raman effect fingerprinting of universe

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Raman Effect: fingerprinting the universe

At school, we were taught that Sir CV Raman won the 1930 Nobel Prize for Physics for discovering the "Raman effect". But when we asked what exactly the Raman Effect was, our

science teacher fobbed us off, saying "it's very complicated." Clearly, even he didn't know.Cynical students wondered why a complicated discovery without any obvious use had won the Nobel Prize.

But today, Raman's discovery has finally become a breakthrough technology. Hand-heldscanners called Raman scanners, weighing just one-third of a kilo, are being used by USnarcotics squads and airports to detect drugs. Security experts think that Raman scanners may bethe best devices to detect explosives carried by terrorists. Safety inspectors are using Ramanscanners to detect hazardous chemicals and gases. Police forces are using Raman scanners for

forensic work.

The scanners work by detecting the molecular structure of the object they are scanning. If youshoot a beam of light on an object, a very small part of it interacts with the atoms of the objectand scatters light in a pattern or spectrum unique to that particular molecule. This is the RamanEffect. It is difficult to detect, and typically needs lasers to amplify the signal. Every moleculehas a different Raman pattern. This is why Raman scanning has been called the fingerprinting of the universe: it can identify substances as surely as fingerprints can identify humans.

Identifying the chemical composition of a substance typically requires chemical and physicaltests that take time, maybe days. They typically require a sample to be extracted and destroyedwhile testing. But Raman scanning can take just 20 seconds. It does not require cutting,extracting or destroying a substance. Scanners have a laser, spectroscope and an electronic heartthat can recognize Raman patterns. This yields almost instant recognition of target substances.

For instance, narcotics squads in the US are using Raman scanners programmed to detect up to

100 drugs. At the scene of a crime, or during airport security checks, the scanner can tell whether a substance is heroin, crack cocaine, amphetamine, or plain chalk. Security experts can programme scanners to detect different sorts of explosives such as RDX or nitroglycerine.

For decades, Raman's discovery could not be converted into easily usable or affordable tools. Inhis time, equipment for lasers and spectrum separation and scanning were primitive, bulky and



costly. Only in the 1980s did laser technology progress to the point where it was compact andeconomic. This new technology was most popularly established in the CD player: a laser couldscan a disc to play music.

Scientists in many fields, including space and telecom, began to research applications for theRaman Effect. Some found ways to enhance the Raman Effect by adding surface metals, makingthe effect easier to detect. This led ultimately to the invention of scanners that could detect traceelements of less than one part per billion. Such scanners can identify minute quantities of

bacteria, chemical pollutants, or explosive elements.

A recent article in The Atlantic, a US monthly, says that Raman scanners are gradually becoming big business. It cites officials at Delta Nu, a manufacturer of Raman scanners, as saying that

scanners are already a $150 million business, and growing fast. The company's scannerscurrently cost $15,000 each, but it hopes to cut the cost to just $5,000 in the next five to tenyears.

Researchers at UCLA and Intel have incorporated the Raman Effect on silicon. Because of itscrystalline structure, the Raman Effect is 10,000 times stronger in silicon than glass. Researchersat JPL and Caltech have found other ways to increase laser efficiency. This has driven down sizeand costs.

Researchers at Stanford University are experimenting with Raman scanners to diagnose cancersin various organs. River Diagnostics in Rotterdam is marketing a bacteria analyzer that hospitalscan use to instantly detect deadly pathogens. One day, Raman scanners may make blood testsobsolete: a scan may suffice to tell you the content of glucose, cholesterol, uric acid and other elements in your blood.

Scientists aim ultimately to create a database of Raman patterns of every substance for easy

identification. This is similar to Nandan Nilekani creating a national database for fingerprints andirises to identify every Indian. Databases have already been created for narcotics, pollutants andexplosives, which is why scanners have already become practical tools. Every time they areused to catch a drug smuggler or terrorist, or to detect a cancer or pollutant, we can give thanksto CV Raman. School teachers can now teach students why exactly the Raman Effect is soimportant: it fingerprints the universe.



Raman scatteringRaman scattering or the Raman effect (pronounced / r m n/) is the inelastic scattering of a

photon. Discovered by Sir Chandrasekhara Venkata Raman in liquids and by Grigory Landsberg

and Leonid Mandelstam in crystals.

When light is scattered from an atom or molecule, most photons are elastically scattered(Rayleigh scattering), such that the scattered photons have the same energy (frequency) andwavelength as the incident photons. However, a small fraction of the scattered light(approximately 1 in 10 million photons) is scattered by an excitation, with the scattered photonshaving a frequency different from, and usually lower than, the frequency of the incident photons.In a gas, Raman scattering can occur with a change in vibrational, rotational or electronic energyof a molecule (see energy level). Chemists are concerned primarily with the vibrational Raman

effect.



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 which ultimately led to hisdiscovery (on 28 February 1928) of the radiation effect which bears his name. The Raman effectwas first reported by C. V. Raman and K. S. Krishnan, and independently by Grigory Landsbergand Leonid Mandelstam, in 1928. Raman received the Nobel Prize in 1930 for his work on the

scattering of light. In 1998 the Raman Effect was designated an ACS National HistoricalChemical Landmark in recognition of its significance as a tool for analyzing the composition of liquids, gases, and solids.

scattering Stokes and anti-Stokes

The different possibilities of visual light scattering: Rayleigh scattering (no Raman effect), Stokesscattering (molecule absorbs energy) and anti-Stokes scattering (molecule loses energy)

There are two types of Raman scattering, Stokes scattering and anti-Stokes scattering.

The interaction of light with matter in a linear regime allows the absorption or simultaneousemission of light precisely matching the difference in energy levels of the interacting electrons.

The Raman effect corresponds, in perturbation theory, to the absorption and subsequent emissionof a photon via an intermediate electron state, having a virtual energy level (see also: Feynmandiagram). There are three possibilities:



y no energy exchange between the incident photons and the molecules (and hence no Ramaneffect)

y energy exchanges occur between the incident photons and the molecules. The energydifferences are equal to the differences of the vibrational and rotational energy-levels of themolecule. In crystals only specific phonons are allowed (solutions of the wave equations whichdo not cancel themselves) by the periodic structure, so Raman scattering can only appear atcertain frequencies. In amorphous materials like glasses, more phonons are allowed and therebythe discrete spectral lines become broad.

y molecule absorbs energy: Stokes scattering. The resulting photon of lower energygenerates a Stokes line on the red side of the incident spectrum.

y molecule loses energy: anti-Stokes scattering. Incident photons are shifted to the blueside of the spectrum, thus generating an anti-Stokes line.

These differences in energy are measured by subtracting the energy of the mono-energetic laser light from the energy of the scattered photons. The absolute value, however, doesn't depend onthe process (Stokes or anti-Stokes scattering), because only the energy of the different vibrationallevels is of importance. Therefore, the Raman spectrum is symmetric relative to the Rayleigh

band. In addition, the intensities of the Raman bands are only dependent on the number of molecules occupying the different vibrational states, when the process began. If the sample is inthermal equilibrium, the relative numbers of molecules in states of different energy will be given

by the Boltzmann distribution:

where:

N0 : number of atoms in the lower vibrational state

N1: number of atoms in the higher vibrational state

g 0 : degeneracy of the lower vibrational state (numberof orbitals of the same energy)

g1: degeneracy of the higher vibrational state

E v : energy difference between these two vibrational

states

k : Boltzmann constant

T : thermodynamic (absolute) temperature

Thus lower energy states will have more molecules in them than will higher (excited) energystates. Therefore, the Stokes spectrum will be more intense than the anti-Stokes spectrum.



D istinction with fluorescence

The Raman effect differs from the process of fluorescence. For the latter, the incident light iscompletely absorbed and the system is transferred to an excited state from which it can go tovarious lower states only after a certain resonance lifetime. The result of both processes is

essentially the same: A photon with the 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 differenceis that the Raman effect can take place for any frequency of the incident light. In contrast to thefluorescence effect, the Raman effect is therefore not a resonant effect. In practice, this meansthat a fluorescence peak is anchored at a specific excitation frequency, whereas a Raman peak maintains a constant separation from the excitation frequency. Another, related distinction is thatRaman scattering is a coherent process, whereas fluorescence is not This means that themeasured intensity is the square of a coherent sum of scattering amplitudes. In practice, thismeans that different paths to the excitation of the same mode may interfere, leading to Fanoeffects: asymmetries in the shape of the scattering peaks.

Selection rules

The distortion of a molecule in an electric field, and therefore the vibrational Raman crosssection, is determined by its polarizability .

A Raman transition from one state to another, and therefore a Raman shift, can be activatedoptically only in the presence of non-zero polarizability derivative with respect to the normalcoordinate (that is, the vibration or rotation):

Raman-active vibrations/rotations can be identified by using almost any textbook that treatsquantum mechanics or group theory for chemistry. Then, Raman-active modes can be found for molecules or crystals that show symmetry by using the appropriate character table for thatsymmetry group.



Classical Raman Physics

raman effect fingerprinting of universe

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