Spectroscopic Techniques Flashcards
What is photon scattering?
What are the two types of general scattering?
- photons (little packets of energy) from the EM spectrum (specifically in visible light, UV, IR region) interact with molecular vibrations in a material and are scattered
ELASTIC:
- photon that interacts with molecules leaves in same state (exactly the same energy, wavelength and frequency)
- RAYLEIGH SCATTERING
INELASTIC
- photon that interacts with molecules and leaves with a different quantum state (either higher or lower)
- RAMAN SCATTERING
What type of scattering occurs?
- majority is RAYLEIGH scattering (elastic)
- for every 1 in 10^7 scattering, there will be 1 RAMAN scattering
- there are two types of raman scattering (stokes and anti-stokes)
Define the two types of raman scattering?
What does it depend on to be either type?
stokes: photon has lost energy to the molecule
- light comes in and leaves with less energy as has lost energy to the molecule
- courier dropping off parcel
anti-stokes: photon has gained energy from the molecule
- light comes in and leaves with more energy as has gained energy from the molecule
- courier picking up parcel
- when photons interact with molecules, they only go to virtual state then drop back down again to ground state
- depends what ground state they drop to as to whether stokes/anti-stokes
What are the six types of molecular vibrations?
1 - symmetric stretching
2 - antisymmetric stretching
3 - in-plane scissoring
4 - in place rocking
5 - out of plane wagging
6 - out of place twisting
What is required for a molecule to produce a raman signal?
- it must be raman active:
- this means it must have anisotropic polarisability
- anisotropic = non uniform
- polarisability = ease of electron cloud distortion
- need a change in polarisability to be able to pick up raman signals
When considering linear molecules, what does the polarisability occur as a function of?
polarisability occurs as a function of the internuclear distance
Are H2 and HCl molecules raman/IR active?
H2:
- RAMAN ACTIVE: change in polarisation so
- IR INACTIVE: no change in dipole as two hydrogens have the same charge
HCl:
- RAMAN ACTIVE: change in polarisation so raman active
- IR ACTIVE: change in dipole as it has a charge
Define the mutual exclusion rule
in a centrosymmetric molecule (symmetrical around centre), a vibrational mode may be either raman or IR active but cannot be both
- where a mode is active in both raman and IR, one tends to be weak and one tends to be strong
- e.g. water is a weak raman scatterer but a strong IR interaction
Describe the difference between raman and IR (method, what it requires, sample prep, evidence bags, types of sample)
raman involves light scattering (laser visible light predominantly) whereas IR involves IR absorption
raman requires a change in polarisability whereas IR requires a change in dipole moment
raman requires little or no sample prep necessary (good for forensics as maintains integrity of sample) whereas IR requires sample prep
light can measure through evidence bags using raman but IR requires short optical path length so cannot do this with IR
raman is suitable for aqueous samples whereas IR is suitable for non-aqueous samples
Describe how a raman spectrometer works?
- laser source (blue, green, red, or infrared) produces light that hits focusing mirrors, goes through beam splitter, hits more focusing mirrors then through microscope objective
- light hits sample and interacts with molecule which scatters light
- the scattered light goes back to the same objective, focusing mirrors, beam splitter and focusing lens
- then goes through very important Rayleigh filter (contains edge and notch filter) to filter out Rayleigh scattered light
- raman scattered radiation then goes through diffraction grating (piece of metal with tiny grooves cut into it to allow light to go through) to break up light into its component parts
- this is captured onto charged coupled device detector (CCD)
- this takes light and converts it into electrical charge which then gives you a signal, then clever algorithm within software creates spectrum for you
What is compromised in smaller raman spectrometers?
smaller spectrometer = more compromise on spectral resolution
What does a raman spectra tell you?
- peaks can be used to provide structural information
- simple molecule e.g. diamond only has carbon so 1 type of bond = one intense narrow peak
- more complex molecule e.g. polystyrene has lots of different bonds = more complex spectrum
What can be said about raman of inorganic materials?
it is a well-rounded technique for both organic and inorganic
raman is typically superior to IR for investigations of inorganics. why?
- can’t see anything below 500 cm -1 on IR but metal-ligand bond vibrations around 100 - 700 cm-1
- on raman, the metal ligand bonds of metal oxides which makeup is full of is visible on raman spectra
what can be said about raman and IR spectra?
they can be complementary techniques as they can do different things and therefore allow for functional group assignments
Why is raman effect weak?
Through what seven techniques can raman signal intensity be improved?
raman effect is weak as only looking at 1 in 10^7 signals - this can cause problems with intensity but it can be enhanced
1 - stimulated raman spectroscopy (SRS)
2 - coherent anti-stokes raman (CARS)
3 - resonance raman spectroscopy (RRS)
4 - surface enhanced raman spectroscopy (SERS)
5 - tip-enhanced raman spectroscopy (TERS)
6 - coherent stokes raman spectroscopy (CSRS)
7 - surface enhanced resonance raman (SERRS)
