Further on Raman scattering of polyatomic molecules and implementations of Raman spectroscopy

Here we see raman spectrums of air and ethanol (nvm the cow). What you see is stokes shifts for different components.
Measurable parameters in raman spectral bands are wavenumber, intensity, width, depolarization factor.
What is depolarization factor?

Here, we have I_{in_polarizationout_polarization} illustration, and for isotropic samples we have I_VH=I_HV=I_HH where VH is vertical polarized laser and horizontal polarized Raman; and vice versa.
And then we defined or depolarization factor as I_VH / I_VV which tells us about how much Raman light loses the polarization of incident laser. Then we defined b and g as above, and final defined ρ.

Now looking at the limits, we have:

Totally symmetric modes

Non-totally symmetric modes

We conclude ρ ≤ 0.75. Since we can measure I_VH and I_VV, we can talk about symmetry via Placzek rules.
Recall: b: isotropic part, g: anisotropic part

Placzek rules

Raman spectral bands related to not fully symmetric normal vibrations have depolarization factor ρ = 0.75

For molecules of high symmetry a vibration with the full symmetry leads to polarized or partially polarized Raman band (ρ=0)

For molecules of low symmetry a vibration with the full symmetry leads to polarized or partially polarized Raman band (ρ < 0.75)

In short, ρ ~ 0 → strongly polarized Raman line, ρ = 0.75 → completely depolarized Raman line

implementations of Raman spectroscopy

There are types of Raman spectroscopy.

Conventional (spontaneous) Raman scattering

Resonance Raman scattering

Nonlinear Raman scattering

Let's go.

Resonance Raman spectroscopy

Incident laser frequency is very close to electronic excitation energy. The virtual state is nearly an electronic state. Strong vibronic coupling occurs. Resonance raman usually include fluoresence (transitions from real electronic states occurance) background. The closeness in energy leads to greatly increased insensity of Raman scattering.

Nonlinear Raman spectroscopy

Coherent anti-stokes raman scattering (CARS)

Stimulated Raman scattering

IMPORTANT!!!! Understand stimulation, amplification and coherency. Come back!!!

Hyper-rayleigh and hyper-raman scattering

The distinction from normal raman is the response. Hyper-raman responses are nonlinear.

Hyper-rayleigh absorbs two photon and emits 2 photon. Same energy. Higher virtual levels compared to normal raman.
On the other hand, in Hyper-raman, two photons are absorbed, molecule reaches a virtual state, one photon is emitted with less energy, the missing energy excites a vibrational mode and shift occurs.

Surface enhanced raman scattering (SERS)

explain... Also mention that HYPERION 3000 and what you can do with it.

Chemical imaging using ATR FTIR technique

Used when the preperation of a thin film of the sample is not possible.