Raman Scattering Flashcards
What is Raman spectroscopy?
Related to vibrational spectroscopy, but is a scattering technique
What does Raman spectroscopy do?
Looks at molecular vibrations (and rotations), with different gross selection rule, need to change the “polarisability”, not the dipole moment – related to how easily the molecule can distort; a two-dimensional effect, [more formally, the change in polarisability alpha leads to a induced dipole moment during the transition], can be very helpful – some vibrational modes that are IR inactive will be Raman active
Gross selection rule for Raman spectroscopy?
Polarisability, needs to have polarisability to be Raman active
delta v = ± 1 molecule must show a change of polarisability
homonuclear diatomics do show up (unlike infra-red), Raman spectra can show lines that do not occur in IR spectra, can find force constants and other information as with IR
What does Raman scattering rely on?
Still relies on our understanding of energy levels, but in this case incoming energy does not need to be matched to the transition we want to observe, instead, energy differences are measured
When a molecule can be irradiated what can the energy do?
be absorbed; be transmitted; be scattered at the same energy (ie elastic; Rayleigh) [small amount, 10-3], be scattered with a different energy (ie inelastic) [very small amount : < 10-7]
What is important when the energy changes?
When the energy changes, the shift of energy E is important – corresponds to vibrational (or rotational) energies
What happens during Raman spectroscopy?
Spectrometer uses UV or visible monochromatic radiation from a laser, sample is irradiated with a laser, some is scattered and exchanges energy with vibrational modes but most light passes straight through unchanged
Scattering modes what is Rayleigh scattering?
Most is scattered with no change in energy – Rayleigh Scattering
Scattering modes when the energy is shifted?
But some is shifted by the energy that is gained or lost by the incident photon, the difference in energy from the incident radiation is measured
Scattering modes, Stokes line?
Sometimes, the photon energy excites vibrations in a molecule so that the scattered photon emerges with lower energy, Energy to the molecule —> “Stokes lines”
Scattering modes, Anti Stokes line?
Less common - a photon hits a molecule that is already in an excited state and gains extra energy to emerge with a higher energy
Energy from the molecule –> “anti-Stokes lines”
On a spectrum what do we observe?
Stokes lines the photon loses some energy to excite the molecule, Main peak photon does not exchange energy with the molecule, Anti-Stokes lines the photon gains energy from an excited molecule - Weaker
Presentation of Raman spectra?
To present a diagram of intensity versus wavenumber of the vibrational transition, the Raman shift is plotted on the x-axis
Why is lambda0 the incident wavelength and the lambda 1 the scatter wavelength used?
This convention is chosen so that the Stokes lines (energy loss from the incident beam) have a positive wavenumber value, They are the strongest lines, so this makes sense
Comparison of Raman and IR spectra?
The vibrations present are the same but the different gross selection rules and the different radiation used in Raman and IR mean the spectra show some differences, these can help in interpreting the spectra since Raman spectra can look cleaner
How do different gross selection rules affect IR and Raman spectra?
Different gross selection rules (polarizability compared to dipole change) means that IR is very good for heteronuclear functional group variations (e.g terminal group stretches), while Raman is very good for homonuclear bonds (e.g. distinguish between C-C, C=C and C triple C bonds)
Water as solvent in IR and Raman?
Water is a weak Raman scatterer, but absorbs IR strongly, so its use as a solvent is Ok in Raman, not so good in IR
Sample prepration in IR and Raman?
Raman requires very little sample preparation, while IR has constraints on sample thickness, uniformity and dilution
Disadvantages of Raman?
Raman suffers from the effects of fluorescence, which can affect the pattern; IR does not have this problem
Raman scattering is weaker, and intense laser beam can damage samples
IR is cheaper
Advantages of both spectra?
Used together, they provide complementary information
Rotational Raman spectra?
As with Vibration-Rotation Spectroscopy, we also observe rotational peaks in high resolution Raman spectra, As a molecule rotates, the polarisability seen by the EM field changes; induced dipole is modulated, rotational transitions can be seen
What molecules show Raman spectra?
All molecules except spherical rotors show rotational Raman, this includes homonuclear diatomics and those without a permanent dipole
Specific selection rule for Raman of linear molecules?
Different specific selection rule
delta J = (0), ± 2 for rotational Raman of linear molecules
Rotational Raman spectra?
Rotational Raman lines centred around the relevant vibrational frequency v0; with specific selection rule delta J = ± 2
Rotational Raman spectra Stokes line?
Stokes at v0 – B(4J + 6)
– First Stokes line is from J=0 (to
J=2); 6B from v0
Rotational Raman spectra anti Stokes line?
Anti-Stokes at v0 + B(4J – 2)
– First anti-Stokes line from J=2 (to J=0); again 6B from v0
How are subsequent rotational lines separated?
Subsequent rotational lines in each branch separated by 4B because of delta J = ± 2 selection rule
Reason for frequency differences?
When incident radiation collides with a molecule in the sample there is an instantaneous interaction involving transfer of energy, most molecules return to their original state and the scattered radiation has exactly the same energy as the incident radiation this is Rayleigh scattering, but a few molecules return to a state that is either of higher or lower energy than the one they started from the energy difference results in Raman shift of the scattered radiation to either a higher or lower energy and therefore frequency than the incident radiation
What does energy difference correspond to?
Difference between two energy levels in the molecules, energy difference is very small so laser light of very precise frequency is needed, laser light is directed at the sample and the Raman shift id measured, the frequency difference provides information about the energy levels in the molecules, these include both rotational and vibtriaon levels
Difference between IR and Raman?
Raman can provide similar information to both microwave and IR spectroscopy, unlike IR spectroscopy Raman spectroscopy gives peaks with homonuclear as well as heteronuclear molecules, also a non invasive technique since it involves scattering can b used to analyse sample without having to remove any of the material
