lecture19 clh class - raman spectroscopy
DESCRIPTION
Raman Spectroscopy.TRANSCRIPT
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:
C6H6 Scattering
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
Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
Rayleigh Scattering
•Elastic (λ does not change)
•Random direction of emission
•Little energy loss 4 2 2
04 2
8 ( ') (1 cos )( )scEE
dθπ α θ
λ+
=
Raman Spectroscopy 1 in 107 photons is scattered inelastically
Infrared (absorption)
Raman (scattering)
v” = 0
v” = 1
virtual state
Exci
tatio
n
Scat
tere
d
Rotational Raman Vibrational Raman Electronic Raman
Classical Theory of Raman Effect
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
scattered flux/unit solid angleindident flux/unit solid angle
dd
ddd
σ
σσ
=Ω
= ΩΩ∫
Process Cross-Section of σ (cm2) absorption UV 10-18
absorption IR 10-21 emission Fluorescence 10-19 scattering Rayleigh 10-26 scattering Raman 10-29 scattering RR 10-24 scattering SERRS 10-15 scattering SERS 10-16 Table adapted from Aroca, Surface Enhanced
Vibrational Spectroscopy, 2006
Raman Scattering Cross-Section
λex (nm) σ ( x 10-28 cm2) 532.0 0.66
435.7 1.66 368.9 3.76 355.0 4.36 319.9 7.56 282.4 13.06 Table adapted from Aroca, Surface Enhanced Vibrational Spectroscopy, 2006
CHCl3: 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 α 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 105 => 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
http://www.photobiology.com/v1/udaltsov/udaltsov.htm
λex = 441.6 nm
λex = 514.5 nm
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 and 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