Raman Spectroscopy Flashcards
Rotational energy levels compared to vibrational
Both quantised but rotational are smaller
Raman spectroscopy involves …
Emitting light of a certain frequency and observing the light scattering off
3 types of scattering
Rayleigh scattering and stoke and anti-stokes Raman scattering
Rayleigh scattering
Frequency of light remains the same so no change in energy level.
hv_0—>hv_0
Raman scattering: stokes
Frequency of light is reduced since the bond is raised in energy level.
hv_0—> hv_0-hv_s
Where hv_s is the vibrational frequency of the molecule
Raman scattering: anti-stokes
Frequency of light is increased since the bond is dropped in energy level.
hv_0—> hv_0+hv_s
Where hv_s is the vibrational frequency of the molecule
What are the most likely forms of scattering and why?
Rayleigh and Stokes as anti-stokes must have bond at a high energy level which is unlikely due to maxwell-boltmann distribution.
Condition for Raman Spectroscopy
Bond must have changing polarizability
What is polarizability
The ease an electric field can induce a dipole moment
Polarizability depends on:
Flexibility of electrons and nuclei within the bond
Short compact molecule means…..(in terms of polarizability)
electron and nuclei don’t have much room to move so it is hard to induce a dipole moment
Long more stretched molecule means….(in terms of polarizability)
electron and nuclei have room to move so it is easy to induce a dipole moment
What can vibrations do in terms of polarizability?
Alter the shape making it more or less polarizable.
Mathematical description of condition for Raman spectroscopy:
dα/dr ≠ 0
Which vibration(s) for CO2 is/are polarizable?
Only symmetric stretch
