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Raman Spectroscopy

Raman 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

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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

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Representation in terms of energy levels:

Arrow up = laser photon in; Arrow down = Raman scattering out

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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

Relativeintensity

Raman shift (cm-1)

Stokes

Rayleigh

anti-Stokes

-277

-112

226

279

114

Cs2NaBiCl6-Raman

shows 3vibrations of

octahedralBiCl6

3-

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Selection rule for Raman spectrum

Vibration is active if it has a change in

polarizability, .Polarizability is the ease of distortion of a bond. ForRaman-active vibrations, the incident radiation does

not cause a change in the dipole moment of themolecule, but instead a change in polarizability.

In starting the vibration going, the electric field ofthe radiation at time t, E, induces a separation of

charge (i.e. between the nuclear protons and thebonding electrons). This is called the induceddipole moment, P. (Dont confuse it with the

molecules dipole moment, or change in dipole

moment, because this is often zero).

P = E

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Example: There are 4 normal modes of CO2. Only 1is Raman active

is dipole moment;

is polarizability;

Q is vibration coordinate, The slopes

are measured at Q = 0 (I.e. at the equilibrium position).

Change indipolemoment, ,andpolarizability,, during CO2vibrations1 3

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Uses of Raman Spectroscopy

Raman spectroscopy has become more widely used

since the advent of FT-Raman systems and remoteoptical fibre sampling. Previous difficulties with

laser safety, stability and precision have largelybeen overcome.

Basically, Raman spectroscopy is complementaryto IR spectroscopy, but the sampling is moreconvenient, since glass containers may be usedand solids do not have to be mulled or pressed intodiscs.

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Applications of Raman spectroscopy

Qualitativetool for identifying molecules from theirvibrations, especially in conjunction with infraredspectrometry.

Quantitative Raman measurements

a) not sensitive since Raman scattering is weak. Butresonance 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 tosample alignment, collection efficiency, laser power.

Overcome by using internal standard.

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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.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?

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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)

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More about polarizability

, polarizability of molecule, related to mobility ofelectrons (under applied radiation field in our presentcase).

For atoms, same distortion is obtained for field in anydirection. Polarizability is Isotropic

For many molecules, polarizability depends ondirection of applied field, e.g. HH easier to distortalong bond than bond. Polarizability is anisotropicVariation of with direction is described bypolarizability tensor.

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Calculation of Stokes and anti-Stokes intensity ratio

The Raman spectrum wastaken at 300 K using 1064

nm Nd-YAG radiation.

Check the intensity ratioof the 1 features at278.5 cm-1.What can you say aboutthe intensity ratio of theband at 112 cm-1?

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Instrumentation

Dispersive Raman instruments

Laser Sample Double or triple monochromator

signal processing and output Photomultiplier tubeThe monochromators are required to separate the weak

Raman signal from the intense, nearby Rayleighscattering. Typical lasers are Ar+ (e.g. green line, 514.5nm) or Kr+ (e.g. yellow line, 530.9 nm).

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New Raman systems

http://www.s-and-i.de/

http://www.s-and-i.de/http://www.s-and-i.de/http://www.s-and-i.de/http://www.s-and-i.de/http://www.s-and-i.de/http://www.s-and-i.de/

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Fourier transform Raman spectrometer

The Raman instrument can be on the same bench

as the FTIR. Often, a YAG:Nd3+ laser (1064 nm) isused to excite the sample, so that the excitationenergy is lower than the absorption bandenergies 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.

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Sampling techniques for Raman spectroscopy

If the sample is colourless, it does not absorb a visible laser

Raman spectroscopy is applicable to solids, liquids orgases.

Gases:

use gas cell

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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 changewavelength;

Dilute the sample into a KBr pellet;

Cool the sample

Rotate or oscillate the laser beam on the sample

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High resolution Raman spectra can show splittingsdue to isotopic mass effects, for example: the 1Raman band of CCl4 (corresponding tp the totallysymmetric stretching vibration) is split into 5

components.

461.5 cm-1 is due to 35Cl4C

458.4 cm-1 is due to 35Cl337ClC

455.1 cm-1 is due to 35Cl237Cl2C

What are the two questionmarks?

Why are these bands weak?

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Depolarization ratio of a vibrational mode in the Ramanspectrum

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, see Infrared and RamanSpectraPart A., K. Nakamoto 5th Ed. Wiley 1997. Pp. 97-101.

0p

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Special types of Raman spectroscopy(see Hollas, Modern Spectroscopy, Wiley)

Resonance Raman (RR) scattering

When the laser excitation frequency is near (or coincident) with anelectronic absorption band, intensity enhancement can occur by afactor of 102-106, 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 ingeometry 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 103-106 times) when theanalyte 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.

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Stimulated Raman scattering (SRS)

The Raman scattering from a laser is observed in the forwarddirection 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 thevibrational modes of the sample.

Hyper-Raman spectroscopy

Very weak scattering at twice the laser frequency, 20, is calledHyper-Rayleigh scattering. Similarly, Stokes and anti-Stokes hyper-Raman scattering occur at 20vib, where vib is a vibrationfrequency. The selection rules differ from those of Ramanscattering.

Electronic Raman scattering

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Electronic Raman scattering

Raman scattering can occur from electronic states, as well asfrom vibrations. At room temperature the bands are very broad,and merge into the background. At low temperatures (

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Books on Raman spectroscopy

There are many in CityU library, such as:

FT Raman spectroscopy, P. Hendra et al., EllisHorwood.

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 ofInstrumental Analysis, Saunders.

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Useful websites for Raman spectrometryGreat 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&Dbase=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 Griffiths 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

http://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www-wilson.ucsd.edu/education/pchem/spectroscopy/sptyperaman.htmlhttp://www.aua.gr/georgiou/http://search.eb.com/bol/search?type=topic&query=Optical,+infrared,+and+Raman+spectroscopy&Dbase=Articles&I3.x=44&I3.y=2http://search.eb.com/bol/search?type=topic&query=Optical,+infrared,+and+Raman+spectroscopy&Dbase=Articles&I3.x=44&I3.y=2http://www.andor-tech.com/http://www.ftir.chem.uidaho.edu/http://pcjagg.dbs.aber.ac.uk/index.htmlhttp://ihome.ust.hk/~keckchan/spectroscopy.htmlhttp://ihome.ust.hk/~keckchan/spectroscopy.htmlhttp://pcjagg.dbs.aber.ac.uk/index.htmlhttp://www.ftir.chem.uidaho.edu/http://www.andor-tech.com/http://www.andor-tech.com/http://www.andor-tech.com/http://search.eb.com/bol/search?type=topic&query=Optical,+infrared,+and+Raman+spectroscopy&Dbase=Articles&I3.x=44&I3.y=2http://search.eb.com/bol/search?type=topic&query=Optical,+infrared,+and+Raman+spectroscopy&Dbase=Articles&I3.x=44&I3.y=2http://www.aua.gr/georgiou/http://www-wilson.ucsd.edu/education/pchem/spectroscopy/sptyperaman.htmlhttp://www-wilson.ucsd.edu/education/pchem/spectroscopy/sptyperaman.htmlhttp://www-wilson.ucsd.edu/education/pchem/spectroscopy/sptyperaman.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,9076,6-0-0-0-0-home-0-0,00.html

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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

k

http://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.renishaw.com/client/start/index.asphttp://www.optics.bruker.com/http://www.nicolet.com/http://www.nicolet.com/labsys/products/Raman_theory.htmlhttp://www.isa-gs.co.uk/http://www.scimedia.com/english/camera/index.htmhttp://www.anamap.co.uk/http://www.anamap.co.uk/http://www.scimedia.com/english/camera/index.htmhttp://www.isa-gs.co.uk/http://www.isa-gs.co.uk/http://www.isa-gs.co.uk/http://www.nicolet.com/labsys/products/Raman_theory.htmlhttp://www.nicolet.com/http://www.optics.bruker.com/http://www.renishaw.com/client/start/index.asphttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.htmlhttp://www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,6-1-12-0-0-news_detail-0-3606,00.html