Chem 524 Lecture Notes (Section 17)

Chem 524 Lecture Notes (Section 17)--2009 For pdf of these notes, click here For HTML of 2005 notes, click here XIII. Molecular Light Scattering and Raman Spectroscopy (Read Ch. 16)

A. Elastic Scattering o = s 1) Raleigh Scattering — scatters are small compared to 

-4 2 s - same frequency, Intensity ~ , ~ (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

y r a r t

i 40 b r A

20 Raman

3000 2500 2000 1500 1000 Raman Shift (cm-1)

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)