Lecture 6 - Raman part 2

Question 1

What are the differences of a Raman and IR spectra?

Answer 1

IR is measured from 4000-0.
Raman done from 0-4000.
If a peak is on Raman, it wont be on the same position on the IR, for the same molecule.

Question 2

How do Raman intensities work?

Answer 2

Depend on the equation;
I = KI(L)p2v4

I(L) is the laser intensity
p polarizability
v is frequency
(For anti-stokes, scattering also dependent on population of state)

This shows that intensity, is massively influenced by frequency as its to the 4th power

I is related to wavelength through v = c/wl, which shows that higher itensities happen at lower wavelengths.

Question 3

How does the laser effect intensity?

Answer 3

A lower wavelength laser means more molecules will be able to absorb it and not show up, but the energy is higher so intensity of peaks is also higher. However fluorescence happens. Also because E is higher, burning and degradation can happen.

A higher wavelength laser means less molecules will absorb it, but the energy is lower so the peaks aren’t intense. However there is less fluorescence.

This means lasers in the UV region usually used (400-700nm)

Question 4

How does fluorescence effect Raman?

Answer 4

• Lower wavelengths are higher in energy, which means the virtual state is closer to the first electronic state than higher wavelengths, which causes fluorescence.
• This is why higher wavelength, lower energy has less fluorescence, as there VS closer to the GS.

Question 7

How is Raman sampling done?

Answer 7

• No sample prep needed
• Water isn’t a problem (as water has a strong change in dipole - not polarizability as it has strong asym stretches)
• Can analyse aqueous samples
• Any sample holder can be used
• Can analyse liquids and solids
• Can sample without removing packaging
• Non destructive