Download to read offline
![Collecting Optics – Overview on
Collecting Optics – Overview on
common objectives
common objectives
Objective N.A.
Working distance
[mm]
x100 0.90 0.21
x50 0.75 0.38
x10 0.25 10.6
x100 LWD 0.80 3.4
x50 LWD 0.50 10.6
CASR](https://image.slidesharecdn.com/raman2-250226153144-4fd8f1aa/85/Raman-spectroscopy-for-the-college-students-39-320.jpg)
![200 400 600 800 1000 1200 1400 1600 1800
244 - 269 nm (25 nm)
325 - 371 nm (46 nm)
457 - 553 nm (96 nm)
488 - 599 nm (111 nm)
514 - 639 nm (125 nm)
532 - 667 nm (135 nm)
633 - 833 nm (200 nm)
785 - 1119 nm (334 nm)
830 - 1210 nm (380 nm)
1064 - 1768 nm (704 nm)
Horizontal lines indicate a relative Raman Shift of 3800 cm
-1
Wavelength [nm]
Dispersion as a function of excitation
Dispersion as a function of excitation
wavelength
wavelength
Long wavelength
Short wavelength
Same focal length
Same grating
CCD
Detector
CCD
Detector
CASR](https://image.slidesharecdn.com/raman2-250226153144-4fd8f1aa/85/Raman-spectroscopy-for-the-college-students-46-320.jpg)
![Spatial resolution: penetration depth
Spatial resolution: penetration depth
0
500
1000
1500
2000
2500
3000
244 nm 325 nm 457 nm 488 nm 514 nm 633 nm
Wavelength [nm]
D
e
p
t
h
p
e
n
e
t
r
a
t
io
n
[
n
m
]
General: The larger the excitation wavelength,
the deeper the penetration.
The exact values depend on material.
Penetration depth in Silicon
0
2
4
6
8
10
12
244 nm 325 nm
CASR](https://image.slidesharecdn.com/raman2-250226153144-4fd8f1aa/85/Raman-spectroscopy-for-the-college-students-52-320.jpg)
![400 450 500 550
Intensity
[
a
.u.]
Raman shift (cm-1)
785 nm
Spatial resolution: penetration depth
Spatial resolution: penetration depth
Uniform layer of SiGe
Gradient SiGe layer
Pure Si substrate
Strained silicon layer
400 450 500 550
Intensity
[
a
.u.]
Raman shift (cm-1)
325 nm 488 nm 785 nm
The higher the excitation wavelength, the deeper the penetration.
488 nm
400 450 500 550
Intensity
[
a
.u.]
Raman shift (cm-1)
325 nm
Strained Si
of top layer
Si of silicon
substrate
Si of SiGe
layer
~nm
~nm
~µm
CASR](https://image.slidesharecdn.com/raman2-250226153144-4fd8f1aa/85/Raman-spectroscopy-for-the-college-students-53-320.jpg)
More Related Content
Similar to Raman spectroscopy for the college students
Raman spectroscopy for the college students
- 1. Introduction to Basicsof Introduction to Basics of Raman Spectroscopy Raman Spectroscopy Chandrabhas Narayana Chandrabhas Narayana Chemistry and Physics of Materials Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India cbhas@jncasr.ac.in http://www.jncasr.ac.in/cbhas Lecture at Lecture at MASTANI Summer School, IISER, Pune, June 30, 2014 to July 12, 2014 MASTANI Summer School, IISER, Pune, June 30, 2014 to July 12, 2014 July 11, 2014 July 11, 2014
- 2. What happens whenlight falls on a material? Transmission Reflection Absorption Luminescence Elastic Scattering Inelastic Scattering
- 3. Raman Spectroscopy Raman Spectroscopy 1in 10 1 in 107 7 photons is scattered inelastically photons is scattered inelastically Infrared (absorption) Raman (scattering) v” = 0 v” = 1 virtual state Excitation Scattered Rotational Raman Rotational Raman Vibrational Raman Vibrational Raman Electronic Raman Electronic Raman
- 4. Raman visible throughunaided eye
- 5. Raman, Fluorescence andIR Raman, Fluorescence and IR Scattering Absorption and emission Absorption
- 6. Concept of normalmodes in a molecule • There are 3N possible movements in a molecule made of N atoms, each of which moving in one of three directions, x, y and z. – There are three transitional movements: all atoms in the molecule moving in x, y or z direction at the same time. – There are three rotational movements around x, y or z-axis • Linear molecules are exceptions because two axes that are perpendicular to the molecular axis are identical. – The rest of movements are vibrational movements • For linear molecules, 3N – 5 movements • For non-linear molecules, 3N – 6 movements – All vibrational movements of the sample can be described as linear combinations of vibrational normal modes.
- 7. Vibrations in Molecules Vibrationsin Molecules HCl HCl HF HF H H2 2O O NH NH3 3 SF SF6 6 Sym. Stretching Sym. Stretching Asym. Stretching Asym. Stretching Sym. Bending Sym. Bending Asym. Bending Asym. Bending 1 = 3835 cm-1 2 = 1648 cm-1 3 = 3939 cm-1 = 2991 cm-1 = 4139 cm-1 1 = 3505.7 cm-1 2 = 1022 cm-1 3 = 3573.1 cm-1 1 = 774.55 cm-1 4 = 523.56 cm-1 3 = 947.98 cm-1 5 = 643.35 cm-1 6 = 348.08 cm-1 2 = 615.02 cm-1 4 = 1689.7 cm-1 8086 cm-1 = 1 eV
- 8. Vibrational Spectroscopy Vibrational Spectroscopy e= equilibrium distance between A and B re For a diatomic molecule (A-B), the bond between the two atoms can be approximated by a spring that restores the distance between A and B to its equilibrium value. The bond can be assigned a force constant, k (in Nm-1 ; the stronger the bond, the larger k) and the relationship between the frequency of the vibration, , is given by the relationship: DAB rAB 0 DAB = energy required to dissociate into A and B atoms k 2 c k or, more typically where , c is the speed of light, is the frequency in “wave numbers” (cm-1 ) and is the reduced mass (in amu) of A and B given by the equation: m m m m A B A B
- 9. Vibrational Spectroscopy Molecule (cm-1 ) k (N/m) (amu) HF 3962 878 19/20 HCl 2886 477 35/36 or 37/38 HBr 2558 390 79/80 or 81/82 HI 2230 290 127/128 Cl2 557 320 17.5 Br2 321 246 39.5 CO 2143 1855 6.9 NO 1876 1548 7.5 N2 2331 2240 7 2 c k can be rearranged to solve for k (in N/m): k 5 89 10 5 2 . For a vibration to be active (observable) in an infrared (IR) spectrum, the vibration must change the dipole moment of the molecule. (the vibrations for Cl2, Br2, and N2 will not be observed in an IR experiment) For a vibration to be active in a Raman spectrum, the vibration must change the polarizability of the molecule.
- 10. Classical Picture ofRaman Classical Picture of Raman Stokes Raman Anti-Stokes Raman Induced Polarization Polarizability
- 11. Mutually exclusive principle Mutuallyexclusive principle
- 12. max 0 max max0 max max 0 ( ) cos2 1 cos 2 ( ) 2 1 cos 2 ( ) 2 equil z zz zz vib zz vib t E t d r E t dr d r E t dr www.andor.com www.andor.com Selection rule: Selection rule: v = ±1 v = ±1 Overtones: Overtones: v = ±2, ±3, … v = ±2, ±3, … Raman Scattering Raman Scattering Must also have a change in polarizability Must also have a change in polarizability Classical Description does not suggest any difference Classical Description does not suggest any difference between Stokes and Anti-Stokes intensities between Stokes and Anti-Stokes intensities 1 0 vib h kT N e N
- 13. Calculate the ratioof Anti-Stokes to Stokes scattering Calculate the ratio of Anti-Stokes to Stokes scattering intensity when T = 300 K and the vibrational frequency is intensity when T = 300 K and the vibrational frequency is 1440 cm 1440 cm-1 -1 . . Are you getting the concept? Are you getting the concept? h = 6.63 x 10 h = 6.63 x 10-34 -34 Js Js k = 1.38 x 10 k = 1.38 x 10-23 -23 J/K J/K 1 0 vib h kT N e N ~ 0.5
- 14. Energy diagram and Energydiagram and Quantum picture Quantum picture Vibrational states Electronic states Virtual states g ex photon <eg,p2|Her|p2,eb> <eb,p2|Hep|p1,ea> <ea,p1|Her|p1,eg> |Es-Eb|x|Ei-Ea| a,b Raman cross section If Ei = Ea or Es = Eb We have Resonance Raman effect
- 15. Intensity of NormalRaman Peaks The intensity or power of a normal Raman peak depends in a complex way upon • the polarizability of the molecule, • the intensity of the source, and • the concentration of the active group. • The power of Raman emission increases with the fourth power of the frequency of the source; - photodecomposition is a problem. • Raman intensities are usually directly proportional to the concentration of the active species.
- 16. Raman Depolarization Ratios Polarizationis a property of a beam of radiation and describes the plane in which the radiation vibrates. Raman spectra are excited by plane-polarized radiation. The scattered radiation is found to be polarized to various degrees depending upon the type of vibration responsible for the scattering.
- 18. Raman Depolarization Ratios Thedepolarization ratio p is defined as Experimentally, the depolarization ratio may be obtained by inserting a polarizer between the sample and the monochromator. The depolarization ratio is dependent upon the symmetry of the vibrations responsible for scattering. p I I
- 19. Raman Depolarization Ratios Polarizedband: p = < 0.76 for totally symmetric modes (A1g) Depolarized band: p = 0.76 for B1g and B2g nonsymmetrical vibrational modes Anomalously polarized band: p = > 0.76 for A2g vibrational modes
- 20. Raman spectra ofCCl Raman spectra of CCl4 4 Isotope effect Cl has two isotopes 35 Cl and 37 Cl Relative abundance is 3:1
- 21. CCl CCl4 4 Spectra Spectra • 461.5cm-1 is due to 35 Cl4C • 458.4 cm-1 is due to 35 Cl3 37 ClC • 455.1 cm-1 is due to 35 Cl2 37 Cl2C • What are the two question marks? • Why are these bands weak?
- 22. Raman Spectra ofMethanol and Ethanol Raman Spectra of Methanol and Ethanol 20000 15000 10000 5000 0 500 1000 1500 2000 2500 3000 3500 OH stretching CH stretching CO stretching CH3 deformation Raman Shift (cm-1 ) Raman Intensity (arbitrary unit) CCO stretching CH3 and CH2 deformation Significant identification of alcohols which differ just in one CH2-grou CASR
- 23. Peak position –Chemical identity – Peak position – Chemical identity – Similar Structures Similar Structures 3,4-Methylenedioxymethamphetamine (MDMA) Methamphetamine 500 1000 1500 2000 2500 3000 3500 Raman Shift (cm-1 ) Raman Intensity (arbitrary unit) CASR ecstasy
- 24. 1200 1000 800 600 400 200 0 Intensity (counts/s) Wavenumber (cm-1) 1000 800 600 400 200 Intensity (counts/s) Wavenumber (cm-1) Mg- SO4 Na2 - SO4 The Mass Effect on Raman Spectra The Mass Effect on Raman Spectra Significant identification of salts (SO4 2- ) which differ just in the metal ion employed CASR
- 25. Peak positions –Chemical identity Peak positions – Chemical identity Diasteromers Diasteromers Pseudoephedrine Ephedrine 500 1000 1500 2000 2500 3000 3500 Raman Shift (cm-1 ) Raman Intensity (arbitrary unit) CASR
- 26. Peak Position –Crystal Phases – Polymorphs 200 400 600 800 1 000 1 200 1 400 1 600 1 800 2 000 2 200 2 400 In t e n s it y ( c n t ) 200 400 600 800 1 000 1 200 1 400 Raman Shift (cm-1 ) Both Anatase and Rutile are TiO2 but of different polymorphic forms - identical chemical composition, different crystalline structures. Rutile Anatase
- 27. Peak Shift –Stress and Strain Peak Shift – Stress and Strain Nasdala, L., Harris, J.W. & Hofmeister, W. (2007): Micro-spectroscopy of diamond. Asia Oceania Geosciences Society, 4th Annual Meeting, Bangkok, Thailand, August, 2007. Nasdala, L., Raman barometry of mineral inclusions in diamond crystals. Mitt. Österr. Miner. Ges. 149 (2004) CASR Larnite ( – Ca2SiO4) inclusion in Diamond
- 28. Bandwidth – Crystallinity– Bandwidth – Crystallinity – Structural order/disorder Structural order/disorder Raman spectra of zircon, showing typical amorphous (blue) and crystalline (red) spectra. CASR
- 29. Intensity – Concentration Intensity– Concentration 4-Nitrophenol dissolved in CH2Cl2 0 500 1 000 1 500 2 000 2 500 3 000 3 500 Intensity (cnt/sec) 1 200 1 400 1 600 Raman Shift (cm-1 ) 4-Nitrophenol in CH2Cl2_0,1 M 4-Nitrophenol in CH2Cl2_0,01 M 4-Nitrophenol in CH2Cl2_0,001 M 1341.0 CASR
- 30. Raman technique –what Raman technique – what requirements are needed? requirements are needed? Requirements for Raman technique to determine peak position, peak shift, bandwidth and intensity - Laser Excitation - Reduction of stray light - Collecting Optics - Spectral resolution and spectral coverage - Spatial resolution and confocality - Sensitivity: subject to detector - Light flux: subject to dispersion CASR
- 31. What do weneed to make a What do we need to make a Raman measurement ? Raman measurement ? Laser Sample Filter Grating Detector •Rejection filter (A way of removing the scattered light that is not shifted( changed in colour). (A way of focusing the laser onto the sample and then collecting the Raman scatter.) •Sampling optics •Monochromatic Light source typically a laser •Spectrometer and detector (often a single grating spectrometer and CCD detector.) CASR
- 32. Demonstration of thevery high Demonstration of the very high spectral resolution obtained in the spectral resolution obtained in the triple additive mode triple additive mode 8000 6000 4000 2000 Intensity (a.u.) 1520 1540 1560 1580 Wavenumber (cm-1) Triple additive mode Slit widths= 30 m Rotation-Vibration Spectrum of O2 Triple subtractive mode . Slit=30 m CASR
- 33. Laser excitation –Laser Selection to Laser excitation – Laser Selection to avoid fluorescence avoid fluorescence Laser wavelength, 1 Raman shift, 1 -1 + Laser wavelength, 3 Raman shift, 3 -1 + Laser wavelength, 3 Fluorescence Laser wavelength: 3 < 2 < 1 CASR
- 34. Laser excitation –Laser selection to Laser excitation – Laser selection to avoid fluorescence avoid fluorescence 0 10 000 20 000 30 000 40 000 50 000 60 000 Intensity (cnt) 600 800 1 000 Wavelength (nm) Green spectrum: 532 nm laser Red spectrum: 633 nm laser Dark red spectrum: 785 nm laser Fluorescence is wavelength dependent Ordinary Raman is wavelength independent 0 10 000 20 000 30 000 40 000 50 000 60 000 Intensity (cnt) 1 000 2 000 3 000 Raman Shift (cm-1 ) CASR
- 35. Commercial Hand Cream 785nm – 633 nm – 473 nm x103 5 10 15 20 25 30 35 40 45 Intensity (cnt) 500 1 000 1 500 2 000 2 500 3 000 3 500 Raman Shift (cm-1 ) Reduction of Fluorescence CASR Laser excitation – Laser selection to Laser excitation – Laser selection to avoid fluorescence avoid fluorescence
- 36. Laser excitation –laser radiation Laser excitation – laser radiation power power Laser wavelength: 473 nm Laser power at sample: 25.5 mW Objective N.A. Laser spot size (µm) Radiation power (kW/cm2 ) 100× 0.90 0.64 ~7900 50× 0.75 0.77 ~5400 10× 0.25 2.31 ~600 Laser wavelength: 633 nm Laser power at sample: 12.6 mW Objective N.A. Laser spot size (µm) Radiatio n power (kW/cm2 ) 100× 0.90 0.85 ~2200 50× 0.75 1.03 ~1500 10× 0.25 3.09 ~200 CASR
- 37. Laser excitation –laser radiation Laser excitation – laser radiation power power • Keep in mind: the usage of high numerical objective lenses causes a very small spot size of the laser which results in a high power density • To avoid sample burning radiation power has to be adapted INDIVIDUALY to the sample CASR
- 38. Collecting Optics Collecting Optics Samplingvolume Small for high N.A. lens Large for low N.A. lens Laser spot size Small for high N.A. lens Large for low N.A. lens Collection solid angle Large for high N.A. lens Small for low N.A. lens High N.A. lens θ Low N.A. lens θ NA = n · sin () n: refraction index : aperture angle Working Working distance CASR
- 39. Collecting Optics –Overview on Collecting Optics – Overview on common objectives common objectives Objective N.A. Working distance [mm] x100 0.90 0.21 x50 0.75 0.38 x10 0.25 10.6 x100 LWD 0.80 3.4 x50 LWD 0.50 10.6 CASR
- 40. Collecting Optics –what objective Collecting Optics – what objective should be used? should be used? A distinction between opaque and transparent samples has to be made For opaque samples, high N.A. lens works better because there is almost no penetration of the laser into the sample. High N.A. lens enables - High laser power density (mW/m3 ) increases sensitivity - Wide collection solid angle increases sensitivity 0 5 000 10 000 15 000 20 000 25 000 30 000 Intens ity (c nt/s ec ) 460 480 500 520 540 560 580 600 620 Raman Shif t (cm -1 ) Silicon x100 – NA = 0.9 – 31.350 C/s x50 – NA = 0.75 - 21.995 C/s x10 – NA = 0.25 - 1.462 C/s 100 % 70 % 5 % -20 0 20 Y (µm) 0 X (µm) 10 µm Example for an opaque sample: Silicon wafer CASR
- 41. Collecting Optics –what objective Collecting Optics – what objective should be used? should be used? A distinction between opaque and transparent samples has to be made For transparent samples, low N.A. lens works better because of penetration of the laser into the sample. Low N.A. lens enables - Large sampling volume increases sensitivity x103 0 2 4 6 8 10 12 14 Intensity (cnt) 740 760 780 800 820 840 860 880 Raman Shift (cm-1 ) cyclo_100xLWD cyclo_macro Sample: Cyclohexane Instrument: ARAMIS Red: x100LWD, 7,000 cts/s Blue: Macro lens, 14,500 cts/s 100 % 48 % CASR
- 42. Spectral resolution andspectral Spectral resolution and spectral coverage coverage Slit Detect or Focal Length Collimating mirror Focusing mirror Grating Schematic diagram of a Czerny-Turner spectrograph CASR
- 43. Spectral resolution andspectral Spectral resolution and spectral coverage coverage • Spectral resolution is a function of 1. dispersion, 2. widths of entrance slit and 3. pixel size of the CCD • Dispersion is the relation between refraction of light according to the wavelength of light • Dispersion is a function of the 1. focal length of spectrograph the 2. groove density of the grating and 3. the excitation wavelength • In general, long focal length and high groove density grating offer high spectral resolution. CASR
- 44. Dispersion as afunction of the focal Dispersion as a function of the focal length length Same grating Same excitation wavelength CCD Detector CCD Detector Long focal length Short focal length CASR
- 45. Dispersion as afunction of the focal Dispersion as a function of the focal length vis-a vis wavelength length vis-a vis wavelength Dispersion in cm-1 / pixel 1800 gr/mm Grating LabRAM (F = 300 mm) LabRAM HR (F = 800 mm) CASR
- 46. 200 400 600800 1000 1200 1400 1600 1800 244 - 269 nm (25 nm) 325 - 371 nm (46 nm) 457 - 553 nm (96 nm) 488 - 599 nm (111 nm) 514 - 639 nm (125 nm) 532 - 667 nm (135 nm) 633 - 833 nm (200 nm) 785 - 1119 nm (334 nm) 830 - 1210 nm (380 nm) 1064 - 1768 nm (704 nm) Horizontal lines indicate a relative Raman Shift of 3800 cm -1 Wavelength [nm] Dispersion as a function of excitation Dispersion as a function of excitation wavelength wavelength Long wavelength Short wavelength Same focal length Same grating CCD Detector CCD Detector CASR
- 47. Spectral coverage -dependence from Spectral coverage - dependence from excitation wavelength excitation wavelength Length of CCD Chip x103 0 2 4 6 8 10 12 14 16 18 20 22 Intensity (cnt/sec) 500 600 700 800 900 1 000 Wavelength (nm) Relative Raman shift of 3100 cm-1 corresponds to 81 nm Relative Raman shift of 3100 cm-1 corresponds to 154 nm Relative Raman shift of 3100 cm-1 corresponds to 252 nm 473 nm – 633 nm – 785 nm Same focal length Same grating Length of CCD Chip Length of CCD Chip CASR
- 48. Dispersion as afunction of groove density Dispersion as a function of groove density High density groove grating Low density groove grating CCD Detector CCD Detector Same focal length Same excitation wavelength CASR
- 49. Spectral resolution asa function slit Spectral resolution as a function slit width width Slit Slit Slit One of parameters that determines the spectral resolution is the entrance slit width. The narrower the slit, the narrower the FWHM (full width at half maximum), and higher the spectral resolution. When recording a line whose natural width is smaller than the monochromator’s resolution, the measured width will reflect the spectrograph’s resolution. CASR
- 50. Spectral resolution asa function of pixel Spectral resolution as a function of pixel size size • Because a CCD detector is made of very small pixels, each pixel serves as an exit slit (pixel size = exit slit width) • For the same size CCDs, the CCD with a larger number of smaller pixels produces a larger number of spectral points closer to each other increasing the limiting spectral resolution and the sampling frequency • 26 m pixel vs. 52 m pixel (simulation) Intensity 600 650 700 Raman Shift (cm-1) Detect or Detect or CASR
- 51. Choice of Laserfor Raman Choice of Laser for Raman The choice of laser will influence different parameters: • Signal Intensity: (1/4 rule applies to Raman intensity. • Probing volume: spot size and material penetration depth. • Fluorescence: may overflow Raman signal. • Enhancement: some bounds only react strongly at a certain wavelength. • Coverage range and Resolution: grating dispersion varies along the spectral range. 0,001 0,01 0,1 1 10 100 200 300 400 500 600 700 800 900 Penetration depth (µm) Wavelength (nm) Silicium CASR
- 52. Spatial resolution: penetrationdepth Spatial resolution: penetration depth 0 500 1000 1500 2000 2500 3000 244 nm 325 nm 457 nm 488 nm 514 nm 633 nm Wavelength [nm] D e p t h p e n e t r a t io n [ n m ] General: The larger the excitation wavelength, the deeper the penetration. The exact values depend on material. Penetration depth in Silicon 0 2 4 6 8 10 12 244 nm 325 nm CASR
- 53. 400 450 500550 Intensity [ a .u.] Raman shift (cm-1) 785 nm Spatial resolution: penetration depth Spatial resolution: penetration depth Uniform layer of SiGe Gradient SiGe layer Pure Si substrate Strained silicon layer 400 450 500 550 Intensity [ a .u.] Raman shift (cm-1) 325 nm 488 nm 785 nm The higher the excitation wavelength, the deeper the penetration. 488 nm 400 450 500 550 Intensity [ a .u.] Raman shift (cm-1) 325 nm Strained Si of top layer Si of silicon substrate Si of SiGe layer ~nm ~nm ~µm CASR
- 54. Spatial resolution: penetrationdepth Spatial resolution: penetration depth EXAMPLE 325nm laser results The strain is not homogenous. A characteristic, cross-hatch pattern is observed. CASR
- 55. D = 1.22 / NA Laser spot size D is defined by the Rayleigh criterion: excitation wavelength () objective numerical aperture (NA) With NA=n sinα Spatial resolution and spot size Spatial resolution and spot size Spatial resolution is half of the laser spot diameter CASR
- 56. Sample Length of Laser Focus Nearlyclosed confocal apertur P P ' P ' P P ' P Image P ' of laser focus P matches exactly the confocal hole. Confocality and Spatial Resolution Confocality and Spatial Resolution CASR
- 57. CASR Axial resolution ofa Confocal Raman Microprobe Axial resolution of a Confocal Raman Microprobe
- 58. Confocal z-scan againstsilicon with different hole apertures exc = 633 nm Sampling Volume Wide Hole Laser focus waist diameter Depth of laser focus (d.o.f) Narrow Hole Narrow Hole: Collecting Raman radiation that originates only from within a diffraction limited laser focal volume with a dimension of: Focus waist diameter ~ 1.22 / NA Depth of laser focus ~ 4 / (NA)2 Confocality and spatial Confocality and spatial Resolution Resolution CASR
- 59. Example of applicationusing the confocality Example of application using the confocality principle : depth profile on a multilayer polymer principle : depth profile on a multilayer polymer sample sample 5000 4000 3000 2000 1000 0 Intensity (a.u.) 1000 1200 1400 1600 Wavenumber (cm-1) 3000 2500 2000 1500 1000 500 0 Intensity (a.u.) 1000 1200 1400 1600 Wavenumber (cm-1) 75 m Polyethylene Polyethylene nylon z x CASR
- 60. Thank you foryour attention!
- 61. Symmetry – Identity(E) Symmetry – Identity (E) Identity operation (E) This is a very important symmetry operation which is where the molecule is rotated by 360º. In otherwords a full rotation or doing nothing at all. This appears in all molecules!!!
- 62. Symmetry – Rotation(C Symmetry – Rotation (Cn n) ) Rotations (Cn) These are rotations about the axes of symmetry. n denotes 360º divided by the number for the rotation. C2 = 180º C3 = 120º C4 = 90º C5 = 72º C6 = 60º
- 63. Symmetry – Symmetry –Reflections ( Reflections ( ) ) Reflections (h, d and v) These are reflections in a symmetry planes (x, y and z). h - Horizontal Plane (y) perpendicular to the highest rotation axis v - Vertical Plane (z) parallel to the highest rotation axis d - Diagonal (dihedral) Plane (x) the Diagonal Plane that bisects two axes
- 64. Symmetry – Inversion(i) Symmetry – Inversion (i) Inversion centre (i) These are where the molecule can be inverted through the centre of the molecule (atom or space).
- 65. Symmetry – ImproperRotation (S Symmetry – Improper Rotation (Sn n) ) Improper rotations (Sn) These are rotations about the axes of symmetry followed by reflections.
- 66. Vibrational Spectroscopy Vibrational Spectroscopy Forpolyatomic molecules, the situation is more complicated because there are more possible types of motion. Each set of possible atomic motions is known as a mode. There are a total of 3N possible motions for a molecule containing N atoms because each atom can move in one of the three orthogonal directions (i.e. in the x, y, or z direction). Translation al modes Rotational modes A mode in which all the atoms are moving in the same direction is called a translational mode because it is equivalent to moving the molecule - there are three translational modes for any molecule. A mode in which the atoms move to rotate (change the orientation) the molecule called a rotational mode - there are three rotational modes for any non-linear molecule and only two for linear molecules. The other 3N-6 modes (or 3N-5 modes for a linear molecule) for a molecule correspond to vibrations that we might be able to observe experimentally. We must use symmetry to figure out what how many signals we expect to see and what atomic motions contribute to the particular vibrational modes.
- 67. Vibrational Spectroscopy andSymmetry Vibrational Spectroscopy and Symmetry 1. Determine the point group of the molecule. 2. Determine the Reducible Representation, tot, for all possible motions of the atoms in the molecule. 3. Identify the Irreducible Representation that provides the Reducible Representation. 4. Identify the representations corresponding to translation (3) and rotation (2 if linear, 3 otherwise) of the molecule. Those that are left correspond to the vibrational modes of the molecule. 5. Determine which of the vibrational modes will be visible in an IR or Raman experiment. We must use character tables to determine how many signals we will see in a vibrational spectrum (IR or Raman) of a molecule. This process is done a few easy steps that are similar to those used to determine the bonding in molecules.
- 68. Finding the PointGroup Finding the Point Group
- 69. Example, the vibrationalmodes in water. The point group is C2v so we must use the appropriate character table for the reducible representation of all possible atomic motions, tot. To determine tot we have to determine how each symmetry operation affects the displacement of each atom the molecule – this is done by placing vectors parallel to the x, y and z axes on each atom and applying the symmetry operations. As with the bonds in the previous examples, if an atom changes position, each of its vectors is given a value of 0; if an atom stays in the same place, we have to determine the effect of the symmetry operation of the signs of all three vectors. The sum for the vectors on all atoms is placed into the reducible representation. Vibrational Spectroscopy and Symmetry Vibrational Spectroscopy and Symmetry Make a drawing of the molecule and add in vectors on each of the atoms. Make the vectors point in the same direction as the x (shown in blue), the y (shown in black) and the z (shown in red) axes. We will treat all vectors at the same time when we are analyzing for molecular motions. H O H H O H top view
- 70. The E operationleaves everything where it is so all nine vectors stay in the same place and the character is 9. The C2 operation moves both H atoms so we can ignore the vectors on those atoms, but we have to look at the vectors on the oxygen atom, because it is still in the same place. The vector in the z direction does not change (+1) but the vectors in the x, and y directions are reversed (-1 and -1) so the character for C2 is -1. The v (xz) operation leaves each atom where it was so we have to look at the vectors on each atom. The vectors in the z and x directions do not move (+3 and +3) but the vectors in the y direction are reversed (-3) so the character is 3. The ’v (yz) operation moves both H atoms so we can ignore the vectors on those atoms, but we have to look at the vectors on the oxygen atom, because it is still in the same place. The vectors in the z and y directions do not move (+1 and +1) but the vectors in the x direction is reversed (-1) so the character is 1. C2V E C2 v (xz) ’v (yz) tot 9 -1 3 1 Example, the vibrational modes in water. Vibrational Spectroscopy and Symmetry Vibrational Spectroscopy and Symmetry H O H C2 H O H H O H H O H H O H v (xz) H O H H O H ’v (yz) z y x
- 71. C2V E C2v (xz) ’v (yz) A1 1 1 1 1 z x2 ,y2 ,z2 A2 1 1 -1 -1 Rz xy B1 1 -1 1 -1 x, Ry xz B2 1 -1 -1 1 y, Rx yz From the tot and the character table, we can figure out the number and types of modes using the same equation that we used for bonding: n 1 order # of operations in class (character of RR) character of X X n 1 4 A1 1 9 1 1 1 1 1 3 1 1 1 1 This gives: n 1 4 B1 1 9 1 1 1 1 1 3 1 1 1 1 n 1 4 B2 1 9 1 1 1 1 1 3 1 1 1 1 n 1 4 A 2 1 9 1 1 1 1 1 3 1 1 1 1 Which gives: 3 A1’s, 1 A2, 3 B1’s and 2 B2’s or a total of 9 modes, which is what we needed to find because water has three atoms so 3N = 3(3) =9. Vibrational Spectroscopy and Symmetry Vibrational Spectroscopy and Symmetry C2V E C2 v (xz) ’v (yz) tot 9 -1 3 1
- 72. Vibrational Spectroscopy andSymmetry Vibrational Spectroscopy and Symmetry Now that we have found that the irreducible representation for tot is (3A1 + A2 + 3B1+ 2B2), the next step is to identify the translational and rotational modes - this can be done by reading them off the character table! The three translational modes have the symmetry of the functions x, y, and z (B1, B2, A1) and the three rotational modes have the symmetry of the functions Rx, Ry and Rz (B2, B1, A2). Translation al modes Rotational modes The other three modes (3(3)-6 = 3) that are left over for water (2A1 + B1) are the vibrational modes that we might be able to observe experimentally. Next we have to figure out if we should expect to see these modes in an IR or Raman vibrational spectrum. C2V E C2 v (xz) ’v (yz) A1 1 1 1 1 z x2 ,y2 ,z2 A2 1 1 -1 -1 Rz xy B1 1 -1 1 -1 x, Ry xz B2 1 -1 -1 1 y, Rx yz
- 73. Remember that fora vibration to be observable in an IR spectrum, the vibration must change the dipole moment of the molecule. In the character table, representations that change the dipole of the molecule are those that have the same symmetry as translations. Since the irreducible representation of the vibrational modes is (2A1 + B1) all three vibrations for water will be IR active (in red) and we expect to see three signals in the spectrum. For a vibration to be active in a Raman spectrum, the vibration must change the polarizability of the molecule. In the character table, representations that change the polarizability of the molecule are those that have the same symmetry as the second order functions (with squared and multiplied variables). Thus all three modes will also be Raman active (in blue) and we will see three signals in the Raman spectrum. Vibrational Spectroscopy and Symmetry Vibrational Spectroscopy and Symmetry C2V E C2 v (xz) ’v (yz) A1 1 1 1 1 z x2 ,y2 ,z2 A2 1 1 -1 -1 Rz xy B1 1 -1 1 -1 x, Ry xz B2 1 -1 -1 1 y, Rx yz The three vibrational modes for water. Each mode is listed with a (Greek letter ‘nu’) and a subscript and the energy of the vibration is given in parentheses. 1 is called the “symmetric stretch”, 3 is called the “anti-symmetric stretch” and 2 is called the “symmetric bend”.
- 74. The Geometry ofthe Sulfur Dioxide The Geometry of the Sulfur Dioxide Molecule Molecule
- 75. Cs structure: 3normal modes, all having A' symmetry The Cs structure should have 3 IR active fundamental transitions. These three fundamental transitions also should be Raman active. We would expect to observe three strong peaks in the IR and three strong peaks in the Raman at the same frequency as in the IR. All of the Raman lines would be polarized because they are totally symmetric (A' symmetry).
- 76. C2v structure: 3normal modes, two with A1 symmetry, one with B2 The C2v structure should have 3 IR active fundamental transitions. These three fundamental transitions also should be Raman active.We would expect to observe three strong peaks in the IR and three strong peaks in the Raman at the same frequency as in the IR. Two of the Raman lines are totally symmetric (A1 symmetry) and would be polarized. One Raman line would be depolarized.
- 77. The Dooh structureshould have two IR active fundamental transitions. It will have one Raman active fundamental transition at a different frequency than either of the IR peaks.. The Raman line will be polarized.
- 78. Experimental Observation Experimental Observation Fundamental2 1 3 IR (cm-1 ) 519 1151 1336 Raman (cm-1 ) 524 1151 1336 The experimental infrared and Raman bands of liquid and gaseous sulfur dioxide have been reported in a book by Herzberg 7 . Only the strong bands corresponding to fundamental transitions are shown below. The polarized Raman bands are in red.
- 79. Conclusion Conclusion The existence ofthree experimental bands in the IR and Raman corresponding to fundamental transitions weighs strongly against the symmetrical linear (Dooh) structure. We usually do not expect more strong bands to exist than are predicted by symmetry. Group theory predicts that both bent structures would have three fundamental transitions that are active in both the IR and Raman. However all three of the Raman lines would be polarized if the structure were unsymmetrical (Cs symmetry). The fact that one Raman line is depolarized indicates that the structure must be bent and symmetrical (C2v symmetry). The sulfur dioxide molecule has C2v symmetry.
- 80. Problems with Raman: a)VeryWeak – for every 106 photons only 1 photon Raman a)Resonant Raman not feasible with every sample. b)Absorption a better process than scattering
- 81. Raman Spectrometers Raman Spectrometers Micro–Ramansetup Micro–Raman setup International and National Patent (2007), G.V. Pavan Kumar et al Current Science (2007) 93, 778. Stage Objective lens Dichroic Mirror Camera Edge filter Focusing lens Computer Mono- chromator CCD Optical fiber LASER