Submitted by: Ms.Bushra Qamar Ms63-10-815
• Raman spectroscopy provides information about molecular vibrations that can be used for sample identification and quantitation. • Raman spectroscopy is a spectroscop spectroscopic ic technique based on inelastic scattering of monochromatic light, usually from a laser source. • Inelastic scattering means that the frequency of photons in monochromatic light changes upon interaction with a sample.
• Photons of the laser light are absorbed by the sample and then reemitted. • Frequency of the reemitted photons is shifted up or down in comparison with original monochromatic frequency, which is called the Raman effect. effect . • This shift provides information about vibrational, rotational and other low frequency transitions in molecules.
• The Raman effect is based on molecular deformations in electric field E determined by molecular polarizability α. • The laser beam can be considered as an oscillating electromagnetic wave with electrical vector E. • Upon interaction with the sample it induces electric dipole moment P = αE which deforms molecules. • Because of periodical deformation, molecules start vibrating with characterist characteristic ic frequency υm
• Amplitude of vibration is called a nuclear displacement. • monochromatic laser light with frequency υ0 excites molecules and transforms them into oscillating dipoles. • Such oscillating dipoles emit light of three different frequencies
1. Elas Elasti tic c Ra Ray yle leig igh h sca scatt tter erin ing. g.:: where a molecule with no Raman-active modes absorbs a photon with the frequency υ0 and resturns back to the same basic vibrational state ,emits light with the same frequency υ0 as an excitation source. 2. “Stokes”. Where a photon with frequency υ0 is absorbed by Raman-active molecule which at the time of interaction is in the basic vibrational state. state . Part of the photon’s energy is transferred to the Raman-active mode with frequency υm and the resulting frequency of scattered light is reduced to υ0 - υm. 3. AntiStokes frequency .A photon with frequency υ0 is absorbed by a Raman-active molecule, which, at the time of interaction, is already in the excited vibrational state. Excessive energy of excited Ramanactive mode is released, molecule returns to the basic vibrational state and the resulting frequency of scattered light goes up to υ0 + υm .
• About 99.999% of all incident photons in spontaneous Raman undergo elastic Rayleigh scattering. • This type of signal is useless for practical purposes of molecular characterization. • Only about 0.001% of the incident light produces inelastic Raman signal with frequencies υ0 ± υm.
• It can be obtained from most molecular Samples – solids, liquids, mixtures, aqueous solutions, gels, slurries, powders, films, etc. and even with some metals. metals.
• Through many containers such as glass bottles, Pyrex®, reaction vessels, plastic containers, blister packs, bags • Raman is usually not destructive, • Temperatures up to 500 ˚C and pressures of up to 3000 PSI can be easily accommodated.
• Plotting the intensity of "shifted" light versus frequency results in a Raman spectrum of the sample. • Generally, Raman spectra are plotted with respect to the laser frequency such that the Rayleigh band lies at 0 cm -1. • On this scale, the band positions will lie at frequencies that correspond to the energy levels of different functional group vibrations
• A Raman system typically consists of four major components: • 1. Excitation source (Laser). • 2. Sample illumination system and light collection optics. • 3. Wavelength selector (Filter or Spectrophotometer). • 4. Detector (Photodiode array, CCD or PMT).
• Before the development of laser, mercury lamps were user to tranmist a single or limited wavelength range of light. • Now Laser beam is used because of following properties: – Monochromatic – small diameters – high focusing radiant flux – linearly polarized
• Focusing nature of laser illuminate the sample • Excitation and the collection from the sample is obtained by using many optical settings such as 90 and 180 scattering • It consists of an achromatic lense system with a collecting lense and a fosuing lense
• The monochromator must disperse the scattered light across a slit for sequential presentation to the detector or across a solid-state diode array. • The standard dispersing device in modern Raman spectrometers is the double monochromator or the dual grating system. • The disadvantage of this system is the transmission loss and, as a consequence, the Diode Array system is now the more popular
•Raman spectrum can be photographed with an ordinary spectrograph. •Basically there are two different ways to detect and record Raman lines. •The easiest way is to gather the scattered light emerging through a glass window at the end of the Raman sample tube. •It is passed through a .prism or grating and then focused on a photographic plate. • The plate is then developed and both the line frequencies and intensities can be measured using external equipments
• Modern spectrometers have photo multiplier tubes are direct measurements and automatic scanning of a spectrum. • The spectrum produced by the monochromator is passed through a slit which allows a narrow wavelength region to pass through which is focused on to a photo multiplier type detector.
• The electron pulsed from the pm tube are averaged over time and the resulting dc current is amplified directly and measured by picometer.
• The electron pulses caused by individual photons reaching the photocathode are measured • A substantial portion of dark signal is electronically discriminated from photon pulses making detector more sensitive. sensitive . • but the maximum signal is limited to a photon count rate at which phonon event do not overlap
• The CCD (charge coupled device )are extensively now used in RS • CCD is a silicon bases array of photosensitive elements , each one of which generates photoelectrons and stores them as a small charge • Major advantage of CCD is low readout noise, high quantum efficieny and sensitivity in wide range
• They can be classified into several categories. • Interference filter – Simple – Variable – wedge shaped – Acoustic – liquid crystal tunable filters
• Prism and grating monochromators and spectrographs
•
•
• •
•
•
Very strong laser pulse with electric field strength > 109 V·cm-1 transforms up to 50% of all laser pulse energy into coherent beam at Stokes frequency υ0 - υm . The Stokes beam is unidirectional with the incident laser beam. Strongest mode υm in the regular Raman spectrum is greatly amplified.. amplified Weaker Raman active modes are eliminated High frequency acts a secondary excitation source generating the second Stokes line with frequency υ0 - 2υm. The second Stokes line generates the third one with the frequency υ0 3υm etc. 4-5 orders of magnitude
• It is non-linear” Raman spectroscopy • Instead of the traditional one laser, two very strong collinear lasers irradiate a sample. sample. • Frequency of the first laser is constant, while the frequency of the second one can be tuned in a way that the frequency difference between the two lasers equals exactly the frequency of some Ramanactive mode of interest
•
•
•
If the frequency difference between two lasers υ1 - υ2 is equal to the frequency um of a Raman-active rotational, vibrational or any other mode then a strong light of frequency υ1 + υm is emitted. To obtain strong Raman signal the second laser frequency should be tuned in a way that υ2 = υ1 – υ Then the frequency of strong scattered light will be 2υ1 - υ2 = 2υ1- (υ1 - υm) = υ1 + υm, which is higher then the excitation frequency u1 and therefore considered to be Anti-Stokes frequency.
• Some colored substance may absorb laser beam energy and generate strong fluorescence which contaminates Raman spectrum • But that under certain conditions some types of colored molecules can produce strong Raman scattering instead of fluorescence. • The Resonance Raman effect takes place when the excitation laser frequency is chosen in a way that it crosses frequencies of electronic excited states and resonates with them
• Intensity of Raman enhanced 3-5 orders • Chromophoric group(having the has the highest level of light absorption), responsible for the molecule’s coloration, col oration, experiences the highest level of enhancement. • The highest intensity of Resonance Raman signal is obtained when laser frequency equals to the first or the second electronic excited state
• Raman signal from molecules adsorbed on certain metal surfaces can be 5-6 orders of magnitude stronger then the Raman signal from the same molecules in bulk volume.
• As intensity of Raman signal isproportional to the square of electric dipole moment P = αE, there are two possible reasons – - the enhancement of polarizability α – the enhancement of electrical field E.
• One disadvantage of SERS is the difficulty of spectra
interpretation. • The signal enhancement is so dramatic that Raman bands that are very weak and unnoticeable in spontaneous Raman spectra can appear in SERS • Some trace contaminants can also contribute additional peaks
• Angle Resolved Raman Spectroscopy - Not only are standard Raman results recorded but also the angle with respect to the incident laser. If the orientation of the sample is known then detailed information about the phonon dispersion relation can also be gleamed from a single test • Transmission Raman - Allows probing of a significant bulk of a turbid material, such as powders, capsules, living tissue, etc • There are also medical diagnostic applications
• Optical Tweezers Raman Spectroscopy (OTRS) - Used to study individual particles, and even biochemical processes in single cells trapped by optical tweezers. • Hyper Raman - A non-linear effect in which the vibrational modes interact with the second harmonic of the excitation beam. This requires very high power, but allows the observation of vibrational modes that are normally "silent". It frequently relies on SERS-type enhancement to boost the sensitivity
• Tip-Enhanced Raman Spectroscopy (TERS) - Uses a metallic (usually silver-/gold-coated AFM or STM) tip to enhance the Raman signals of molecules situated in its vicinity. With resolution apex (20-30 nm). TERS has been shown to have sensitivity down to the single molecule. • Spontaneous Raman Spectroscopy (SRS) - Used to study the temperature dependence of the Raman spectra of molecules. •
Spatially-offset Raman spectroscopy (SORS), which is less sensitive to surface layers than conventional Raman, can be used to discover counterfeit drugs without opening their packaging, and for non-invasive monitoring of biological tissue
• Characterize materials, measure temperature, and find the crystallographic orientation of a sample • Gives information on the crystal cryst al orientation • To detect explosives for airport security • Used in medicine for real-time monitoring of anaesthetic and respiratory gas mixtures during surgery
Raman offers several advantages over mid-IR and near-IR spectroscopy, including
:
• Little or no sample preparation is required • Water is a weak scatterer - no special accessories are needed for measuring aqueous solutions • Inexpensive glass sample holders are ideal in most cases • The standard spectral range reaches well below 400 cm-1, making the technique ideal for both organic and inorganic species
Raman spectra are "cleaner than mid-IR spectr spectr
• - Raman bands are narrower, and overtone and combination bands are generally weak
• Fiber optics (up to 100's of meters in length) can be used for remote analyses • Since fundamental modes are measured, Raman bands can be easily related to chemical structure • Raman spectroscopy can be used to measure bands of symmetric linkages which are weak in an infrared spectrum (e.g. -S-S-, -C-S-, -C=C-) - C=C-)
• Princeton Instruments • PerkinElmer instruments • HORIBA • bayspec
•
•
•
Laurence D. Barron discovered during hisdoctoral work in Oxford with Peter Atkins of a new optical process involving interference between light waves scattered via the polarizability and optical activity tensors of a chiral molecule. This leads to a dependence of the scattered intensity on the degree of circular polarization of the incident beam and to a small circularly polarized component in the scattered beam. He subsequently developed the definitive theory of ROA with David Buckingham in Cambridge in 1971, and made the first observations (in the form of a small difference in the intensity of Raman scattering in right- and left-circularly polarized incident light) with David Buckingham and Martin Bogaard 1973.
Raman optical activity
Raman optical activity • Measures vibrational vibrational optical activity by means of a small difference in the intensity of Raman scattering from chiral molecules in right- and left-circularly polarized incident light or, equivalently, a small circularly polarized component component in the scattered light
Intr Introd oduc ucto tory ry Ra Rama man n spe spect ctro rosc scop opy, y, By John R. Ferraro, Kazuo Nakamoto, Chris W. ii. Analyt nalytica icall applic applicati ations ons of Ra Raman man spectr spectrosc oscopy opy,, By Michael J. PelletierBrownhttp://www PelletierBrownhttp://www.analyticalspe .analyticalspectroscopy.n ctroscopy.net/spectropae et/spectropaedia.htm#Diode dia.htm#Diode iii.. Molec iii Molecula ularr spect spectro rosc scop opy, y, By Chemical Society (Great Britain) iv. Characte Characterizat rization ion in Compoun Compound d Semicondu Semiconductor ctor Processing Processing,, By: Gary McGuire;; Yale E. Strasser McGuire v. http://chemwiki.ucdavis.edu vi. htt http:/ p://w /www ww.an .analy alytic ticals alspe pectr ctros oscop copy. y.net net vii. vii. htt http:/ p://w /www ww.ba .baysp yspec. ec.com com viii. http http://w ://www ww.inph .inphotoni otonics.c cs.com om ix. ix. www ww.p .pri rin ncet eton on instruments .com instruments .com x. www.horiba.com i.
