Raman Scattering

Question 1

What is Raman spectroscopy?

Answer 1

Related to vibrational spectroscopy, but is a scattering technique

Question 2

What does Raman spectroscopy do?

Answer 2

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

Question 3

Gross selection rule for Raman spectroscopy?

Answer 3

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

Question 4

What does Raman scattering rely on?

Answer 4

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

Question 5

When a molecule can be irradiated what can the energy do?

Answer 5

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]

Question 6

What is important when the energy changes?

Answer 6

When the energy changes, the shift of energy E is important – corresponds to vibrational (or rotational) energies

Question 7

What happens during Raman spectroscopy?

Answer 7

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

Question 8

Scattering modes what is Rayleigh scattering?

Answer 8

Most is scattered with no change in energy – Rayleigh Scattering

Question 9

Scattering modes when the energy is shifted?

Answer 9

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

Question 10

Scattering modes, Stokes line?

Answer 10

Sometimes, the photon energy excites vibrations in a molecule so that the scattered photon emerges with lower energy, Energy to the molecule —> “Stokes lines”

Question 11

Scattering modes, Anti Stokes line?

Answer 11

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”

Question 12

On a spectrum what do we observe?

Answer 12

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

Question 13

Presentation of Raman spectra?

Answer 13

To present a diagram of intensity versus wavenumber of the vibrational transition, the Raman shift is plotted on the x-axis

Question 14

Why is lambda0 the incident wavelength and the lambda 1 the scatter wavelength used?

Answer 14

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

Question 15

Comparison of Raman and IR spectra?

Answer 15

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

Question 16

How do different gross selection rules affect IR and Raman spectra?

Answer 16

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)

Question 17

Water as solvent in IR and Raman?

Answer 17

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

Question 18

Sample prepration in IR and Raman?

Answer 18

Raman requires very little sample preparation, while IR has constraints on sample thickness, uniformity and dilution

Question 19

Disadvantages of Raman?

Answer 19

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

Question 20

Advantages of both spectra?

Answer 20

Used together, they provide complementary information

Question 21

Rotational Raman spectra?

Answer 21

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

Question 22

What molecules show Raman spectra?

Answer 22

All molecules except spherical rotors show rotational Raman, this includes homonuclear diatomics and those without a permanent dipole

Question 23

Specific selection rule for Raman of linear molecules?

Answer 23

Different specific selection rule

delta J = (0), ± 2 for rotational Raman of linear molecules

Question 24

Rotational Raman spectra?

Answer 24

Rotational Raman lines centred around the relevant vibrational frequency v0; with specific selection rule delta J = ± 2

Question 25

Rotational Raman spectra Stokes line?

Answer 25

Stokes at v0 – B(4J + 6)
– First Stokes line is from J=0 (to
J=2); 6B from v0

Question 26

Rotational Raman spectra anti Stokes line?

Answer 26

Anti-Stokes at v0 + B(4J – 2)

– First anti-Stokes line from J=2 (to J=0); again 6B from v0

Question 27

How are subsequent rotational lines separated?

Answer 27

Subsequent rotational lines in each branch separated by 4B because of delta J = ± 2 selection rule

Question 28

Reason for frequency differences?

Answer 28

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

Question 29

What does energy difference correspond to?

Answer 29

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

Question 30

Difference between IR and Raman?

Answer 30

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