Typically this is vibrational energy and the incident photons are of visible light, although rotational energy if gas samples are used and electronic energy levels if an X-ray source is used may also be probed. The Raman effect forms the basis for Raman spectroscopy which is used by chemists and physicists to gain information about materials.
The inelastic scattering of light was predicted by Adolf Smekal in  and in German-language literature it may be referred to as the Smekal-Raman effect .
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In , Indian physicist C. Raman published his work on the "Molecular Diffraction of Light," the first of a series of investigations with his collaborators that ultimately led to his discovery on 28 February of the radiation effect that bears his name. The Raman effect was first reported by C. In the former Soviet Union, Raman's contribution was always disputed; thus in Russian scientific literature this effect is usually referred to as "combination scattering" or "combinatory scattering".
Raman received the Nobel Prize in for his work on the scattering of light.
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Although modern Raman spectroscopy nearly always involves the use of lasers , which were not available until more than three decades later, Raman and Krishnan used a mercury lamp and photographic plates to record spectra. In the Raman effect was designated a National Historic Chemical Landmark by the American Chemical Society in recognition of its significance as a tool for analyzing the composition of liquids, gases, and solids. For any given chemical compound, there are a total of 3N degrees of freedom , where N is the number of atoms in the compound. This number arises from the ability of each atom in a molecule to move in three different directions x, y, and z.
Consequently, the 3N degrees of freedom are partitioned into molecular translational, rotational , and vibrational motion. Three of the degrees of freedom correspond to translational motion of the molecule as a whole along each of the three spatial dimensions. Linear molecules only have two rotations because rotations along the bond axis do not change the positions of the atoms in the molecule.
The remaining degrees of freedom correspond to molecular vibrational modes. These modes include stretching and bending motions of the chemical bonds of the molecule. For a linear molecule, the number of vibrational modes is: The frequencies of molecular vibrations range from less than 10 12 to approximately 10 14 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 of the molecules will be in the lowest energy state , which is known as the ground state. A few molecules will be in higher energy states, which are 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. Such a molecule can be excited to a higher vibrational mode through the direct absorption of a photon of the appropriate energy.
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This is the mechanism by which IR spectroscopy operates: A reduction in intensity at a given wavelength of light indicates the absorption of energy by a vibrational transition. Thus, the energy required for such a 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. In inelastic Raman scattering, an absorbed photon is re-emitted with lower energy; the difference in energy between the incident photons and scattered photons corresponds to the energy required to excite a molecule to a higher vibrational mode.
Typically, in Raman spectroscopy high intensity laser radiation with wavelengths in either the visible or near-infrared regions of the spectrum is passed through a sample.
Photons from the laser beam produce an oscillating polarization in the molecules, exciting them to a virtual energy state. The oscillating polarization of the molecule can couple with other possible polarizations of the molecule, including vibrational and electronic excitations. If the polarization in the molecule does not couple to these other possible polarizations, then it will not change the vibrational state that the molecule started in and the scattered photon will have the same energy as the original photon.
This type of scattering is known as Rayleigh scattering. When the polarization in the molecules couples to a vibrational state that is higher in energy than the state they started in, then the original photon and the scattered photon differ in energy by the amount required to vibrationally excite the molecule.
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In perturbation theory , the Raman effect corresponds to the absorption and subsequent emission of a photon via an intermediate quantum state of a material. The intermediate state can be either a "real", i. The energy difference between the absorbed and emitted photon corresponds to the energy difference between two resonant states of the material and is independent of the absolute energy of the photon.
The spectrum of the scattered photons is termed the Raman spectrum.
The frequency shifts are symmetric because they correspond to the energy difference between the same upper and lower resonant states. The intensities of the pairs of features will typically differ, though. They depend on the populations of the initial states of the material, which in turn depend on the temperature. In thermodynamic equilibrium , the lower state will be more populated than the upper state.
Therefore, the rate of transitions from the more populated lower state to the upper state Stokes transitions will be higher than in the opposite direction anti-Stokes transitions. Correspondingly, Stokes scattering peaks are stronger than anti-Stokes scattering peaks. Their ratio depends on the temperature, and can therefore be exploited to measure it. The Raman effect differs from the process of fluorescence in that it is a scattering process.
For fluorescence, the incident light is completely absorbed, transferring the system to an excited state. After a certain resonance lifetime, the system de-excites to lower energy states via emission of photons. The result of both processes is in essence the same: A photon with a frequency different from that of the incident photon is produced and the molecule is brought to a higher or lower energy level. But the major difference is that the Raman effect can take place for any frequency of incident light. In contrast to the fluorescence effect, the Raman effect is therefore not a resonant effect.
In practice, this means that a fluorescence peak is anchored at a specific frequency, whereas a Raman peak maintains a constant separation from the excitation frequency. A Raman transition from one state to another is allowed only if the molecular polarizability of those states is different.
For a vibration, this means that the derivative of the polarizability with respect to the normal coordinate associated to the vibration is non-zero: Tumkin and Elizaveta O. Kirillova and Vladimir Lukianov and N. Kirillova and Vladimir A. Grechukha , Kseniya O. Kochemirovsky Published This work is devoted to the extremely popular but poorly developed scientific and forensic problem of the estimation of the actual dates of inscriptions placed on paper and made by ballpoint pens.
It is shown that the degradation of writing inks with time may be controlled via Raman spectroscopy and gas chromatography. From This Paper Figures, tables, and topics from this paper. Raman scattering Search for additional papers on this topic. Elegant degradation Digital pen Gases.
References Publications referenced by this paper. Showing of 30 references. The investigation of dye aging dynamics in writing inks using Raman spectroscopy.
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