Raman scattering

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"Raman Effect" redirects here. Raman scattering_sentence_0

For the 2008 film, see Raman (film). Raman scattering_sentence_1

Raman scattering or the Raman effect /ˈrɑːmən/ is the inelastic scattering of photons by matter, meaning that there is an exchange of energy and a change in the light's direction. Raman scattering_sentence_2

Typically this involves vibrational energy being gained by a molecule as incident photons from a visible laser are shifted to lower energy. Raman scattering_sentence_3

This is called normal Stokes Raman scattering. Raman scattering_sentence_4

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. Raman scattering_sentence_5

Many other variants of Raman spectroscopy allow rotational energy to be examined (if gas samples are used) and electronic energy levels may be examined if an X-ray source is used in addition to other possibilities. Raman scattering_sentence_6

More complex techniques involving pulsed lasers, multiple laser beams and so on are known. Raman scattering_sentence_7

Light has a certain probability of being scattered by a material. Raman scattering_sentence_8

When photons are scattered, most of them are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency, wavelength and color) as the incident photons but different direction. Raman scattering_sentence_9

Rayleigh scattering usually has an intensity in the range 0.1% to 0.01% relative to that of a radiation source. Raman scattering_sentence_10

An even smaller fraction of the scattered photons (approximately 1 in 10 million) can be scattered inelastically, with the scattered photons having an energy different (usually lower) from those of the incident photons—these are Raman scattered photons. Raman scattering_sentence_11

Because of conservation of energy, the material either gains or loses energy in the process. Raman scattering_sentence_12

Rayleigh scattering was discovered and explained in the 19th century. Raman scattering_sentence_13

The Raman effect is named after Indian scientist C. Raman scattering_sentence_14 V. Raman, who discovered it in 1928 with assistance from his student K. Raman scattering_sentence_15 S. Krishnan. Raman scattering_sentence_16

Raman was awarded the Nobel prize in Physics in 1930 for his discovery. Raman scattering_sentence_17

The effect had been predicted theoretically by Adolf Smekal in 1923. Raman scattering_sentence_18

History Raman scattering_section_0

The elastic light scattering phenomena called Rayleigh scattering, in which light retains its energy, was described in the 19th century. Raman scattering_sentence_19

The intensity of Rayleigh scattering is about 10 to 10 compared to the intensity of the exciting source. Raman scattering_sentence_20

In 1908, another form of elastic scattering, called Mie scattering was discovered. Raman scattering_sentence_21

The inelastic scattering of light was predicted by Adolf Smekal in 1923 and in older German-language literature it has been referred to as the Smekal-Raman-Effekt. Raman scattering_sentence_22

In 1922, Indian physicist C. Raman scattering_sentence_23 V. 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 1928) of the radiation effect that bears his name. Raman scattering_sentence_24

The Raman effect was first reported by Raman and his coworker K. Raman scattering_sentence_25 S. Krishnan, and independently by Grigory Landsberg and Leonid Mandelstam, in Moscow on 21 February 1928 (one week earlier than Raman and Krishnan). Raman scattering_sentence_26

In the former Soviet Union, Raman's contribution was always disputed; thus in Russian scientific literature the effect is usually referred to as "combination scattering" or "combinatory scattering". Raman scattering_sentence_27

Raman received the Nobel Prize in 1930 for his work on the scattering of light. Raman scattering_sentence_28

In 1998 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. Raman scattering_sentence_29

Instrumentation Raman scattering_section_1

Main article: Raman spectroscopy § Instrumentation Raman scattering_sentence_30

Modern Raman spectroscopy nearly always involves the use of lasers as an exciting light source. Raman scattering_sentence_31

Because lasers were not available until more than three decades after the discovery of the effect, Raman and Krishnan used a mercury lamp and photographic plates to record spectra. Raman scattering_sentence_32

Early spectra took hours or even days to acquire due to weak light sources, poor sensitivity of the detectors and the weak Raman scattering cross-sections of most materials. Raman scattering_sentence_33

The most common modern detectors are charge-coupled devices (CCDs). Raman scattering_sentence_34

Photodiode arrays and photomultiplier tubes were common prior to the adoption of CCDs. Raman scattering_sentence_35

Theory Raman scattering_section_2

The following focuses on the theory of normal (non-resonant, spontaneous, vibrational) Raman scattering of light by discrete molecules. Raman scattering_sentence_36

X-ray Raman spectroscopy is conceptually similar but involves excitation of electronic, rather than vibrational, energy levels. Raman scattering_sentence_37

Molecular vibrations Raman scattering_section_3

Main article: Molecular vibration Raman scattering_sentence_38

Raman scattering generally gives information about vibrations within a molecule. Raman scattering_sentence_39

In the case of gases, information about rotational energy can also be gleaned. Raman scattering_sentence_40

For solids, phonon modes may also be observed. Raman scattering_sentence_41

The basics of infrared absorption regarding molecular vibrations apply to Raman scattering although the selection rules are different. Raman scattering_sentence_42

Degrees of freedom Raman scattering_section_4

Main article: Degrees of freedom (physics and chemistry) Raman scattering_sentence_43

Vibrational energy Raman scattering_section_5

Main article: Quantum harmonic oscillator Raman scattering_sentence_44

Molecular vibrational energy is known to be quantized and can be modeled using the quantum harmonic oscillator (QHO) approximation or a Dunham expansion when anharmonicity is important. Raman scattering_sentence_45

The vibrational energy levels according to the QHO are Raman scattering_sentence_46

The energy range for vibrations is in the range of approximately 5 to 3500 cm. Raman scattering_sentence_47

The fraction of molecules occupying a given vibrational mode at a given temperature follows a Boltzmann distribution. Raman scattering_sentence_48

A molecule can be excited to a higher vibrational mode through the direct absorption of a photon of the appropriate energy, which falls in the terahertz or infrared range. Raman scattering_sentence_49

This forms the basis of infrared spectroscopy. Raman scattering_sentence_50

Alternatively, the same vibrational excitation can be produced by an inelastic scattering process. Raman scattering_sentence_51

This is called Stokes Raman scattering, by analogy with the Stokes shift in fluorescence discovered by George Stokes in 1852, with light emission at longer wavelength (now known to correspond to lower energy) than the absorbed incident light. Raman scattering_sentence_52

Conceptually similar effects can be caused by neutrons or electrons rather than light. Raman scattering_sentence_53

An increase in photon energy which leaves the molecule in a lower vibrational energy state is called anti-Stokes scattering. Raman scattering_sentence_54

Raman scattering Raman scattering_section_6

A classical physics based model is able to account for Raman scattering and predicts an increase in the intensity which scales with the fourth-power of the light frequency. Raman scattering_sentence_55

Light scattering by a molecule is associated with oscillations of an induced electric dipole. Raman scattering_sentence_56

The oscillating electric field component of electromagnetic radiation may bring about an induced dipole in a molecule which follows the alternating electric field which is modulated by the molecular vibrations. Raman scattering_sentence_57

Oscillations at the external field frequency are therefore observed along with beat frequencies resulting from the external field and normal mode vibrations. Raman scattering_sentence_58

The spectrum of the scattered photons is termed the Raman spectrum. Raman scattering_sentence_59

It shows the intensity of the scattered light as a function of its frequency difference Δν to the incident photons, more commonly called a Raman shift. Raman scattering_sentence_60

The locations of corresponding Stokes and anti-Stokes peaks form a symmetric pattern around the RayleighΔν=0 line. Raman scattering_sentence_61

The frequency shifts are symmetric because they correspond to the energy difference between the same upper and lower resonant states. Raman scattering_sentence_62

The intensities of the pairs of features will typically differ, though. Raman scattering_sentence_63

They depend on the populations of the initial states of the material, which in turn depend on the temperature. Raman scattering_sentence_64

In thermodynamic equilibrium, the lower state will be more populated than the upper state. Raman scattering_sentence_65

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). Raman scattering_sentence_66

Correspondingly, Stokes scattering peaks are stronger than anti-Stokes scattering peaks. Raman scattering_sentence_67

Their ratio depends on the temperature, and can therefore be exploited to measure it: Raman scattering_sentence_68

Selection rules Raman scattering_section_7

A selection rule relevant only to ordered solid materials states that only phonons with zero phase angle can be observed by IR and Raman, except when phonon confinement is manifest. Raman scattering_sentence_69

Symmetry and polarization Raman scattering_section_8

Main article: Depolarization ratio Raman scattering_sentence_70

Monitoring the polarization of the scattered photons is useful for understanding the connections between molecular symmetry and Raman activity which may assist in assigning peaks in Raman spectra. Raman scattering_sentence_71

Light polarized in a single direction only gives access to some Raman–active modes, but rotating the polarization gives access to other modes. Raman scattering_sentence_72

Each mode is separated according to its symmetry. Raman scattering_sentence_73

Stimulated Raman scattering and Raman amplification Raman scattering_section_9

Main article: Stimulated Raman spectroscopy Raman scattering_sentence_74

The Raman-scattering process as described above takes place spontaneously; i.e., in random time intervals, one of the many incoming photons is scattered by the material. Raman scattering_sentence_75

This process is thus called spontaneous Raman scattering. Raman scattering_sentence_76

On the other hand, stimulated Raman scattering can take place when some Stokes photons have previously been generated by spontaneous Raman scattering (and somehow forced to remain in the material), or when deliberately injecting Stokes photons ("signal light") together with the original light ("pump light"). Raman scattering_sentence_77

In that case, the total Raman-scattering rate is increased beyond that of spontaneous Raman scattering: pump photons are converted more rapidly into additional Stokes photons. Raman scattering_sentence_78

The more Stokes photons that are already present, the faster more of them are added. Raman scattering_sentence_79

Effectively, this amplifies the Stokes light in the presence of the pump light, which is exploited in Raman amplifiers and Raman lasers. Raman scattering_sentence_80

Requirement for space-coherence Raman scattering_section_10

Suppose that the distance between two points A and B of an exciting beam is x. Raman scattering_sentence_81

Generally, as the exciting frequency is not equal to the scattered Raman frequency, the corresponding relative wavelengths λ and λ' are not equal. Raman scattering_sentence_82

Thus, a phase-shift Θ = 2πx(1/λ − 1/λ') appears. Raman scattering_sentence_83

For Θ = π, the scattered amplitudes are opposite, so that the Raman scattered beam remains weak. Raman scattering_sentence_84

Raman scattering_unordered_list_0

  • A crossing of the beams may limit the path x.Raman scattering_item_0_0

Several tricks may be used to get a larger amplitude: Raman scattering_sentence_85

Raman scattering_unordered_list_1

  • In an optically anisotropic crystal, a light ray may have two modes of propagation with different polarizations and different indices of refraction. If energy may be transferred between these modes by a quadrupolar (Raman) resonance, phases remain coherent along the whole path, transfer of energy may be large. It is an Optical parametric generation.Raman scattering_item_1_1
  • Light may be pulsed, so that beats do not appear. In Impulsive Stimulated Raman Scattering (ISRS), the length of the pulses must be shorter than all relevant time constants. Interference of Raman and incident lights is too short to allow beats, so that it produces a frequency shift roughly, in best conditions, inversely proportional to the cube of the pulse length.Raman scattering_item_1_2

In labs, femtosecond laser pulses must be used because the ISRS becomes very weak if the pulses are too long. Raman scattering_sentence_86

Thus ISRS cannot be observed using nanosecond pulses making ordinary time-incoherent light. Raman scattering_sentence_87

Inverse Raman effect Raman scattering_section_11

The inverse Raman effect is a form of Raman scattering first noted by W. J. Jones and B.P. Raman scattering_sentence_88 Stoicheff. Raman scattering_sentence_89

In some circumstances, Stokes scattering can exceed anti-Stokes scattering; in these cases the continuum (on leaving the material) is observed to have an absorption line (a dip in intensity) at νL+νM. Raman scattering_sentence_90

This phenomenon is referred to as the inverse Raman effect; the application of the phenomenon is referred to as inverse Raman spectroscopy, and a record of the continuum is referred to as an inverse Raman spectrum. Raman scattering_sentence_91

In the original description of the inverse Raman effect, the authors discuss both absorption from a continuum of higher frequencies and absorption from a continuum of lower frequencies. Raman scattering_sentence_92

They note that absorption from a continuum of lower frequencies will not be observed if the Raman frequency of the material is vibrational in origin and if the material is in thermal equilibrium. Raman scattering_sentence_93

Supercontinuum generation Raman scattering_section_12

For high-intensity continuous wave (CW) lasers, stimulated Raman scattering can be used to produce a broad bandwidth supercontinuum. Raman scattering_sentence_94

This process can also be seen as a special case of four-wave mixing, wherein the frequencies of the two incident photons are equal and the emitted spectra are found in two bands separated from the incident light by the phonon energies. Raman scattering_sentence_95

The initial Raman spectrum is built up with spontaneous emission and is amplified later on. Raman scattering_sentence_96

At high pumping levels in long fibers, higher-order Raman spectra can be generated by using the Raman spectrum as a new starting point, thereby building a chain of new spectra with decreasing amplitude. Raman scattering_sentence_97

The disadvantage of intrinsic noise due to the initial spontaneous process can be overcome by seeding a spectrum at the beginning, or even using a feedback loop as in a resonator to stabilize the process. Raman scattering_sentence_98

Since this technology easily fits into the fast evolving fiber laser field and there is demand for transversal coherent high-intensity light sources (i.e., broadband telecommunication, imaging applications), Raman amplification and spectrum generation might be widely used in the near-future. Raman scattering_sentence_99

Applications Raman scattering_section_13

Main article: Raman spectroscopy § Applications Raman scattering_sentence_100

Raman spectroscopy employs the Raman effect for substances analysis. Raman scattering_sentence_101

The spectrum of the Raman-scattered light depends on the molecular constituents present and their state, allowing the spectrum to be used for material identification and analysis. Raman scattering_sentence_102

Raman spectroscopy is used to analyze a wide range of materials, including gases, liquids, and solids. Raman scattering_sentence_103

Highly complex materials such as biological organisms and human tissue can also be analyzed by Raman spectroscopy. Raman scattering_sentence_104

For solid materials, Raman scattering is used as a tool to detect high-frequency phonon and magnon excitations. Raman scattering_sentence_105

Raman lidar is used in atmospheric physics to measure the atmospheric extinction coefficient and the water vapour vertical distribution. Raman scattering_sentence_106

Stimulated Raman transitions are also widely used for manipulating a trapped ion's energy levels, and thus basis qubit states. Raman scattering_sentence_107

Raman spectroscopy can be used to determine the force constant and bond length for molecules that do not have an infrared absorption spectrum. Raman scattering_sentence_108

Raman amplification is used in optical amplifiers. Raman scattering_sentence_109

The Raman effect is also involved in producing the appearance of the blue sky (see Rayleigh Scattering: 'Rayleigh scattering of molecular nitrogen and oxygen in the atmosphere includes elastic scattering as well as the inelastic contribution from rotational Raman scattering in air'). Raman scattering_sentence_110

Raman spectroscopy has been used to chemically image small molecules, such as nucleic acids, in biological systems by a vibrational tag. Raman scattering_sentence_111

See also Raman scattering_section_14

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Raman scattering.