Raman scattering
"Raman Effect" redirects here.
For the 2008 film, see Raman (film).
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.
Typically this 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 (if gas samples are used) 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.
Light has a certain probability of being scattered by a material.
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.
Rayleigh scattering usually has an intensity in the range 0.1% to 0.01% relative to that of a radiation source.
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.
Because of conservation of energy, the material either gains or loses energy in the process.
Rayleigh scattering was discovered and explained in the 19th century.
The Raman effect is named after Indian scientist C. V. Raman, who discovered it in 1928 with assistance from his student K. S. Krishnan.
Raman was awarded the Nobel prize in Physics in 1930 for his discovery.
The effect had been predicted theoretically by Adolf Smekal in 1923.
History
The elastic light scattering phenomena called Rayleigh scattering, in which light retains its energy, was described in the 19th century.
The intensity of Rayleigh scattering is about 10 to 10 compared to the intensity of the exciting source.
In 1908, another form of elastic scattering, called Mie scattering was discovered.
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.
In 1922, Indian physicist C. 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.
The Raman effect was first reported by Raman and his coworker K. S. Krishnan, and independently by Grigory Landsberg and Leonid Mandelstam, in Moscow on 21 February 1928 (one week earlier than Raman and Krishnan).
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 received the Nobel Prize in 1930 for his work on the scattering of light.
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.
Instrumentation
Main article: Raman spectroscopy § Instrumentation
Modern Raman spectroscopy nearly always involves the use of lasers as an exciting light source.
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.
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.
The most common modern detectors are charge-coupled devices (CCDs).
Photodiode arrays and photomultiplier tubes were common prior to the adoption of CCDs.
Theory
The following focuses on the theory of normal (non-resonant, spontaneous, vibrational) Raman scattering of light by discrete molecules.
X-ray Raman spectroscopy is conceptually similar but involves excitation of electronic, rather than vibrational, energy levels.
Molecular vibrations
Main article: Molecular vibration
Raman scattering generally gives information about vibrations within a molecule.
In the case of gases, information about rotational energy can also be gleaned.
For solids, phonon modes may also be observed.
The basics of infrared absorption regarding molecular vibrations apply to Raman scattering although the selection rules are different.
Degrees of freedom
Main article: Degrees of freedom (physics and chemistry)
Vibrational energy
Main article: Quantum harmonic oscillator
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.
The vibrational energy levels according to the QHO are
The energy range for vibrations is in the range of approximately 5 to 3500 cm.
The fraction of molecules occupying a given vibrational mode at a given temperature follows a Boltzmann distribution.
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.
This forms the basis of infrared spectroscopy.
Alternatively, the same vibrational excitation can be produced by an inelastic scattering process.
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.
Conceptually similar effects can be caused by neutrons or electrons rather than light.
An increase in photon energy which leaves the molecule in a lower vibrational energy state is called anti-Stokes scattering.
Raman scattering
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.
Light scattering by a molecule is associated with oscillations of an induced electric dipole.
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.
Oscillations at the external field frequency are therefore observed along with beat frequencies resulting from the external field and normal mode vibrations.
The spectrum of the scattered photons is termed the Raman spectrum.
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.
The locations of corresponding Stokes and anti-Stokes peaks form a symmetric pattern around the RayleighΔν=0 line.
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:
Selection rules
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.
Symmetry and polarization
Main article: Depolarization ratio
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.
Light polarized in a single direction only gives access to some Raman–active modes, but rotating the polarization gives access to other modes.
Each mode is separated according to its symmetry.
Stimulated Raman scattering and Raman amplification
Main article: Stimulated Raman spectroscopy
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.
This process is thus called spontaneous Raman scattering.
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").
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.
The more Stokes photons that are already present, the faster more of them are added.
Effectively, this amplifies the Stokes light in the presence of the pump light, which is exploited in Raman amplifiers and Raman lasers.
Requirement for space-coherence
Suppose that the distance between two points A and B of an exciting beam is x.
Generally, as the exciting frequency is not equal to the scattered Raman frequency, the corresponding relative wavelengths λ and λ' are not equal.
Thus, a phase-shift Θ = 2πx(1/λ − 1/λ') appears.
For Θ = π, the scattered amplitudes are opposite, so that the Raman scattered beam remains weak.
- A crossing of the beams may limit the path x.
Several tricks may be used to get a larger amplitude:
- 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.
- 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.
In labs, femtosecond laser pulses must be used because the ISRS becomes very weak if the pulses are too long.
Thus ISRS cannot be observed using nanosecond pulses making ordinary time-incoherent light.
Inverse Raman effect
The inverse Raman effect is a form of Raman scattering first noted by W. J. Jones and B.P. Stoicheff.
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.
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.
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.
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.
Supercontinuum generation
For high-intensity continuous wave (CW) lasers, stimulated Raman scattering can be used to produce a broad bandwidth supercontinuum.
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.
The initial Raman spectrum is built up with spontaneous emission and is amplified later on.
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.
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.
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.
Applications
Main article: Raman spectroscopy § Applications
Raman spectroscopy employs the Raman effect for substances analysis.
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 spectroscopy is used to analyze a wide range of materials, including gases, liquids, and solids.
Highly complex materials such as biological organisms and human tissue can also be analyzed by Raman spectroscopy.
For solid materials, Raman scattering is used as a tool to detect high-frequency phonon and magnon excitations.
Raman lidar is used in atmospheric physics to measure the atmospheric extinction coefficient and the water vapour vertical distribution.
Stimulated Raman transitions are also widely used for manipulating a trapped ion's energy levels, and thus basis qubit states.
Raman spectroscopy can be used to determine the force constant and bond length for molecules that do not have an infrared absorption spectrum.
Raman amplification is used in optical amplifiers.
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 spectroscopy has been used to chemically image small molecules, such as nucleic acids, in biological systems by a vibrational tag.
See also
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Raman scattering.