Chem 524 Lecture Notes (Section 17)

XIII. Molecular Light Scattering and Raman Spectroscopy (Read Ch. 16)

A. Elastic Scattering lo = ls

1) Raleigh Scattering — scatters are small compared to l

ls - same frequency, Intensity ~l-4, ~a2 (polarizability)

2) Debye/Mie — more anisotropic

spatial variation indicates size and shape

B. Inelastic - lo ‚ ls

1) Brillouin — scatter from phorous (thermal density fluctuation)

2) Raman — scatter from molecular excited states — vibs

C. Raman — ns = no ± nvib(rot)

(+) = anti stokes (-) = stokes (energy into molecule)

-- qualitative big use (like IR) identify/characterize

-- qualitative particularly difficult — standards

-- capable of small volumes/but require relatively high concentration

1. Instruments

a. Rebuilt emission — higher res needed/very high sensitive?

-- Ar laser typ excite/now YAG (Ti-supp)

-- PMT and photon court typical/now CCD

-- 90º Scatter typical (include back reflection), 180º sometimes

-- microscope (180º) possible, fine focus

-- polarization important (Polaroid and depolarize)

-- monochromator–double (even triple) 1/2 à 1m typical (scatter light)

/now single plus holographic filter for laser

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 t < ms

c. FT Raman — near IR laser (700 nm à 1m)

lose as l4 -- gain from multiplex

-- big advantage — eliminate. fluorescence

-- big application — materials/bio/complex sample

-- YAG: need InGaAs (~1.8 m) on Ge (~1.6 m) detector — limit Dn

-- 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 — curvette, 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)