Lecture 7 - Resonance raman Flashcards
What is the difference between Raman and resonance Raman?
- The virtual state of resonance Raman is above the first electronic excited state.
How does resonance Raman scattering work?
- The laser is tuned to match the energy or electronic transition of the molecule being analysed (should match maximum absorbance).
- Absorption and scattering both happen
- Scattering is much faster than absorption, also a lot faster than scattering in normal Raman.
- Its a lot more sensitive than normal Raman as the scattering can be increased by 10E4 or more.
What is a chromophore? and why is it needed for resonance?
- Chromophores have conjugation (so usually has a ring), this makes the ring coloured.
- Now, applies to any conjugated system which absorbs UV or visible light.
- The laser wavelength must match this maximum absorbance
What is the difference between resonance Raman and a normal electronic absorbance?
- The time that the molecule stays in the excited state.
- Absorption is much slower than the scattering as the nuclei relaxes into the equilibrium geometry of the excited state.
- It will sit in the electronic excited state for longer than scattering and cause fluorescence.
How does resonance Raman get overtones? and what are they like compared to IR and normal Raman?
- Due to KHD, In resonance the majority of scattering is coming from 1 energy state, so A is no longer 0 and can be very large so can give overtones.
- The peaks are very large in intensity and there are lots, opposite from IR.
- Raman cant have overtones.
How do we get electronic info from resonance Raman?
- The electronic transition that it closest to the laser wavelength will be enhanced the most (UV-vis is done first to match the absorbance).
- So, some bands are enhanced more than others - Raman spec is simplified with dominating peaks
What are the sample decomposition problems of resonance Raman? and how are they reduced?
- Sample decomposition
Using an intense laser which has the same absorbance, could cause damage and photodegradation.
This can be reduced depending on the sample type.
For SOLIDS - the spin the sample in a disc or use a spinning sample holder.
For LIQUIDS - also using a spinning sample holder or flowing it past the beam. - Can also use a pre-resonance frequency
- Dilute the sample
-Less time as more time will cause burning
-Can also turn the sample so laser is not focused on 1 spot
What are the fluorescence considerations for resonance Raman?
- This comes from emission of light when degradation absorbs the light
- In resonance Raman, the virtual state is a lot higher so fluorescence is a lot worse.
Fixed by;
- Diluting sample
- Pulsed lasers
- Can use anti-stokes as the fluo occurs at lower E than the excitation frequency.
- Can use a higher wl laser, but might no longer see resonance enhancement so cant be very far
What are the main differences between resonance Raman and Raman?
Raman
- More vibrations seen
- No electronic info
- Weak
- More peaks in spectrum
- No overtones
Resonance
- Less vibrations seen as specific
- Electronic info
- More sensitive (10E4e more enhanced)
- Less peaks as specific
- Overtones
How can the overtones from resonance Raman spectra be put into a morse curve?
By measuring the difference between the peaks.
What is surfaced enhanced Raman scattering?
- Huge increase (106) in Raman scattering
- Requires close proximity of analyte to roughened metal surface
- A surface plasmon created on a smooth surface does not scatter light since it is confined to the metal surface. To create scattering, the surface requires to be roughened to create a component of the plasmon with a vector perpendicular to the surface.
- Consequently vibrations perpendicular to the surface experience largest enhancement
How did SERS come about?
- Unusually high Raman signals were obtained from pyridine adsorbed onto the surface of a silver electrode which had been electrochemically roughened by successive oxidation-reduction cycles
What is the SERS enhancement?
Comes from 2 effects
- Electrochemical mechanism - Interaction between the molecule and the surface plasmon at the metal surface
THIS IS THE DOMINANT EFFECT - Charge transfer mechanism - Requires the scattering molecule
to be chemically bonded to the surface
What roughened metal surfaces are used in SERS?
- Gold and silver give greatest enhancement but can also use Cu, Li or Na.
- Usually thin metals which are roughened on the nano scale.
- They can have specially engineered surfaces to give roughness.
- Can have colloidal suspensions (like dips) around 35 nm in diameter.
How does nanoparticle SERS work?
- The analyte is absorbed onto gold or silver nanoparticles which are then aggregated.
- Requires good surface attachment of analyte
- A Raman spectrometer used to get signals
Can be used for detection of multiple things like drugs.
