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