Raman SpectroscopyRaman effect is a 2-photon scattering process These processes are inelastic scattering: Stokes scattering: energy lost by photon: ÷ -— - (( -— - )) ÷ Photon in Photon out No vibration Vibration Anti-Stokes scattering: energy gained by photon: ÷(( -— - )) -— - ÷ Photon in Photon out Vibration No vibration But dominant process is elastic scattering: Rayleigh scattering ÷ -— - -— - ÷ Photon in Photon out No vibration No vibration If incident photon energy E; vibration energy v, then in terms of energy, photon out has energy: E-v Stokes scattering E+v anti-Stokes scattering E Rayleigh scattering Representation in terms of energy levels: Arrow up = laser photon in; Arrow down = Raman scattering out Explanation of Raman effect: click under scattering http://gepasi.dbs.aber.ac.uk/roy/ftir/emspec.htm Here is another explanation, and a Raman spectrometer http://www- personal.umich.edu/~jshaver/virtual/labeled/desc.html Or http://www- personal.umich.edu:80/~jshaver/virtual/explain.html Typical Raman spectrum Plot of signal intensity vs Raman shift (Raman shift, in cm -1 = energy of photon in-energy of photon out) -400 -300 -200 -100 0 100 200 300 400 0 10 R e l a t i v e i n t e n s i t y Raman shift (cm -1 ) Stokes Rayleigh anti-Stokes - 2 7 7 - 1 1 2 2 2 6 2 7 9 1 1 4 Cs 2 NaBiCl 6 -Raman shows 3 vibrations of octahedral BiCl 6 3- Selection rule for Raman spectrum Vibration is active if it has a change in polarizability, o. Polarizability is the ease of distortion of a bond. For Raman-active vibrations, the incident radiation does not cause a change in the dipole moment of the molecule, but instead a change in polarizability. In starting the vibration going, the electric field of the radiation at time t, E, induces a separation of charge (i.e. between the nuclear protons and the bonding electrons). This is called the induced dipole moment, P. (Don’t confuse it with the molecule’s dipole moment, or change in dipole moment, because this is often zero). P = oE Example: There are 4 normal modes of CO 2 . Only v 1 is Raman active µ is dipole moment; o is polarizability; Q is vibration coordinate, The slopes are measured at Q = 0 (I.e. at the equilibrium position). Change in dipole moment, µ, and polarizability, o, during CO 2 vibrations v 1 v 3 Uses of Raman Spectroscopy Raman spectroscopy has become more widely used since the advent of FT-Raman systems and remote optical fibre sampling. Previous difficulties with laser safety, stability and precision have largely been overcome. Basically, Raman spectroscopy is complementary to IR spectroscopy, but the sampling is more convenient, since glass containers may be used and solids do not have to be mulled or pressed into discs. Applications of Raman spectroscopy Qualitative tool for identifying molecules from their vibrations, especially in conjunction with infrared spectrometry. Quantitative Raman measurements a) not sensitive since Raman scattering is weak. But resonance Raman spectra offer higher sensitivity, e.g. fabric dyes studied at 30-50 ppb. b) beset by difficulties in measuring relative intensities of bands from different samples, due to sample alignment, collection efficiency, laser power. Overcome by using internal standard. Raman and fluorescence spectra The diagram shows some of the energy levels of the uranyl ion. UO 2 2+ 20000 cm -1 | Energy ___________ 800 cm -1 ___________ 0 cm -1 The vibrational level at 800 cm -1 is the totally symmetric stretch. The electronic levels are fairly continuous above 20000 cm -1 . What happens if you excite with a laser at (a) 15000 cm -1 ? (b) 21000 cm -1 ? (c) 22000 cm -1 ? Raman vs IR spectroscopy RAMAN IR Sample preparation usually simpler Liquid/ Solid samples must be free from dust Biological materials usually fluoresce, masking scattering Spectral measurements on vibrations Halide optics must be used- made in the visible region-glass cells expensive, easily broken, may be used water soluble Depolarization studies are easily made IR spectrometers not usually (laser radiation almost totally linearly equipped with polarizers polarized) More about polarizability o, polarizability of molecule, related to mobility of electrons (under applied radiation field in our present case). For atoms, same distortion is obtained for field in any direction. Polarizability is Isotropic For many molecules, polarizability depends on direction of applied field, e.g. H—H easier to distort along bond than ± bond. Polarizability is anisotropic Variation of o with direction is described by polarizability tensor. Calculation of Stokes and anti-Stokes intensity ratio The Raman spectrum was taken at 300 K using 1064 nm Nd-YAG radiation. Check the intensity ratio of the v 1 features at ±278.5 cm -1. What can you say about the intensity ratio of the band at 112 cm -1 ? Instrumentation Dispersive Raman instruments Laser ÷Sample ÷Double or triple monochromator + signal processing and output ÷ Photomultiplier tube The monochromators are required to separate the weak Raman signal from the intense, nearby Rayleigh scattering. Typical lasers are Ar + (e.g. green line, 514.5 nm) or Kr + (e.g. yellow line, 530.9 nm). New Raman systems http://www.s-and-i.de/ Fourier transform Raman spectrometer The Raman instrument can be on the same bench as the FTIR. Often, a YAG:Nd 3+ laser (1064 nm) is used to excite the sample, so that the excitation energy is lower than the absorption band energies of organic systems. Fluorescence is then minimized. Instruments may be combined with a microscope, or optical fibre, so that scanning over a few (microns) 2 of surface area, and Raman mapping is easily performed. Sampling techniques for Raman spectroscopy If the sample is colourless, it does not absorb a visible laser Raman spectroscopy is applicable to solids, liquids or gases. Gases: use gas cell Liquids and solids can be sealed in a glass capillary: If the compound is colored, it can absorb the laser, get hot and decompose. Some techniques are: • Reduce the laser power (defocus) and/or change wavelength; • Dilute the sample into a KBr pellet; • Cool the sample • Rotate or oscillate the laser beam on the sample Number of bands in a Raman spectrum As for an IR spectrum, the number of bands in the Raman spectrum for an N-atom non-linear molecule is seldom 3N-6, because: polarizability change is zero or small for some vibrations; bands overlap; combination or overtone bands are present; Fermi resonances occur; some vibrations are highly degenerate; etc… High resolution Raman spectra can show splittings due to isotopic mass effects, for example: the v 1 Raman band of CCl 4 (corresponding tp the totally symmetric stretching vibration) is split into 5 components. 461.5 cm -1 is due to 35 Cl 4 C 458.4 cm -1 is due to 35 Cl 3 37 ClC 455.1 cm -1 is due to 35 Cl 2 37 Cl 2 C What are the two question marks? Why are these bands weak? Depolarization ratio of a vibrational mode in the Raman spectrum may give information about the symmetry of a vibration. µ p = depolarization ratio for polarized light = I y /I z = I ± /I || This is different from the depolarization ratio for unpolarized light, see Infrared and Raman Spectra…Part A., K. Nakamoto 5th Ed. Wiley 1997. Pp. 97-101. 0s µ p <0.75; Raman line is polarized (p). Vibration is totally symmetric µ p = 0.75. Raman line is depolarized (dp). Vibration is not totally symmetric. CCl 4 Special types of Raman spectroscopy (see Hollas, Modern Spectroscopy, Wiley) Resonance Raman (RR) scattering When the laser excitation frequency is near (or coincident) with an electronic absorption band, intensity enhancement can occur by a factor of 10 2 -10 6 , compared with normal Raman scattering. Electronic transitions are often localized in one part of a molecule, so that RR provides information about vibrations of the chromophore, especially those exhibiting a large change in geometry between the two electronic states. RR is used in analytical chemistry to achieve detection limits 10 -6 - 10 -8 M. Surface enhanced Raman spectroscopy (SERS): Raman scattering is enhanced (typically by 10 3 -10 6 times) when the analyte is adsorbed on colloidal metallic surfaces, e.g. on colloidal Ag prepared by reduction of Ag + with citrate, in particle size range 25-500 nm. Stimulated Raman scattering (SRS) The Raman scattering from a laser is observed in the forward direction from the sample (i.e. in the same direction, or at a small angle to the incident laser radiation). Vibrational progressions are observed for certain modes. Coherent anti-Stokes Raman spectroscopy (CARS) Radiation from two lasers is incident on the sample, and the intensity of the outgoing wave energy gives information about the vibrational modes of the sample. Hyper-Raman spectroscopy Very weak scattering at twice the laser frequency, 2v 0 , is called Hyper-Rayleigh scattering. Similarly, Stokes and anti-Stokes hyper- Raman scattering occur at 2v 0 ±v vib , where v vib is a vibration frequency. The selection rules differ from those of Raman scattering. Electronic Raman scattering Raman scattering can occur from electronic states, as well as from vibrations. At room temperature the bands are very broad, and merge into the background. At low temperatures (<80 K), the bands sharpen, and give information about the energies of electronic states of the molecule. 5000 6000 7000 0 800 1600 Part of electronic Raman spectrum of PrCl 6 3- at 10 K I n t e n s i t y Raman shift (cm -1 ) Books on Raman spectroscopy There are many in CityU library, such as: FT Raman spectroscopy, P. Hendra et al., Ellis Horwood. Raman and IR spectroscopy in biology and chemistry, J. Twardowski and P. Anzenbacher, Ellis Horwood. Also, there are some good short chapters: Ch 18 in Skoog, Holler, Nieman, Principles of Instrumental Analysis, Saunders. Useful websites for Raman spectrometry Great site covering all aspects http://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.html Introduction to Raman spectroscopy http://www-wilson.ucsd.edu/education/pchem/spectroscopy/sptyperaman.html Analysis of pesticides and pharmaceuticals http://www.aua.gr/georgiou/ From Encyclopaedia Brittanica: Raman http://search.eb.com/bol/search?type=topic&query=Optical,+infrared,+and+Raman+spectroscopy&Dba se=Articles&I3.x=44&I3.y=2 Andor Technology: good site comparing Raman and ir; FT-Raman and applications http://www.andor-tech.com/ then click on Contents, then Raman on-line tour Peter Griffith’s website about FT http://www.ftir.chem.uidaho.edu/ Raman analysis of olive oils (get the English page) by Adrian Shaw http://pcjagg.dbs.aber.ac.uk/index.html Study of molecular structures in supersaturated solutions http://ihome.ust.hk/~keckchan/spectroscopy.html Useful websites for Raman spectrometry Instrument manufacturers Thermo http://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.html Renishaw instruments, do a search at http://www.renishaw.com/client/start/index.asp Bruker instruments http://www.optics.bruker.com/ Nicolet instruments http://www.nicolet.com/ and http://www.nicolet.com/labsys/products/Raman_theory.html Jobin-Yvon instruments website http://www.isa-gs.co.uk/ Scimedia company: about CCD camera detector http://www.scimedia.com/english/camera/index.htm ukap www.anamap.co.uk But dominant process is elastic scattering: Rayleigh scattering — — Photon in No vibration Photon out No vibration If incident photon energy E; vibration energy v, then in terms of energy, photon out has energy: E-v Stokes scattering E+v anti-Stokes scattering E Rayleigh scattering Representation in terms of energy levels: Arrow up = laser photon in. Arrow down = Raman scattering out . html Or http://wwwpersonal.Explanation of Raman effect: click under scattering http://gepasi.ac.edu:80/~jshaver/virtual/explain.aber.umich.html .dbs.umich.htm Here is another explanation. and a Raman spectrometer http://wwwpersonal.uk/roy/ftir/emspec.edu/~jshaver/virtual/labeled/desc. in cm-1 = energy of photon in-energy of photon out) Rayleigh 10 Cs2NaBiCl6-Raman shows 3 vibrations of octahedral BiCl63Stokes Relative intensity 114 anti-Stokes -277 -112 0 -400 -300 -200 -100 0 100 -1 200 226 300 279 400 Raman shift (cm ) .Typical Raman spectrum Plot of signal intensity vs Raman shift (Raman shift. For Raman-active vibrations.Selection rule for Raman spectrum Vibration is active if it has a change in polarizability. E. or change in dipole moment. . This is called the induced dipole moment. P. P = E . induces a separation of charge (i. but instead a change in polarizability. In starting the vibration going. the electric field of the radiation at time t. between the nuclear protons and the bonding electrons). (Don’t confuse it with the molecule’s dipole moment.e. the incident radiation does not cause a change in the dipole moment of the molecule. because this is often zero). Polarizability is the ease of distortion of a bond. Q is vibration coordinate.Example: There are 4 normal modes of CO2. . Only 1 is Raman active is dipole moment.e. . at the equilibrium position). during CO2 vibrations 1 3 . is polarizability. and polarizability. The slopes are measured at Q = 0 (I. Change in dipole moment. . Basically. Previous difficulties with laser safety. since glass containers may be used and solids do not have to be mulled or pressed into discs.Uses of Raman Spectroscopy Raman spectroscopy has become more widely used since the advent of FT-Raman systems and remote optical fibre sampling. Raman spectroscopy is complementary to IR spectroscopy. but the sampling is more convenient. stability and precision have largely been overcome. But resonance Raman spectra offer higher sensitivity. b) beset by difficulties in measuring relative intensities of bands from different samples.g. especially in conjunction with infrared spectrometry. laser power. due to sample alignment. collection efficiency. Overcome by using internal standard. fabric dyes studied at 30-50 ppb. Quantitative Raman measurements a) not sensitive since Raman scattering is weak. e.Applications of Raman spectroscopy Qualitative tool for identifying molecules from their vibrations. . What happens if you excite with a laser at (a) 15000 cm-1? (b) 21000 cm-1? (c) 22000 cm-1? . The electronic levels are fairly continuous above 20000 cm-1.Raman and fluorescence spectra The diagram shows some of the energy levels of the uranyl ion. UO22+ 20000 cm-1 Energy ___________ ___________ 800 cm-1 0 cm-1 The vibrational level at 800 cm-1 is the totally symmetric stretch. Raman vs IR spectroscopy RAMAN Sample preparation usually simpler Liquid/ Solid samples must be free from dust Biological materials usually fluoresce. masking scattering IR Spectral measurements on vibrations made in the visible region-glass cells may be used Depolarization studies are easily made (laser radiation almost totally linearly polarized) Halide optics must be usedexpensive. easily broken. water soluble IR spectrometers not usually equipped with polarizers . . related to mobility of electrons (under applied radiation field in our present case). polarizability of molecule. Polarizability is anisotropic Variation of with direction is described by polarizability tensor. H—H easier to distort along bond than bond. e.g. Polarizability is Isotropic For many molecules. same distortion is obtained for field in any direction. polarizability depends on direction of applied field.More about polarizability . For atoms. . Calculation of Stokes and anti-Stokes intensity ratio The Raman spectrum was taken at 300 K using 1064 nm Nd-YAG radiation. Check the intensity ratio of the 1 features at 278.5 cm-1. What can you say about the intensity ratio of the band at 112 cm-1? . 514. Typical lasers are Ar+ (e. nearby Rayleigh scattering. 530. green line.9 nm).g.g. .5 nm) or Kr+ (e. yellow line.Instrumentation Dispersive Raman instruments Laser Sample Double or triple monochromator signal processing and output Photomultiplier tube The monochromators are required to separate the weak Raman signal from the intense. New Raman systems http://www.de/ .s-and-i. Instruments may be combined with a microscope. and Raman mapping is easily performed. or optical fibre. Often. so that the excitation energy is lower than the absorption band energies of organic systems. a YAG:Nd3+ laser (1064 nm) is used to excite the sample. so that scanning over a few (microns)2 of surface area. Fluorescence is then minimized. .Fourier transform Raman spectrometer The Raman instrument can be on the same bench as the FTIR. liquids or gases. it does not absorb a visible laser Raman spectroscopy is applicable to solids.Sampling techniques for Raman spectroscopy If the sample is colourless. Gases: use gas cell . Some techniques are: • Reduce the laser power (defocus) and/or change wavelength. get hot and decompose.Liquids and solids can be sealed in a glass capillary: If the compound is colored. • Dilute the sample into a KBr pellet. it can absorb the laser. • Cool the sample • Rotate or oscillate the laser beam on the sample . the number of bands in the Raman spectrum for an N-atom non-linear molecule is seldom 3N-6.Number of bands in a Raman spectrum As for an IR spectrum. some vibrations are highly degenerate. because: polarizability change is zero or small for some vibrations. etc… . bands overlap. Fermi resonances occur. combination or overtone bands are present. 4 cm-1 is due to 35Cl337ClC 455.1 cm-1 is due to 35Cl237Cl2C What are the two question marks? Why are these bands weak? . for example: the 1 Raman band of CCl4 (corresponding tp the totally symmetric stretching vibration) is split into 5 components. 461.High resolution Raman spectra can show splittings due to isotopic mass effects.5 cm-1 is due to 35Cl4C 458. Wiley 1997.Depolarization ratio of a vibrational mode in the Raman spectrum may give information about the symmetry of a vibration. p = depolarization ratio for polarized light = Iy/Iz = I/I|| This is different from the depolarization ratio for unpolarized light. CCl4 . Vibration is totally symmetric p = 0.. 0 p <0. see Infrared and Raman Spectra…Part A. Nakamoto 5th Ed. Vibration is not totally symmetric. Raman line is depolarized (dp). 97-101. Raman line is polarized (p).75. K. Pp.75. compared with normal Raman scattering. RR is used in analytical chemistry to achieve detection limits 10-610-8 M. Modern Spectroscopy. especially those exhibiting a large change in geometry between the two electronic states.g. intensity enhancement can occur by a factor of 102-106.Special types of Raman spectroscopy (see Hollas. in particle size range 25-500 nm. on colloidal Ag prepared by reduction of Ag+ with citrate. Wiley) Resonance Raman (RR) scattering When the laser excitation frequency is near (or coincident) with an electronic absorption band. e. . Electronic transitions are often localized in one part of a molecule. Surface enhanced Raman spectroscopy (SERS): Raman scattering is enhanced (typically by 103-106 times) when the analyte is adsorbed on colloidal metallic surfaces. so that RR provides information about vibrations of the chromophore. or at a small angle to the incident laser radiation). Stokes and anti-Stokes hyperRaman scattering occur at 20vib. Coherent anti-Stokes Raman spectroscopy (CARS) Radiation from two lasers is incident on the sample. where vib is a vibration frequency. Hyper-Raman spectroscopy Very weak scattering at twice the laser frequency. and the intensity of the outgoing wave energy gives information about the vibrational modes of the sample. 20. Vibrational progressions are observed for certain modes. Similarly. .Stimulated Raman scattering (SRS) The Raman scattering from a laser is observed in the forward direction from the sample (i. is called Hyper-Rayleigh scattering. in the same direction.e. The selection rules differ from those of Raman scattering. At low temperatures (<80 K). 1600 Part of electronic Raman spectrum of PrCl 6 3- at 10 K Intensity 800 0 5000 6000 7000 -1 Raman shift (cm ) . and give information about the energies of electronic states of the molecule. the bands sharpen. At room temperature the bands are very broad. as well as from vibrations.Electronic Raman scattering Raman scattering can occur from electronic states. and merge into the background. Nieman. Ellis Horwood. Anzenbacher. Also. Principles of Instrumental Analysis. such as: FT Raman spectroscopy. Raman and IR spectroscopy in biology and chemistry. Saunders. Ellis Horwood. Holler. there are some good short chapters: Ch 18 in Skoog. .Books on Raman spectroscopy There are many in CityU library. J. Twardowski and P.. P. Hendra et al. ftir.com/bol/search?type=topic&query=Optical.html Study of molecular structures in supersaturated solutions http://ihome.00.andor-tech. FT-Raman and applications http://www.ucsd.html Introduction to Raman spectroscopy http://www-wilson.chem.com/ then click on Contents.Useful websites for Raman spectrometry Great site covering all aspects http://www.hk/~keckchan/spectroscopy.uk/index.edu/ Raman analysis of olive oils (get the English page) by Adrian Shaw http://pcjagg.gr/georgiou/ From Encyclopaedia Brittanica: Raman http://search. then Raman on-line tour Peter Griffith’s website about FT http://www.y=2 Andor Technology: good site comparing Raman and ir.+and+Raman+spectroscopy&Dba se=Articles&I3.com/Spy/basehtml/SpyH/1.x=44&I3.ac.aber.html .+infrared.html Analysis of pesticides and pharmaceuticals http://www.uidaho.9076.dbs.6-0-0-0-0-home-0-0.ust.edu/education/pchem/spectroscopy/sptyperaman.aua.eb.spectroscopynow. isa-gs.uk .asp Bruker instruments http://www.1181.nicolet.anamap.spectroscopynow.optics.com/ Nicolet instruments http://www.com/Spy/basehtml/SpyH/1.6-1-12-0-0-news_detail-0-3606.co.html Renishaw instruments.co.html Jobin-Yvon instruments website http://www.Useful websites for Raman spectrometry Instrument manufacturers Thermo http://www.scimedia.uk/ Scimedia company: about CCD camera detector http://www.com/client/start/index.nicolet.com/ and http://www.00.bruker.htm ukap www.renishaw.com/labsys/products/Raman_theory. do a search at http://www.com/english/camera/index.