Raman Spectroscopy 1923 – Inelastic light scattering is predicted by A. Smekel 1928 – 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: http://www.springerlink.com/content/u4d7aexmjm8pa1fv/fulltext.pdf Filtered Hg arc lamp spectrum: C 6 H 6 Scattering
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Raman Spectroscopy 1923 – Inelastic light scattering is predicted by A. Smekel 1928 – 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
Raman Spectroscopy 1923 – Inelastic light scattering is predicted by A. Smekel 1928 – 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 – C.V. Raman wins Nobel Prize in Physics 1961 – Invention of laser makes Raman experiments reasonable 1977 – Surface-enhanced Raman scattering (SERS) is discovered 1997 – Single molecule SERS is possible
Colthup et al., Introduction to Infrared and Raman Spectroscopy, 3rd ed., Academic Press, Boston: 1990
µind = αE
polarizability
Kellner et al., Analytical Chemistry
max 0
max max 0
max max 0
( ) cos 21 cos 2 ( )21 cos 2 ( )2
equilz zz
zzvib
zzvib
t E td r E tdr
d r E tdr
µ α πνα π ν ν
α π ν ν
= +
∆ + +
∆ −
When light interacts with a vibrating diatomic molecule, the induced dipole moment has 3 components:
Photon-Molecule Interactions
Rayleigh scatter
Anti-Stokes Raman scatter
Stokes Raman scatter
www.andor.com
max 0
max max 0
max max 0
( ) cos 21 cos 2 ( )21 cos 2 ( )2
equilz zz
zzvib
zzvib
t E td r E tdr
d r E tdr
µ α πνα π ν ν
α π ν ν
= +
∆ + +
∆ −
Selection rule: ∆v = 1 Overtones: ∆v = ±2, ±3, …
Raman Scattering
Must also have a change in polarizability
Classical Description does not suggest any difference between Stokes and Anti-Stokes intensities
1
0
vibhkTN e
N
ν−
=
Calculate the ratio of Anti-Stokes to Stokes scattering intensity when T = 300 K and the vibrational frequency is 1440 cm-1.
Are you getting the concept?
h = 6.63 x 10-34 Js k = 1.38 x 10-23 J/K
Presentation of Raman Spectra
λex = 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
σ(νex) – Raman scattering cross-section (cm2) νex – excitation frequency E0 – incident beam irradiance ni – number density in state i exponential – Boltzmann factor for state i
40 α ( )
iEkT
R ex ex iE n eσ ν ν−
Φ
Radiant power of Raman scattering:
σ(νex) - target area presented by a molecule for scattering
Raman Scattering Cross-Section
σ(νex) - target area presented by a molecule for scattering