Spectroscopic Techniques

Question 1

What are the two types of scattering?

Answer 1

• Rayleigh: elastic - leaves the molecule in the same state
• Raman: inelastic - leaves the molecule in a different quantum state

Question 2

What are photons produced from?

Answer 2

• electromagnetic radiation

Question 3

What do photons do in electromagnetic radiation?

Answer 3

interact with molecular vibrations in a material and are scattered

Question 4

What is Raman scattering photon?

Answer 4

has a different energy to that of the incident photon

Question 5

What are the two types of Raman scattering?

Answer 5

• Stokes: the photon has lost energy to the molecule
• Anti-Stokes: the photon has gained energy from the molecule

Question 6

What happens when a photon interacts with a molecule in Raman scattering?

Answer 6

they go into a virtual state and then drop back down to the ground state

Question 7

What happens in stokes scattering?

Answer 7

start at a lower vibrational state, then enter a virtual state, then fall back down to a higher vibrational state than at the start

Question 8

What happens in anti-stokes scattering?

Answer 8

starts at a higher vibrational state, then enters a virtual state, then falls back down to a lower vibrational state than at the start

Question 9

What is the type of scattering that majority of light scattered with?

Answer 9

Rayleigh

Question 10

What are the different molecular vibrations?

Answer 10

• symmetric stretching
• antisymmetric stretching
• in-plane scissoring
• in-plane rocking
• out-of-plane wagging
• out-of-plane twisting

Question 11

What must a molecule be to be Raman active?

Answer 11

anisotropic polarisability - change in polarisation

Question 12

What must a molecule be to be IR active?

Answer 12

change in dipole

Question 13

What is the mutual exclusion rule?

Answer 13

in a centrosymmetric molecule a vibrational mode may be either Raman or IR active but it cannot be both

Question 14

What happens when a mode is active in both Raman and IR?

Answer 14

one tends to be weak and the other tends to be strong - e.g. water

Question 15

Raman vs IR

Answer 15

Raman
* light scattering
* requires a change in polarisability
* little or no sample prep needed
* measure through transparent packaging
* aqueous samples
IR
* IR absorption
* requires change in dipole moment
* sample preparation needed
* short optical path length required
* non-aqueous samples

Question 16

How does a Raman spectrometer work?

Answer 16

• laser source (blue, green, red) produces light which hits focusing mirror, goes through a beam splitter and hits more mirrors to hit the sample
• light interacts with sample and scatters the light
• Rayleigh filter filters out the Rayleigh svattered light
• charged couple detector takes light and converts it inot an electrical change to produce a signal, algorithm turns it into a spectrum

Question 17

Order the forms of scattering in terms of strength

Answer 17

• Rayleigh
• Stokes
• anti-stokes

Question 18

What does Raman ususally look at?

Answer 18

inorganics but can also look at organics

Question 19

How do you assign functional groups with Raman?

Answer 19

complementary to IR spectra

Question 20

Why would you use Raman microspectroscopy?

Answer 20

• rapid
• non-destructive
• high spatial resolution
• inorganic/organic profiling
• sensitive
• minimal sample preparations
• suitable for databasing
• chemometrics
• parameter optimisation
• automated stage mapping
• material identification
• sample matching and heterogeneity
• structure determination
• forgery detection

Question 21

Why is chemometrics used for Raman and IR spectrums?

Answer 21

• group similar samples
• gets rid of bias and difference of opinions
• use it to identify an unknown
• useful for small spectral differences that are difficult to distinguish visually