Chem 524 Lecture Notes (Section 17)--2009
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XIII. Molecular Light Scattering and Raman Spectroscopy (Read Ch. 16)
A. Elastic Scattering o = s
1) Raleigh Scattering — scatters are small compared to 
s - same frequency, Intensity ~-4, ~2 (polarizability)
2) Debye/Mie — more anisotropic
spatial variation indicates size and shape
B. Inelastic - o ≠ s
1) Brillouin — scatter from phonons (thermal density fluctuation)
2) Raman — scatter from molecular excited states
— vibrations most often (also rotations, low lying electronic states)
C. Raman — s
= o ± vib(rot)
(+) = anti stokes (-) = stokes (energy into molecule)
-- qualitative big use (like IR) identify/characterize
-- qualitative particularly difficult — standards
80
IR
Arbitrary Y
60
40
Raman
20
0
3000
2500
2000
Raman Shift (cm-1)
1500
1000
File # 2 : SILICONE
- capable of small volumes/but require relatively high concentration
--Complementary to IR, tend to opposite selection rules, different intensities ~[∂/∂Q]2
1. Instruments – many used to be homemade, construct from components
Commercial components
a. Rebuilt emission — higher res needed/very high sensitive?
-- Ar laser typical excite – visible, 488 or 514 nm
/now YAG (doubled at 532) or diodes various places,
and special FRED lasers, double to UV
-- PMT and photon count typical
/now CCD universal with spectrograph – simultaneous detection spectrum
-- 90º Scatter typical (include back reflection),
180º sometimes more efficient, better for imaging or microscopy
-- monochromator–double (even triple) 1/2  1m typical (scatter light)
/now single spectrograph + CCD + holographic filter for laser
-- microscope (180º) possible, fine focus possible due to shorter wavelength, 0
-- polarization important (polarized, ||, and depolarized transitions, ┴)
Raman is 2-photon, so relative polarization of ex and s beams important
Polarization ratio:  = I┴/I|| --  < 7/8 (or  < ¾ for different excitation geometry)
means mode is polarized, can tell symmetry of transition
b. Multichannel systems — fixed resolution
-- go for speed/ S/N by averaging
-- diode array works/CCD can be better, bigger slit image
-- can do time dependence with gate  < s
c. FT Raman — near IR laser (700 nm  1) -- lose as 4 -- gain from multiplex
Link to write-up on methods, Henry Bujis
-- big advantage — eliminate. fluorescence
-- big application — materials/bio/complex sample
-- YAG: need InGaAs (~1.8 ) on Ge (~1.6) detector — limit 
-- Ti: Sapphire — GaAs PMT works
-- back scatter aid illuminate round pattern
-- filter Rayleigh is vital importance
2. Sampling, multiple styles
a) 90º — capillary or tube is fine/optical quality
b) 180º — cuvette, polish bottom
c) flow — narrow device/jet
d) cool — stir/spin/blow cold N2
-- crystal mount in dewar, on a cold finger (platform)
3. Resonance Raman — less analytically important
a) excite a real absorption state
b) seek info about vibrations and excited state
c) enhance intensity — more dilute samples
4. SERS — surface enhancement by analyte on metal (Au typ) surface — rougher
5. CARS, etc — many non-linear possibilities utilize multiple photon excitations
6. Time dependent — if signal enhanced can excite w/ps or ns laser and see time dependent
processes
7. Raman optical activity — differential scatter of left and right circular polarized light (only for
molecules that are chiral) -- instrument figures, 180o back scatter, transmission spectrograph, very
good conventional design: L.D. Barron (note: W.Hug developed a more complex design, currently
marketed by BioTools that has improvements)
Data: helicene (W. Hug below)
Lysozyme in water (BioTools)