Ceri Hammond

Question 1

4 disadvantages of classic Raman

Answer 1

1. only 1 in 10^8 incident photons are raman scattered
2. Raman background (fluorescence)
3. spectral crowding and insensitivity
4. sample degradation and photolysis

Question 2

How can you avoid Raman background? (2 answers)

Answer 2

1. Irradiate sample with a laser outside of visible region preferably UV but IR will also work
2. pulse Raman

Question 3

How can you minimize insensitivity?

Answer 3

resonance enhanced raman

Question 4

4 advantages of Raman?

Answer 4

1. Provides info on a wide variety of sample types/phases. i.e solid liquid and gas, slurries, suspensions, and gels.
2. Fingerprint region information
3. tolerant of water
4. Direct structural information

Question 5

What causes the increased intensity of certain raman shifts during resonance raman?

Answer 5

when a sample is irradiated with a wavelength that matches one of the electronic transitions of the sample the Raman shift that the electronic transition corresponds to has an increased intensity due to the vibronic nature of resonance Raman

Question 6

what does the Krameis-Hesienburg Dirac equation show?

Answer 6

How polarizability changes according to the excitation line, when the energy difference and wavelength of excitation become close the equation tends to infinity. it never reaches infinty due to the dampening factor but it causes the polarizibilty to be very large and as intensity is related to polarizibilty that too becomes very large

Question 7

Four sources of Fluorescence

Answer 7

• fluorophores such as transition metal impurities
• defect sites
• hydrocarbon impurities
• anharmonic interactions

Question 8

How Pulse Raman methodology works?

Answer 8

• pulsed methods minimize the amount of fluorescent signal detected, by measuring Raman signals over very short timescales
• Raman 10^-11 - 10-^13 s
• Fluorensense 10^-6 - 10^-9 s

Question 9

Why UV raman avoids fluoresence?

Answer 9

• fluorescence is (predominantly) a visible region effect (ca. 350-700 nm)
• irradiation with a UV laser allows us to bypass fluorescence
• IR laser would also work but due to the 1/wavelength^4 relationship with intensity it is far more desirable for UV laser as smaller wavelength

Question 10

What is resonance enhancement Raman

Answer 10

-resonance enhancement occurs when the sample is irradiated with an exciting line whose energy corresponds to that of the electronic transitions of a chromophoric group in the molecular system

Question 11

Disadvantages of UV raman?

Answer 11

-Sample degradation

Question 12

-Primary parameters that influence the extent of degradation observed by UV Raman beam.

Answer 12

• the residence time of the material in the beam
• the excitation wavelength used (increasing degradation with decreasing λ)
• power density i.e. E0 (increasing power increases degradation)
• The physical/chemical properties of the material of interest
• optical properties
• thermal conductivity
• specific heat capacity
• photo/thermal stability

Question 13

challenges that are encountered whilst applying spectroscopic methods to the study of heterogeneous catalytic systems.

Answer 13

• The need to probe multiple phases (solid,liquid,gas)
• The need for obtaining surface/interface specificity
• The presence of interference effects (from e.g. bulk solvent)
• The non-uniform distribution of active sites
• For in situ study, obtaining kinetically-relevant data

Question 14

Minimise the sample degradation

Answer 14

• Use a fluid bed reactor to minimise the heat build up on the sample,
• minimises the length of time each particle is exposed to the beam