Spectroscopic Techniques
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What are the two types of scattering?
• Rayleigh: elastic - leaves the molecule in the same state • Raman: inelastic - leaves the molecule in a different quantum state
What are photons produced from?
• electromagnetic radiation
What do photons do in electromagnetic radiation?
interact with molecular vibrations in a material and are scattered
What is Raman scattering photon?
has a different energy to that of the incident photon
What are the two types of Raman scattering?
• Stokes: the photon has lost energy to the molecule • Anti-Stokes: the photon has gained energy from the molecule
What happens when a photon interacts with a molecule in Raman scattering?
they go into a virtual state and then drop back down to the ground state
What happens in stokes scattering?
start at a lower vibrational state, then enter a virtual state, then fall back down to a higher vibrational state than at the start
What happens in anti-stokes scattering?
starts at a higher vibrational state, then enters a virtual state, then falls back down to a lower vibrational state than at the start
What is the type of scattering that majority of light scattered with?
Rayleigh
What are the different molecular vibrations?
• symmetric stretching • antisymmetric stretching • in-plane scissoring • in-plane rocking • out-of-plane wagging • out-of-plane twisting
What must a molecule be to be Raman active?
anisotropic polarisability - change in polarisation
What must a molecule be to be IR active?
change in dipole
What is the mutual exclusion rule?
in a centrosymmetric molecule a vibrational mode may be either Raman or IR active but it cannot be both
What happens when a mode is active in both Raman and IR?
one tends to be weak and the other tends to be strong - e.g. water
Raman vs IR
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
How does a Raman spectrometer work?
• 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
Order the forms of scattering in terms of strength
• Rayleigh • Stokes • anti-stokes
What does Raman ususally look at?
inorganics but can also look at organics
How do you assign functional groups with Raman?
complementary to IR spectra
Why would you use Raman microspectroscopy?
• 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
Why is chemometrics used for Raman and IR spectrums?
• 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