Sir Chandrasekhara Venkata Raman . November 7, 1888 - November 21, 1970 . Won the Nobel prize in 1930 for Physics . Discovered the “Raman effect” . Besides discovering the Raman effect he studied extensively in X-ray Diffractions, Acoustics, Optics, Dielectrics and Colloidal solutions.
When a monochromatic radiation of frequency ʋ is passedthrough a non absorbing medium,it is found that most of it istransmitted without any change, and some of it is scattered. Ifthe scattered energy is analyzed by means of aspectrometer, the bulk of the energy is found at the frequencyof the incident beam ʋ˳ but a small portion of the scatteredenergy will befound at frequencies ʋ =ʋ˳ . The scattering of radiationwith change of frequency is called Raman scattering.
In Raman spectroscopy, by varying the frequency ofthe radiation, a spectrum can be produced, showing theintensity of the exiting radiation for each frequency.This spectrum will show which frequencies ofradiation have been absorbed by the molecule to raiseit to higher vibrational energy states.
When Light hits a sample, It is Excited, and isforced to vibrate and move. It is these vibrationswhich we are measuring.
Atoms are at a certain energy level at any given time. As a laser light hits the atom, it is excited and reaches a higher level of energy, and then is brought back down. If an atom is at a given energy level, it can be excited then fall below the original level. Anti-stokes spectrum are mirror spectrums of Stokes Raman Spectrums
Energy Scheme for Photon Scattering Virtual State h 0 h 0+h h h h mEnergy 0 0 0 h m E0+h m E0 IR Rayleigh Stokes Anti-Stokes Absorption Scattering Scattering Scattering (elastic) Raman (inelastic) The Raman effect comprises a very small fraction, about 1 in 107 of the incident photons.
Raman SpectrumA Raman spectrum is a plot of the intensity of Ramanscattered radiation as a function of its frequencydifference from the incident radiation (usually in unitsof wavenumbers, cm-1). This difference is called theRaman shift.
Days before Laser..Commonly used sources were 435.8nm and 253.6nmemission lines of mercury vapourDisadvantages the source is an extended one and the brightness available per unit area is very small the relatively high frequency of mercury radiation often causes the sample to fluorescence as colored samples absorb in this high frequency region, it is not possible to record their spectra
With the discovery of lasers …Advantages excellent monochromaticity good beam focusing capabilities and small line widths the second order Raman spectra can be recorded the broadening due to Doppler effect can be minimized
Lasers using in Raman spectroscopyLaser sources for Raman spectroscopy include laserdiodes, diode-pumped lasers and ion lasers.The Innova 300C and 70C series of small-frame argon orkrypton ion lasers are also well suited for Ramanexperiments in the visible region of the spectrum. Innova 70C Spectrum is a mixed gas lasers that can generatea number of laser lines from the UV to the near IR
Laser wavelengths ranging from ultra-violet throughvisible to near infra-red can be used for Ramanspectroscopy.Typical examples include,Ultra-violet: 244 nm, 257 nm, 325 nm, 364 nmVisible: 457 nm, 473 nm, 488 nm, 514 nm, 532nm, 633 nm, 660 nmNear infra-red: 785 nm, 830 nm, 980 nm, 1064 nm
The Invictus 785-nm NIR laser is the excitation laserof choice for the majority of Raman spectroscopyapplications from pharmaceutical to polymers.The Invictus 830-nm NIR laser has been developedfor biomedical applications of Raman spectroscopywhere sample absorption characteristics require longerexcitation wavelengths and reduced spectral range.The Invictus 532-nm VIS laser is used for specificclasses of Raman spectroscopy including gas phasemeasurements.
The choice of laser wavelength has animportant impact on experimental capabilities:SensitivitySpatial resolutionOptimisation of resulting based on samplebehaviour.
Laser filters using in Raman spectroscopyOptical filtersEdgeHolographic notch
Gratings using in Raman spectroscopyTypical gratings used for Raman vary from perhaps 300gr/mm(low resolution) through to 1800gr/mm (high resolution) – morespecialised gratings (including 2400gr/mm and 3600gr/mm) arealso available, but have certain limitations, and should not beconsidered general purpose.Raman spectrometers typically use holographic gratings, whichnormally have much less manufacturing defects in theirstructure than ruled gratings. Stray light produced byholographic gratings is about an order of magnitude less intensethan from the ruled gratings of the same groove density.
Detectors used in Raman spectroscopyCharge Coupled Device (CCD) detector is the “camera” usedto detect the Raman spectrum. A CCD detector is a twodimensional array of very low noise, silicon detectors. Typical CCD chip.
Applications of Raman spectroscopyRaman spectroscopy is commonly used in chemistry, since vibrationalinformation is specific to the chemical bonds and symmetry ofmolecules. Therefore, it provides a fingerprint by which the moleculecan be identified.In solid-state physics, spontaneous Raman spectroscopy is usedto, characterize materials, measure temperature, and find thecrystallographic orientation of a sample.Raman spectroscopy can be used to observe other low frequencyexcitations of the solid, such as plasmons, magnons, andsuperconducting gap excitations
Spatially-offset Raman spectroscopy (SORS), which is lesssensitive to surface layers than conventional Raman, can be used todiscover counterfeit drugs without opening their packaging, and fornon-invasive monitoring of biological tissue Raman spectroscopy can be used to investigate the chemicalcomposition of historical documents such as and contribute toknowledge of the social and economic conditions at the time thedocuments were produced. Raman spectroscopy is being investigated as a means to detectexplosives for airport security.
Raman spectroscopy can be used as a techniquefor identification of seafloor hydrothermal andcold seep mineralsUsed to discriminate between healthy andunhealthy tissues, or to determine the degree ofprogress of a certain disease.Used in medicine , aiming to the development ofnew therapeutic drugs and in the diagnosis ofarteriosclerosis and cancer.
References Colin N. Banwell, Elaine M.McCash, 1994.Fundamentals of Spectroscopy, Tata McGraw-HillPublishing Company Limited, New Delhi, 308p. B B Laud, 1991.Lasers and non linearoptics, New age International(P) Limited, NewDelhi,261p. H S Randhawa, 2003. Modern MolecularSpectroscopy, Macmillan India LTD, NewDelhi,584p.