a point fixed, like rotations or reflections, which leave the crystal unchanged. ...

Raman active fibers, such as aramid and carbon, have vibrational modes that show a shift in Raman frequency with applied stress. Polypropylene fibers also exhibit similar shifts.

The radial breathing mode is a commonly used technique to evaluate the diameter of carbon nanotubes.

Spatially Offset Raman Spectroscopy (SORS), which is less sensitive to surface layers than conventional Raman, can be used to discover counterfeit drugs without opening their internal packaging, and for non-invasive monitoring of biological tissue.^[1][2] A counterfeit drug or a counterfeit medicine is a medication which is produced and sold with the intent to deceptively represent its origin, authenticity or effectiveness. ...

Raman microspectroscopy

Raman spectroscopy offers several advantages for microscopic analysis. Since it is a scattering technique, specimens do not need to be fixed or sectioned. Raman spectra can be collected from a very small volume (< 1 µm in diameter); these spectra allow the identification of species present in that volume. Water does not interfere very strongly. Thus, Raman spectroscopy is suitable for the microscopic examination of minerals, materials such as polymers and ceramics, cells and proteins. A Raman microscope begins with a standard optical microscope, and adds an excitation laser, a monochromator, and a sensitive detector (such as a charge-coupled device (CCD) or photomultiplier tube (PMT)). FT-Raman has also been used with microscopes. This article or section is not written in the formal tone expected of an encyclopedia article. ... Minerals are natural compounds formed through geological processes. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hook from Micrographia which is the origin of the word cell. POOP Cells in culture, stained for keratin (red) and DNA (green). ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light chosen from a wider range of wavelengths available at the input. ... Photomultipliers, or photomultiplier tubes (PMT) are extremely sensitive detectors of light in the ultraviolet, visible and near infrared. ... FTIR of a thin film of ethanol in the liquid phase. ...

In direct imaging, the whole field of view is examined for scattering over a small range of wavenumbers (Raman shifts). For instance, a wavenumber characteristic for cholesterol could be used to record the distribution of cholesterol within a cell culture.

The other approach is hyperspectral imaging, in which thousands of Raman spectra are acquired from all over the field of view. The data can then be used to generate images showing the location and amount of different components. Taking the cell culture example, a hyperspectral image could show the distribution of cholesterol, as well as proteins, nucleic acids, and fatty acids. Sophisticated signal- and image-processing techniques can be used to ignore the presence of water, culture media, buffers, and other interferents. Imaging spectroscopy is defined as being the simultaneous acquisition of spatially coregistered images, in many, spectrally contiguous bands, in an internationally recognized system of units from a remotely operated platform (Schaepman, 2005). ...

Raman microscopy, and in particular confocal microscopy, has very high spatial resolution. For example, the lateral and depth resolutions were 250 nm and 1.7 µm, respectively, using a confocal Raman microspectrometer with the 632.8 nm line from a He-Ne laser with a pinhole of 100 µm diameter. Confocal microscopy is an imaging technique used to increase micrograph contrast and/or to reconstruct three-dimensional images by using a spatial pinhole to eliminate out-of-focus light or flare in specimens that are thicker than the focal plane. ... Experiment with a laser (likely an argon type) (US Military) In physics, a laser is a device that emits light through a specific mechanism for which the term laser is an acronym: light amplification by stimulated emission of radiation. ...

Since the objective lenses of microscopes focus the laser beam to several micrometres in diameter, the resulting photon flux is much higher than achieved in conventional Raman setups. This has the added benefit of enhanced fluorescence quenching. However, the high photon flux can also cause sample degradation, and for this reason some setups require a thermally conducting substrate (which acts as a heat sink) in order to mitigate this process. Quenching refers to any process which decreases the fluorescence intensity of a given substance. ...

By using Raman microspectroscopy, in vivo time- and space-resolved Raman spectra of microscopic regions of samples can be measured. As a result, the fluorescence of water, media, and buffers can be removed. Consequently in vivo time- and space-resolved Raman spectroscopy is suitable to measure cells, proteins, organs, and erythrocytes. Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hook from Micrographia which is the origin of the word cell. POOP Cells in culture, stained for keratin (red) and DNA (green). ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... In biology, an organ (Latin: organum, instrument, tool) is a group of tissues that perform a specific function or group of functions. ... Human red blood cells Red blood cells are the most common type of blood cell and are the vertebrate bodys principal means of delivering oxygen to body tissues via the blood. ...

Raman microscopy for biological and medical specimens generally uses near-infrared (NIR) lasers (785 nm diodes and 1064 nm Nd:YAG are especially common). This reduces the risk of damaging the specimen by applying high power. However, the intensity of NIR Raman is low (owing to the ω^-4 dependence of Raman scattering intensity), and most detectors required very long collection times. Recently, more sensitive detectors have become available, making the technique better suited to general use. Raman microscopy of inorganic specimens, such as rocks and ceramics and polymers, can use a broader range of excitation wavelengths.^[3] Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ...

History

Inelastic scattering of light is sometimes called the Raman effect, named after one of its discoverers, the Indian scientist Sir C. V. Raman (1928, together with K. S. Krishnan and independently by Grigory Landsberg and Leonid Mandelstam). Raman won the Nobel Prize in Physics in 1930 for this discovery accomplished using sunlight, a narrow band photographic filter to create monochromatic light and a "crossed" filter to block this monochromatic light. He found that light of changed frequency passed through the "crossed" filter. Subsequently the mercury arc became the principal light source, first with photographic detection and then with spectrophotometric detection. Currently lasers are used as light sources. When light is scattered from a molecule most photons are elastically scattered. ... Sir Chandrasekhara Venkata Raman, CBE (Tamil: சந்திரசேகர வெங்கடராமன்) (November 7, 1888 – November 21, 1970) was an Indian physicist, who was awarded the 1930 Nobel Prize in Physics for his work on the scattering of light and for the discovery of the Raman effect, which is named after him. ... Grigory Samuilovich Landsberg (&#1043;&#1088;&#1080;&#1075;&#1086;&#1088;&#1080;&#1081; &#1057;&#1072;&#1084;&#1091;&#1080;&#1083;&#1086;&#1074;&#1080;&#1095; &#1051;&#1072;&#1085;&#1076;&#1089;&#1073;&#1077;&#1088;&#1075;) (January 10, 1890 - February 2, 1957) Russian physicist. ... Leonid Isaakovich Mandelshtam (Леонид Исаакович Мандельштам, last name more often spelled as Mandelstam) (May 4, 1879 - November 27, 1944) was a Russian/Soviet physicist of Jewish background. ... Hannes Alfvén (1908–1995) accepting the Nobel Prize for his work on magnetohydrodynamics [1]. List of Nobel Prize laureates in Physics from 1901 to the present day. ...

Other Types

Several variations of Raman spectroscopy have been developed. The usual purpose is to enhance the sensitivity (e.g., surface-enhanced Raman), to improve the spatial resolution (Raman microscopy), or to acquire very specific information (resonance Raman).

• Surface Enhanced Raman Spectroscopy (SERS) - Normally done in a silver or gold colloid or a substrate containing silver or gold. Surface plasmons of silver and gold are easily excited by the laser, and the resulting electric fields cause other nearby molecules to become Raman active. The result is amplification of the Raman signal (by up to 10¹¹). This effect was originally observed by Fleishman but the prevailing explanation was proposed by Van Duyne in 1977.

• Hyper Raman - A non-linear effect in which the vibrational modes interact with the second harmonic of the excitation beam. This requires very high power, but allows the observation of vibrational modes which are normally "silent". It frequently relies on SERS-type enhancement to boost the sensitivity.

• Resonance Raman spectroscopy - The excitation wavelength is matched to an electronic transition of the molecule or crystal, so that vibrational modes associated with the excited electronic state are greatly enhanced. This is useful for studying large molecules such as polypeptides, which might show hundreds of bands in "conventional" Raman spectra. It is also useful for associating normal modes with their observed frequency shifts.

• Spontaneous Raman Spectroscopy - Used to study the temperature dependence of the Raman spectra of molecules.

• Optical Tweezers Raman Spectroscopy (OTRS) - Used to study individual particles, and even biochemical processes in single cells trapped by optical tweezers.

• Stimulated Raman Spectroscopy - A two color pulse transfers the population from ground to a rovibrationally excited state, if the difference in energy corresponds to an allowed Raman transition. Two photon UV ionization, applied after the population transfer but before relaxation, allows the intra-molecular or inter-molecular Raman spectrum of a gas or molecular cluster (indeed, a given conformation of molecular cluster) to be collected. This is a useful molecular dynamics technique.

• Spatially Offset Raman Spectroscopy (SORS) - The Raman scatter is collected from regions laterally offset away from the excitation laser spot, leading to significantly lower contributions from the surface layer than with traditional Raman spectroscopy.^[4]

Surface Enhanced Raman Spectroscopy was discovered by Martin Fleischmann (later of cold fusion infamy) et al. ... In physics, the plasmon is the quasiparticle resulting from the quantization of plasma oscillations, which are density waves of the charge carriers in a conducting medium such as a metal, semiconductor, or plasma. ... Nonlinear optics is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization P responds nonlinearly to the electric field E of the light. ... Second Harmonic Generation (SHG) is a subcategory of nonlinear optics in physics. ... Overview Resonance Raman (RR) spectroscopy is a specialized implementation of the more general Raman spectroscopy. ... Peptides are the family of molecules formed from the linking, in a defined order, of various amino acids. ... An optical tweezer is a scientific instrument that uses a focused laser beam to provide an attractive or repulsive force, depending on the index mismatch (typically on the order of piconewtons) to physically hold and move microscopic dielectric objects. ...

See also

• Coherent anti-Stokes Raman spectroscopy (CARS)
• Raman laser
• Resonance Raman spectroscopy

The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... The Raman laser is a byproduct of the Raman Effect, discovered in 1928 by Nobel laureate Chandrasekhara Venkata Raman. ... Overview Resonance Raman (RR) spectroscopy is a specialized implementation of the more general Raman spectroscopy. ...

References

1. ^ BBC News report on use of Raman spectroscopy to detect fake drugs (31st January 2007).
2. ^ Science and Technology Facilities Council article.
3. ^ Ellis, D.I. and Goodacre, R. (2006) Metabolic fingerprinting in disease diagnosis: biomedical applications of infrared and Raman spectroscopy, Analyst, 131, 875-885. DOI:10.1039/b602376m
4. ^ P. Matousek, I.P. Clark, E.R.C. Draper, M.D. Morris, A.E. Goodship, N. Everall, M. Towrie, W.F. Finney, A.W. Parker, Subsurface Probing in Diffusely Scattering Media using Spatially Offset Raman Spectroscopy, Appl. Spectrosc. 59 (2005) 393.

A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

External links

• Raman application notes - Raman Spectroscopy application notes
• Raman Resource Pages - Tutorial, FAQs and application notes on various uses for Raman spectroscopy
• Raman Spectroscopy Tutorial - A detailed explanation of Raman Spectroscopy including Resonance-Enhanced Raman Scattering and Surface-Enhanced Raman Scattering.
• The Science of Spectroscopy - supported by NASA. Spectroscopy education wiki and films - introduction to light, its uses in NASA, space science, astronomy, medicine & health, environmental research, and consumer products.
• Slashdot - Discussion on using Raman spectroscopy and confocal laser scanning microscopy for 3D images of fossils embedded in solid rock
• The Science Show, ABC Radio National - Interview with Scientist on NASA funded project to build Raman Spectrometer for 2009 Mars mission: a cellular phone size device to detect almost any substance known, with commercial <USD$5000 commercial spin-off, prototyped by June 2006.
• Power Technology in depth look at Raman Spectroscopy - Applications of Raman Spectroscopy and Distributed Feedback Diodes
• Algorithms used in Spectroscopy