Raman Spectroscopy

1923 – 1928 – Inelastic light scattering is predicted by A. Smekel Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartz 1928 – C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat solvents First Raman Spectra: Filtered Hg arc lamp spectrum: C 6 H 6 Scattering http://www.springerlink.com/content/u4d7aexmjm8pa1fv/fulltext.pdf

Raman Spectroscopy

1923 – 1928 – Inelastic light scattering is predicted by A. Smekel Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartz 1928 – C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat solvents 1930 – 1961 – C.V. Raman wins Nobel Prize in Physics Invention of laser makes Raman experiments reasonable 1977 – Surface-enhanced Raman scattering (SERS) is discovered 1997 – Single molecule SERS is possible

Rayleigh Scattering

•Elastic (  does not change) •Random direction of emission •Little energy loss (

E sc

)   8 2   4

d

2 2  )

E

0 Eugene Hecht,

Optics

, Addison-Wesley, Reading, MA, 1998.

Raman Spectroscopy

1 in 10 7 photons is scattered inelastically

virtual state Infrared (absorption) Raman (scattering) v” = 1 v” = 0

Rotational Raman Vibrational Raman Electronic Raman

Classical Theory of Raman Effect

m ind = 

E

polarizability Colthup et al.,

Introduction to Infrared and Raman Spectroscopy, 3rd ed.

, Academic Press, Boston: 1990

Photon-Molecule Interactions

When light interacts with a vibrating diatomic molecule, the induced dipole moment has 3 components: m

z

1

d



zz

 

r

max

E

max 2 1

dr d



zz



zz equil E

max 

r

max

E

max 2

dr

cos 2  0

t

    0     0 

vib

)

t vib

)

t

 Kellner et al.,

Analytical Chemistry

Rayleigh scatter Anti-Stokes Raman scatter Stokes Raman scatter

Raman Scattering

Selection rule:  v = ±1 Overtones:  v = ±2, ±3, … m

z

1

d



zz

 

r

max

E

max 2 1

dr d



zz



zz equil E

max 

r

max

E

max 2

dr

cos 2  0

t

    0 0    

vib

)

t vib

)

t

 Must also have a change in polarizability Classical Description does not suggest any difference between Stokes and Anti-Stokes intensities

N

1 

e



h



vib kT N

0 www.andor.com

Are you getting the concept?

Calculate the ratio of Anti-Stokes to Stokes scattering intensity when T = 300 K and the vibrational frequency is 1440 cm -1 .

h = 6.63 x 10 -34 k = 1.38 x 10 -23 Js J/K

Presentation of Raman Spectra

 ex = 1064 nm = 9399 cm -1 Breathing mode: 9399 – 992 = 8407 cm -1 Stretching mode: 9399 – 3063 = 6336 cm -1

Mutual Exclusion Principle

For molecules with a center of symmetry, no IR active transitions are Raman active and vice versa 

Symmetric molecules

IR-active vibrations are not Raman-active.

Raman-active vibrations are not IR-active.

O = C = O O = C = O Raman active Raman inactive IR inactive IR active

Raman vs IR Spectra

Ingle and Crouch,

Spectrochemical Analysis

Raman vs Infrared Spectra

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Raman vs Infrared Spectra

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Raman Intensities

Radiant power of Raman scattering: 

R ex

) 4

ex

E n e

0

i



E i kT

s (  ex ) – Raman scattering cross-section (cm 2 )  ex – excitation frequency E 0 n i – incident beam irradiance – number density in state i exponential – Boltzmann factor for state i s (  ex ) - target area presented by a molecule for scattering

Raman Scattering Cross-Section

s

d

s

d

    scattered flux/unit solid angle indident flux/unit solid angle

d



d

s

d

 Process absorption absorption emission scattering scattering scattering scattering scattering Cross-Section of UV IR Fluorescence Rayleigh Raman RR SERRS SERS s (cm 2 ) 10 -18 10 -21 10 -19 10 -26 10 -29 10 -24 10 -15 10 -16 s (  ex ) - target area presented by a molecule for scattering Table adapted from Aroca,

Surface Enhanced Vibrational Spectroscopy

, 2006

Raman Scattering Cross-Section

 ex (nm) 532.0

435.7

368.9

355.0

319.9

282.4

s ( x 10 -28 0.66

1.66

3.76

4.36

7.56

13.06

cm 2 Table adapted from Aroca,

Surface Enhanced Vibrational Spectroscopy

, 2006 ) CHCl 3 : C-Cl stretch at 666 cm -1

Advantages of Raman over IR

• Water can be used as solvent. •Very suitable for biological samples in native state (because water can be used as solvent).

• Although Raman spectra result from molecular vibrations at IR frequencies, spectrum is obtained using visible light or NIR radiation.

=>Glass and quartz lenses, cells, and optical fibers can be used. Standard detectors can be used.

• Few intense overtones and combination bands => few spectral overlaps. • Totally symmetric vibrations are observable.

•Raman intensities  to concentration and laser power.

Advantages of IR over Raman

• Simpler and cheaper instrumentation.

• Less instrument dependent than Raman spectra because IR spectra are based on measurement of intensity

ratio

.

• Lower detection limit than (normal) Raman.

• Background fluorescence can overwhelm Raman.

• More suitable for vibrations of bonds with very low polarizability (e.g. C –F).

Raman and Fraud

Lewis, I. R.; Edwards, H. G. M.,

Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line,

Marcel Dekker, New York: 2001.0

Ivory or Plastic?

Lewis, I. R.; Edwards, H. G. M.,

Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line,

Marcel Dekker, New York: 2001.

The Vinland Map: Genuine or Forged?

Brown, K. L.; Clark, J. H. R., Anal. Chem. 2002, 74, 3658.

The Vinland Map: Forged!

Brown, K. L.; Clark, J. H. R., Anal. Chem. 2002, 74, 3658.

Resonance Raman

Raman signal intensities can be enhanced by resonance by factor of up to 10 5 => Detection limits 10 -6 to 10 -8 M.

Typically requires tunable laser as light source.

Kellner et al.,

Analytical Chemistry

Resonance Raman Spectra

Ingle and Crouch,

Spectrochemical Analysis

Resonance Raman Spectra

 ex = 441.6 nm  ex = 514.5 nm

http://www.photobiology.com/v1/udaltsov/udaltsov.htm

Raman Instrumentation

Tunable Laser System Versatile Detection System

Dispersive and FT-Raman Spectrometry

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Spectra from Background Subtraction

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Rotating Raman Cells

Rubinson, K. A., Rubinson, J. F.,

Contemporary Instrumental Analysis

, Prentice Hall, New Jersey: 2000

Raman Spectroscopy: PMT vs CCD

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Fluorescence Background in Raman Scattering

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Confocal Microscopy Optics

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Confocal Aperture and Field Depth

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000 a nd http://www.olympusfluoview.com/theory/confocalintro.html

Confocal Aperture and Field Depth

McCreery, R. L.,

Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000