Lecture 5 - Raman

Question 1

What is Raman?

Answer 1

• Involves SCATTERING not absorption of light.
• Don’t need a permanent dipole
• Symmetric vibrations strongest
• Water isnt a strong raman scatterer
• Raman is a weak effect only found in 1 of 10E6 photons

Question 2

How does the change in polarizability of a molecule work?

Answer 2

Polarizability describes the mobility of the electrons in a molecule
Every molecule has polarizability , even with no permanent dipole

µIND = α E
where u is the induced dipole
E is electric field
α is polarizability

Raman will NOT happen without a change in polarizability

Question 3

How does the scattering work?

Answer 3

• When a photon interacts with light it can absorb or scatter the light
• When scattered, the photon interacts with the electron cloud and distorts it.
• Scattering occurs when the distorted electron cloud relaxes
• Most scattering happens at the same frequency as the incident photon - known as Rayleigh scattering
• If there is energy transfer to or from the nuclei the scattering is shifted in energy by one vibrational unit. This is Raman scattering

Question 4

How does EM radiation effect raman?

Answer 4

• Raman scattering requires interaction between the incident light and a polarisable analyte molecule
• This induces a dipole in the molecule by the displacement of the nucleus and electrons by the electric component of the incident light
• Possible due to laser light used being much higher in energy than a vibration

Question 5

What is the difference with Raman(Anti-stokes) and Rayleigh? (diagram POF)

Answer 5

• In Raman spec, in both Raman and Rayleigh, the proton is excited to a virtual energy level (the distortion)
• For Rayleigh scattering, the photon can be excited from the GS -> VS-> GS, or 1 ES->VS-> 1ES. So the proton goes back down to the energy level it started in.
• For Raman scattering, the photon can be excited from GS->VS->1 ES, or 1ES->VS->GS

Question 6

What is Stoken vs Anti-Stokes?

Answer 6

• Both are Raman scattering
• Stokes is when the proton goes form GS->VS-> 1 ES, so there’s a +1 difference.
• Anti-Stokes is when the proton goes from 1ES->VS->GS, so there’s a -1 difference.
• For stokes, E = h(v0-vn)
• For anti-stokes, E = h(v0+vn)

Question 7

What is the virtual energy state dependent on?

Answer 7

• The laser energy
• A high laser energy will bring the VS up close to the next electronic level.
• The laser wl is the arrow going up to the VS, then scattering wl is the one going back down.

Question 8

What is the virtual energy state?

Answer 8

• This represents the distorted electron cloud.
• Energy equivalent to the energy of the laser beam
• Can be anywhere between GS and 1st ES.
• Happens very fast, scattering occurs fast as its not a true energy state.

Question 9

What laser is usually used? and how is Raman scattering plotted?

Answer 9

• Usually a visible laser which are much higher in frequency than the vibration.
• Raman scattering is plotted as a shift in energy from the laser frequency (laser wl - energy diff)

Question 10

What are the selection rules?

Answer 10

-DeltaV = (+/-) 1
- Need a change in polarizability.
- Symmetric vibrations largest

Question 11

Why is Stokes more likely to happen than Anti-Stokes?

Answer 11

Molecules less likely to start in the excited state in room temp.
At room temp, most molecules likely to be in the GS - Boltzmann distribution.
So, most likely to be Stokes and not Anti-stokes.

Question 12

How does Stokes vs Rayleigh vs Anti-stokes look on a Raman graph?

Answer 12

• When Intensity is plotted against Raman shift (cm-1), anti-stokes will be a negative number, Rayleigh will be 0, and stokes will be a positive number.

Question 13

How is the energy of each scatter found for Stokes/anti-stokes? and how is the Raman shift found?

Answer 13

• The energy of a stokes scatter is, freq of laser (first arrow) - scatter wl (second arrow)
• The energy of an anti-stokes is, freq of laser + scatter wl.
• The Raman shift is calculated by EI - Es =
  EI - (v0 - v v) for stokes
• Last term is + for anti-stokes
• EI is the energy laser.

Question 14

How do the vibrations work for Raman? (why is symmetrical the best?)

Answer 14

• Result in the largest change in polarizability - largest change in electron cloud.

Question 15

What are the IR vs Raman selection rules?

Answer 15

• For IR, DeltaV = +/- 1
  There needs to be a change in dipole moment.
  Asymmetric vibrations strongest
• For Raman, DeltaV = +/- 1
  There needs to be a change in polarizability.
  Symmetric vibrations strongest

Question 16

How do the group frequencies look for Raman?

Answer 16

• Similar to Raman, they are in groups as they all interact and are so similar - benzene ring
• The C-C and C-H stretches will be grouped as aromatic ring vibrations/stretches
• Symmetric vibration (has a sym peak) around 1000. These are called “breathing modes” and shows the whole ring moving.
• Asymmetric vibrations, known as a quadrant peak, around 1600. This shows that the C’s are moving away/closer to each other at different points (asym)

Question 17

What is a centrosymmetric molecule?

Answer 17

• In a centrosymmetric molecule any point reflected through the centre will appear at an identical point on the other side, so there’s an inversion centre.
• These can be symmetric, meaning all the inversions are going the same way (known as g).
• They can be asymmetric, meaning one set can be inverted out the way while the other centre being inverted in the way (known as u).
• Would need a trans molecule meaning the same molecule is opposite.

Question 18

What is the mutual exclusion rule?

Answer 18

No one vibration in a centrosymmetric molecule can be both IR and Raman active.
- So, g vibrations (symmetric) will likely be Raman and u vibrations (asymmetric) will likely be IR.

Question 19

How does cis vs trans work for IR/Raman?

Answer 19

• Cis molecules are not centrosymmetric as the same molecule is on the same face, doesn’t have inversion centre. So could show a peak on Raman and IR.
• Trans molecules are centrosymmetric as the 2 molecules are on opposite faces. Only 1 peak will show, for Raman OR IR, not both.