Lecture 16: Raman Spectroscopy and Other Scattering Methods

What is optical scattering?

changing the trajectory of light

What are the major classes of scattering processes?

Elastic, quasi-elastic, and inelastic

What is elastic scattering?

no change in energy, rayleigh and mie scattering

what is quasi-electric scattering?

very small energy transfer, dynamic light scattering

What is inelastic scattering?

change in energy, Raman

In mie scattering, what does the magnitude of scattering depend on?

particle size, direction of observation wrt incidence, # denity of particles

Where is Mie scattering used?

turbidity measurements (AgCl titration), nephelometry (bacteria counter)

What are some charactaristics of rayliegh scattering?

mangintude depends on polarizability of particle, scattering near isotopic with same polarization, very fast, power proportional to 1/λ⁴

What is Raman scattering?

Very special case of Rayleigh where polarizability of molecule changes during scattering

What does raman scattering result in?

amplitude modulation of scattered wave

What kind of process is Raman scattering?

inelastic, caused by change in polarizability, bond vibration modulats scattered waves, leading to side-bands

What kind of Raman scattering is this?

Stokes

What kind of Raman scattering is this?

Anti-Stokes

What symmetry labels does IR have?

x, y, z

What symmetry labels does Raman have?

x, y, z PRODUCTS

What shift is Stokes?

red shifted

What shift is anti-stokes?

blue shifted

Charactaristics of Raman Spectroscopy

can excite with any wavelegnth, active for vibration that cause change in polarizability, can examine aqueous samples

Disadvantages of Raman

signals very close to excitation frequency, very weak signals, need to account for absorption/fluorescence in a sample

Raman Sources

Laser (Ar or Kr), Nd:YAG if want no fluorescence

Monochrometers for Raman

Need double monochrometer or band-rejection filter

What is the Raman block diagram?

Raman excitation source

does not need to be monochromatic, must have narrow line width, low background in wavelegnth region, and stable

What is the diagram for a raman instrument?

Raman calculation from stokes

Δv_RS=v_ex-v_st

Raman calculation from anti-stokes

Δv_RS=v_AS-v_ex

Resonance Raman

tunes radiation to absorption band of molecule (10²-10⁶ increase in Raman scattering, simple spectrum, sensitive to vibrations on chromophore) BUT background from fluorescence

Surface-enhanced raman scattering

meqasure scattering of molecules on rough metal surface (acts like an antenna) 10¹⁰-10¹¹ enhancement