Resonance Raman spectroscopy (RR spectroscopy) is a Raman spectroscopy technique in which the incident photon energy is close in energy to an electronic transition of a compound or material under examination.  The frequency coincidence (or resonance ) can lead to greatly enhanced intensity of the Raman scattering, which facilitates the study of chemical compounds present at low concentrations. 
Raman scattering is usually extremely weak, since only about 1 in 10 million photons that hit a sample are scattered with a loss (Stokes) or gain (anti-Stokes) in energy from changes in vibrational energy of the molecules in the sample; the rest of the photons are scattered with no change in energy. Resonance enhancement of Raman scattering requires the incident wavelength (usually from a laser) to be close to that of an electronic transition of the molecules. In larger molecules the change in electron density can be largely confined to one part of the molecule, a chromophore, and in these cases the Raman bands that are enhanced are primarily from those parts of the molecule in which the electronic transition leads to a change in bond length or force constant in the excited state of the chromophore. For large molecules such as proteins, this selectivity helps to identify the observed bands as originating from vibrational modes of specific parts of the molecule or protein, such as the heme unit within myoglobin. 
Raman spectroscopy and RR spectroscopy provide information about the vibrations of molecules, and can also be used for identifying unknown substances. RR spectroscopy has found wide application to the analysis of bioinorganic molecules. The technique measures the energy required to change the vibrational state of a molecule as does infrared (IR) spectroscopy. The mechanism and selection rules are different in each technique, however, band positions are identical and therefore the two methods provide complementary information.
Infrared spectroscopy involves measuring the direct absorption of photons with the appropriate energy to excite molecular bond vibrational modes and phonons. The wavelengths of these photons lie in the infrared region of the spectrum, hence the name of the technique. Raman spectroscopy measures the excitation of bond vibrations by an inelastic scattering process, in which the incident photons are more energetic (usually in the visible, ultraviolet or even X-ray region) and lose (or gain in the case of anti-Stokes Raman scattering) only part of their energy to the sample. The two methods are complementary because some vibrational transitions that are observed in IR spectroscopy are not observed in Raman spectroscopy, and vice versa. RR spectroscopy is an extension of conventional Raman spectroscopy that can provide increased sensitivity to specific (colored) compounds that are present at low concentrations (micro to millimolar) in an otherwise complex mixture of compounds.
An advantage of resonance Raman spectroscopy over (normal) Raman spectroscopy is that the intensity of bands can be increased by several orders of magnitude. An application that illustrates this advantage is the study of the dioxygen unit in cytochrome c oxidase. Identification of the band associated with the O–O stretching vibration was confirmed by using 18O–16O and 16O–16O isotopologues. 
The frequencies of molecular vibrations range from less than 1012 to approximately 1014 Hz. These frequencies correspond to radiation in the infrared (IR) region of the electromagnetic spectrum. At any given instant, each molecule in a sample has a certain amount of vibrational energy. However, the amount of vibrational energy that a molecule has continually changes due to collisions and other interactions with other molecules in the sample.
At room temperature, most molecules are in the lowest energy state—known as the ground state . A few molecules are in higher energy states—known as excited states. The fraction of molecules occupying a given vibrational mode at a given temperature can be calculated using the Boltzmann distribution. Performing such a calculation shows that, for relatively low temperatures (such as those used for most routine spectroscopy), most of the molecules occupy the ground vibrational state (except in the case of low-frequency modes). Such a molecule can be excited to a higher vibrational mode through the direct absorption of a photon of the appropriate energy. This is the mechanism by which IR spectroscopy operates: infrared radiation is passed through the sample, and the intensity of the transmitted light is compared with that of the incident light. A reduction in intensity at a given wavelength of light indicates the absorption of energy by a vibrational transition. The energy, , of a photon is
where is Planck's constant and is the frequency of the radiation. Thus, the energy required for such transition may be calculated if the frequency of the incident radiation is known.
It is also possible to observe molecular vibrations by an inelastic scattering process, Stokes Raman scattering being one such process. A photon is absorbed and then re-emitted (scattered) with lower energy. The difference in energy between the absorbed and re-emitted photons corresponds to the energy required to excite a molecule to a higher vibrational mode. Anti-Stokes Raman scattering is another inelastic scattering process and only occurs from molecules starting in excited vibrational states; it results in light scattered with higher energy. Light scattered elastically (no change in energy between the incoming photons and the re-emitted/scattered photons) is known as Rayleigh scattering.
Generally, the difference in energy is recorded as the difference in wavenumber () between the laser light and the scattered light which is known as the Raman shift. A Raman spectrum is generated by plotting the intensity of the scattered light versus .
Like infrared spectroscopy, Raman spectroscopy can be used to identify chemical compounds because the values of are indicative of different chemical species (their so-called chemical fingerprint). This is because the frequencies of vibrational transitions depend on the atomic masses and the bond strengths. Thus, armed with a database of spectra from known compounds, one can unambiguously identify many different known chemical compounds based on a Raman spectrum. The number of vibrational modes scales with the number of atoms in a molecule, which means that the Raman spectra from large molecules is complicated. For example, proteins typically contain thousands of atoms and therefore have thousands of vibrational modes. If these modes have similar energies (), then the spectrum may be incredibly cluttered and complicated.
Not all vibrational transitions are Raman active, meaning that some vibrational transitions do not appear in the Raman spectrum. This is because of the spectroscopic selection rules for Raman spectra. In contrast to IR spectroscopy, where a transition can only be seen when that particular vibration causes a net change in dipole moment of the molecule, in Raman spectroscopy only transitions where the polarizability of the molecule changes along the vibrational coordinate can be observed. This is the fundamental difference in how IR and Raman spectroscopy access the vibrational transitions. In Raman spectroscopy, the incoming photon causes a momentary distortion of the electron distribution around a bond in a molecule, followed by re-emission of the radiation as the bond returns to its normal state. This causes temporary polarization of the bond and an induced dipole that disappears upon relaxation. In a molecule with a center of symmetry, a change in dipole is accomplished by loss of the center of symmetry, while a change in polarizability is compatible with preservation of the center of symmetry. Thus, in a centrosymmetric molecule, asymmetrical stretching and bending are IR active and Raman inactive, while symmetrical stretching and bending is Raman active and IR inactive. Hence, in a centrosymmetric molecule, IR and Raman spectroscopy are mutually exclusive. For molecules without a center of symmetry, each vibrational mode may be IR active, Raman active, both, or neither. Symmetrical stretches and bends, however, tend to be Raman active.
In resonance Raman spectroscopy, the wavelength of the incoming photons coincides with an electronic transition of the molecule or material. Electronic excitation of a molecule results in structural changes which are reflected in the enhancement of Raman scattering of certain vibrational modes. Vibrational modes that undergo a change in bond length and/or force constant during the electronic excitation can show a large increase in polarizability and hence Raman intensity. This is known as Tsuboi's rule, which gives a qualitative relationship between the nature of an electronic transition and the enhancement pattern in resonance Raman spectroscopy.  The enhancement factor can be by a factor of 10 to > 100,000 and is most apparent in the case of π-π* transitions and least for metal centered (d–d) transitions. 
There are two primary methods used to quantitatively understand resonance Raman enhancement. These are the transform theory, developed by Albrecht and the time-dependent theory developed by Heller. 
The selective enhancement of the Raman scattering from specific modes under resonance conditions means that resonance Raman spectroscopy is especially useful for large biomolecules with chromophores embedded in their structure. In such chromophores, the resonance scattering from charge-transfer (CT) electronic transitions of the metal complex generally result in enhancement of metal-ligand stretching modes, as well as some of the modes associated with the ligands alone. Hence, in a biomolecule such as hemoglobin, tuning the laser to near the charge-transfer electronic transition of the iron center results in a spectrum reflecting only the stretching and bending modes associated with the tetrapyrrole-iron group. Consequently, in a molecule with thousands of vibrational modes, RR spectroscopy allows us to look at relatively few vibrational modes at a time. The Raman spectrum of a protein containing perhaps hundreds of peptide bonds but only a single porphyrin molecule may show only the vibrations associated with the porphyrin. This reduces the complexity of the spectrum and allows for easier identification of an unknown protein. Also, if a protein has more than one chromophore, different chromophores can be studied individually if their CT bands differ in energy. In addition to identifying compounds, RR spectroscopy can also supply structural identification about chromophores in some cases.
The main advantage of RR spectroscopy over non-resonant Raman spectroscopy is the large increase in intensity of the bands in question (by as much as a factor of 106). This allows RR spectra to be obtained with sample concentrations as low as 10−8 M. It also makes it possible to record Raman spectra of short-lived excited state species when pulsed lasers are used.  This is in stark contrast to non-resonant Raman spectra, which usually requires concentrations greater than 0.01 M. RR spectra usually exhibit fewer bands than the non resonant Raman spectrum of a compound, and the enhancement seen for each band can vary depending on the electronic transitions with which the laser is resonant. Since typically, RR spectroscopy are obtained with lasers at visible and near-UV wavelengths, spectra are more likely to be affected by fluorescence. Furthermore, photodegradation (photobleaching) and heating of the sample can occur as the sample also absorbs the excitation light, dissipating the energy as heat.
The instrumentation used for resonance Raman spectroscopy is identical to that used for Raman spectroscopy; specifically, a highly monochromatic light source (a laser), with an emission wavelength in either the near-infrared, visible, or near-ultraviolet region of the spectrum. Since the energy of electronic transitions (i.e. the color) varies widely from compound to compound, wavelength-tunable lasers, which appeared in the early 1970s, are useful as they can be tuned to coincide with an electronic transition (resonance). However, the broadness of electronic transitions means that many laser wavelengths may be necessary and multi-line lasers (Argon and Krypton ion) are commonly used. The essential point is that the wavelength of the laser emission is coincident with an electronic absorption band of the compound of interest. The spectra obtained contain non-resonant Raman scattering of the matrix (e.g., solvent) also.
Sample handling in Raman spectroscopy offers considerable advantages over FTIR spectroscopy in that glass can be used for windows, lenses, and other optical components. A further advantage is that whereas water absorbs strongly in the infrared region, which limits the pathlengths that can be used and masking large region of the spectrum, the intensity of Raman scattering from water is usually weak and direct absorption interferes only when near-infrared lasers (e.g., 1064 nm) are used. Therefore, water is an ideal solvent. However, since the laser is focused to a relatively small spot size, rapid heating of samples can occur. When resonance Raman spectra are recorded, however, sample heating and photo-bleaching can cause damage and a change to the Raman spectrum obtained. Furthermore, if the absorbance of the sample is high (> OD 2) over the wavelength range in which the Raman spectrum is recorded then inner-filter effects (reabsorption of the Raman scattering by the sample) can decrease signal intensity dramatically. Typically, the sample is placed into a tube, which can then be spun to decrease the sample's exposure to the laser light, and reduce the effects of photodegradation. Gaseous, liquid, and solid samples can all be analyzed using RR spectroscopy.
Although scattered light leaves the sample in all directions the collection of the scattered light is achieved only over a relatively small solid angle by a lens and directed to the spectrograph and CCD detector. The laser beam can be at any angle with respect to the optical axis used to collect Raman scattering. In free space systems, the laser path is typically at an angle of 180° or 135° (a so-called back scattering arrangement). The 180° arrangement is typically used in microscopes and fiber optic based Raman probes. Other arrangements involve the laser passing at 90° with respect to the optical axis. Detection angles of 90° and 0° are less frequently used.
The collected scattered radiation is focused into a spectrograph, in which the light is first collimated and then dispersed by a diffraction grating and refocused onto a CCD camera. The entire spectrum is recorded simultaneously and multiple scans can be acquired in a short period of time, which can increase the signal-to-noise ratio of the spectrum through averaging. Use of this (or equivalent) equipment and following an appropriate protocol  can yield better than 10% repeatability in absolute measurements for the rate of Raman scattering. This can be useful with resonance Raman for accurately determining optical transitions in structures with strong Van Hove singularities. 
Resonance hyper-Raman spectroscopy is a variation on resonance Raman spectroscopy in which the aim is to achieve an excitation to a particular energy level in the target molecule of the sample by a phenomenon known as two-photon absorption. In two-photon absorption, two photons are simultaneously absorbed into a molecule. When that molecule relaxes from this excited state to its ground state, only one photon is emitted. This is a type of fluorescence.
In resonance Raman spectroscopy, certain parts of molecules can be targeted by adjusting the wavelength of the incident laser beam to the “color” (energy between two desired electron quantum levels) of the part of the molecule that is being studied. This is known as resonance fluorescence, hence the addition of the term “resonance” to the name “Raman spectroscopy”. Some excited states can be achieved via single or double photon absorption. In these cases, however, the use of double photon excitation can be used to attain more information about these excited states than would a single photon absorption. There are some limitations and consequences to both resonance Raman and resonance hyper Raman spectroscopy. 
Both resonance Raman and resonance hyper Raman spectroscopy employ a tunable laser. The wavelength of a tunable laser can be adjusted by the operator to wavelengths within a particular range. This frequency range, however, is dependent on the laser’s design. Regular resonance Raman spectroscopy, therefore, is only sensitive to the electron energy transitions that match that of the laser used in the experiment. The molecular parts that can be studied by normal resonance Raman spectroscopy is therefore limited to those bonds that happen to have a “color” that fits somewhere into the spectrum of “colors” to which the laser used in that particular device can be tuned. Resonance hyper Raman spectroscopy, on the other hand, can excite atoms to emit light at wavelengths outside the laser’s tunable range, thus expanding the range of possible components of a molecule that can be excited and therefore studied.
Resonance hyper Raman spectroscopy is one of the types of “non-linear” Raman spectroscopy. In linear Raman spectroscopy, the amount of energy that goes into the excitation of an atom is the same amount that leaves the electron cloud of that atom when a photon is emitted and the electron cloud relaxes back down to its ground state. The term non-linear signifies reduced emission energy compared to input energy. In other words, the energy into the system no longer matches the energy out of the system. This is due to the fact that the energy input in hyper-Raman spectroscopy is much larger than that of typical Raman spectroscopy. Non-linear Raman spectroscopy tends to be more sensitive than conventional Raman spectroscopy. Additionally, it can significantly reduce, or even eliminate the effects of fluorescence. 
In the X-ray region, enough energy is available for making electronic transitions possible. At core level resonances, X-ray Raman scattering can become the dominating part of the X-ray fluorescence spectrum. This is due to the resonant behavior of the Kramers-Heisenberg formula in which the denominator is minimized for incident energies that equal a core level. This type of scattering is also known as Resonant inelastic X-ray scattering (RIXS). In the soft X-ray range, RIXS has been shown to reflect crystal field excitations, which are often hard to observe with any other technique. Application of RIXS to strongly correlated materials is of particular value for gaining knowledge about their electronic structure. For certain wide band materials such as graphite, RIXS has been shown to (nearly) conserve crystal momentum and thus has found use as a complementary bandmapping technique.
Infrared spectroscopy is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms. It can be used to characterize new materials or identify and verify known and unknown samples. The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer which produces an infrared spectrum. An IR spectrum can be visualized in a graph of infrared light absorbance on the vertical axis vs. frequency, wavenumber or wavelength on the horizontal axis. Typical units of wavenumber used in IR spectra are reciprocal centimeters, with the symbol cm−1. Units of IR wavelength are commonly given in micrometers, symbol μm, which are related to the wavenumber in a reciprocal way. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer. Two-dimensional IR is also possible as discussed below.
Spectroscopy is the general field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter waves and acoustic waves can also be considered forms of radiative energy, and recently gravitational waves have been associated with a spectral signature in the context of the Laser Interferometer Gravitational-Wave Observatory (LIGO)
Raman spectroscopy is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified.
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a transition from a high energy state to a lower energy state. The photon energy of the emitted photon is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element's emission spectrum is unique. Therefore, spectroscopy can be used to identify elements in matter of unknown composition. Similarly, the emission spectra of molecules can be used in chemical analysis of substances.
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a function of frequency, and this variation is the absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum.
Fluorescence spectroscopy is a type of electromagnetic spectroscopy that analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electrons in molecules of certain compounds and causes them to emit light; typically, but not necessarily, visible light. A complementary technique is absorption spectroscopy. In the special case of single molecule fluorescence spectroscopy, intensity fluctuations from the emitted light are measured from either single fluorophores, or pairs of fluorophores.
In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied after the illumination of a material occurs, but in principle, the technique can be applied to any process that leads to a change in properties of a material. With the help of pulsed lasers, it is possible to study processes that occur on time scales as short as 10−16 seconds. All time-resolved spectra are suitable to be analyzed using the two-dimensional correlation method for a correlation map between the peaks.
Raman scattering or the Raman effect is the inelastic scattering of photons by matter, meaning that there is both an exchange of energy and a change in the light's direction. Typically this effect involves vibrational energy being gained by a molecule as incident photons from a visible laser are shifted to lower energy. This is called normal Stokes Raman scattering. The effect is exploited by chemists and physicists to gain information about materials for a variety of purposes by performing various forms of Raman spectroscopy. Many other variants of Raman spectroscopy allow rotational energy to be examined and electronic energy levels may be examined if an X-ray source is used in addition to other possibilities. More complex techniques involving pulsed lasers, multiple laser beams and so on are known.
Stokes shift is the difference between positions of the band maxima of the absorption and emission spectra of the same electronic transition. It is named after Irish physicist George Gabriel Stokes. Sometimes Stokes shifts are given in wavelength units, but this is less meaningful than energy, wavenumber or frequency units because it depends on the absorption wavelength. For instance, a 50 nm Stokes shift from absorption at 300 nm is larger in terms of energy than a 50 nm Stokes shift from absorption at 600 nm.
Coherent anti-Stokes Raman spectroscopy, also called Coherent anti-Stokes Raman scattering spectroscopy (CARS), is a form of spectroscopy used primarily in chemistry, physics and related fields. It is sensitive to the same vibrational signatures of molecules as seen in Raman spectroscopy, typically the nuclear vibrations of chemical bonds. Unlike Raman spectroscopy, CARS employs multiple photons to address the molecular vibrations, and produces a coherent signal. As a result, CARS is orders of magnitude stronger than spontaneous Raman emission. CARS is a third-order nonlinear optical process involving three laser beams: a pump beam of frequency ωp, a Stokes beam of frequency ωS and a probe beam at frequency ωpr. These beams interact with the sample and generate a coherent optical signal at the anti-Stokes frequency (ωpr+ωp-ωS). The latter is resonantly enhanced when the frequency difference between the pump and the Stokes beams (ωp-ωS) coincides with the frequency of a Raman resonance, which is the basis of the technique's intrinsic vibrational contrast mechanism.
Ultrafast laser spectroscopy is a spectroscopic technique that uses ultrashort pulse lasers for the study of dynamics on extremely short time scales. Different methods are used to examine the dynamics of charge carriers, atoms, and molecules. Many different procedures have been developed spanning different time scales and photon energy ranges; some common methods are listed below.
X-ray absorption near edge structure (XANES), also known as near edge X-ray absorption fine structure (NEXAFS), is a type of absorption spectroscopy that indicates the features in the X-ray absorption spectra (XAS) of condensed matter due to the photoabsorption cross section for electronic transitions from an atomic core level to final states in the energy region of 50–100 eV above the selected atomic core level ionization energy, where the wavelength of the photoelectron is larger than the interatomic distance between the absorbing atom and its first neighbour atoms.
Two-photon absorption or two-photon excitation or non-linear absorption is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state to a higher energy, most commonly an excited electronic state. Absorption of two photons with different frequencies is called non-degenerate two-photon absorption. Since TPA depends on the simultaneous absorption of two photons, the probability of TPA is proportional to the square of the light intensity, thus it is a nonlinear optical process. The energy difference between the involved lower and upper states of the molecule is equal or smaller than the sum of the photon energies of the two photons absorbed. Two-photon absorption is a third-order process, with absorption cross section typically several orders of magnitude smaller than one-photon absorption cross section.
Chemical imaging is the analytical capability to create a visual image of components distribution from simultaneous measurement of spectra and spatial, time information. Hyperspectral imaging measures contiguous spectral bands, as opposed to multispectral imaging which measures spaced spectral bands.
A molecular vibration is a periodic motion of the atoms of a molecule relative to each other, such that the center of mass of the molecule remains unchanged. The typical vibrational frequencies range from less than 1013 Hz to approximately 1014 Hz, corresponding to wavenumbers of approximately 300 to 3000 cm−1 and wavelengths of approximately 30 to 3 µm.
X-ray Raman scattering (XRS) is non-resonant inelastic scattering of X-rays from core electrons. It is analogous to vibrational Raman scattering, which is a widely used tool in optical spectroscopy, with the difference being that the wavelengths of the exciting photons fall in the X-ray regime and the corresponding excitations are from deep core electrons.
Photoelectrochemical processes are processes in photoelectrochemistry; they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism.
The technique of vibrational analysis with scanning probe microscopy allows probing vibrational properties of materials at the submicrometer scale, and even of individual molecules. This is accomplished by integrating scanning probe microscopy (SPM) and vibrational spectroscopy. This combination allows for much higher spatial resolution than can be achieved with conventional Raman/FTIR instrumentation. The technique is also nondestructive, requires non-extensive sample preparation, and provides more contrast such as intensity contrast, polarization contrast and wavelength contrast, as well as providing specific chemical information and topography images simultaneously.
Stimulated Raman spectroscopy, also referred to as stimulated Raman scattering (SRS) is a form of spectroscopy employed in physics, chemistry, biology, and other fields. The basic mechanism resembles that of spontaneous Raman spectroscopy: a pump photon, of the angular frequency , which is scattered by a molecule has some small probability of inducing some vibrational transition, as opposed to inducing a simple Rayleigh transition. This makes the molecule emit a photon at a shifted frequency. However, SRS, as opposed to spontaneous Raman spectroscopy, is a third-order non-linear phenomenon involving a second photon—the Stokes photon of angular frequency —which stimulates a specific transition. When the difference in frequency between both photons resembles that of a specific vibrational transition the occurrence of this transition is resonantly enhanced. In SRS, the signal is equivalent to changes in the intensity of the pump and Stokes beams. Employing a pump laser beam of a constant frequency and a Stokes laser beam of a scanned frequency allows for the unraveling of the spectral fingerprint of the molecule. This spectral fingerprint differs from those obtained by other spectroscopy methods such as Rayleigh scattering as the Raman transitions confer to different exclusion rules than those that apply for Rayleigh transitions.
Raman spectroelectrochemistry (Raman-SEC) is a technique that studies the inelastic scattering or Raman scattering of monochromatic light related to chemical compounds involved in an electrode process. This technique provides information about vibrational energy transitions of molecules, using a monochromatic light source, usually from a laser that belongs to the UV, Vis or NIR region. Raman spectroelectrochemistry provides specific information about structural changes, composition and orientation of the molecules on the electrode surface involved in an electrochemical reaction, being the Raman spectra registered a real fingerprint of the compounds.