19.12: Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

Which type of scattering results in a longer wavelength than the incident light?

a	Rayleigh
b	Stokes
c	Anti-Stokes

Answer

A: Incorrect. Rayleigh scattering is elastic, and the wavelength does not change.

B: Correct. Stokes scattering results in a photon with a lower energy, and a longer wavelength.

C: Incorrect. Anti-Stokes scattering results in a photon with greater energy, and a shorter wavelength.

Which type of scattering is the strongest?

a	Rayleigh
b	Stokes
c	Anti-Stokes

Answer

A

Which of these properties must change for a mode to be Raman active?

a	Volume
b	Dipole moment
c	Polarisability

Answer

A: Incorrect, although the volume can affect the property which matters, and generally provides a useful guide.

B: Incorrect. This affects infrared absorption spectroscopy, not Raman.

C: Correct. The polarisability must not be constant on passing through the equilibrium position if a mode is to be Raman active.

Sulphur hexafluoride (SF6) is centrosymmetric. Which of these statements is true?

Yes	No	a	SF6 has an inversion centre.
Yes	No	b	SF6 obeys the rule of mutual exclusion.
Yes	No	c	Raman active modes will be IR active also.
Yes	No	d	IR active modes will be Raman inactive.
Yes	No	e	Modes will be Raman active if they involve a change in dipole moment.

Answer

A: Yes. This is the definition of centrosymmetry.

B: Yes. As a result of being centrosymmetric.

C: No. The rule of mutual exclusion prevents this.

D: Yes. This is from the rule of mutual exclusion.

E: No Raman activity depends on polarisability.

Can these alternative techniques be used to give a stronger signal than normal Raman spectroscopy?

Yes	No	a	Resonance Raman
Yes	No	b	Fourier transform Raman
Yes	No	c	Coherent Anti-Stokes Raman Spectroscopy
Yes	No	d	Stimulated Raman
Yes	No	e	Raman optical activity

Answer

A: Yes. Resonance effects greatly increase the Raman scattering.

B: No. Fourier transform Raman simply uses interference rather than diffraction to determine the spectrum.

C: Yes. A high intensity coherent beam results from the interaction of two lasers.

D: Yes. Stimulated Raman is 4 to 5 orders of magnitude stronger than normal Raman.

E: No. ROA compares the polarisation of light Raman scattered from chiral molecules.

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

What advantages does Raman spectroscopy have for process monitoring?

Answer

Possible answers:

It does not require any sample preparation.

It is non-destructive.

Raman spectra can be acquired quickly, allowing almost real-time monitoring.

Measurement can take place down fibre optic cables, so the Raman equipment can be located away from the processing equipment.

You may be able to think of others.

Calculate the wavenumber shift for the vibrational mode of Cl₂, given that the force constant k for the bond is 3.23 N cm^-1.

hint:

\[\omega=\sqrt{\frac{k}{\mu}}\]

Answer

\[k=3.23 \mathrm{~N} / \mathrm{cm}=323 \mathrm{Nm}^{-1}\]

\[\mu = \frac{35.45u}{2}=17.73\]

\[\nu = \frac{1}{2 \pi}\omega = \frac{1}{2 \pi} \sqrt{\frac{k}{\mu}}\]

\[\lambda = \frac{c}{\nu} = 2 \pi c \sqrt{\frac{k}{\mu}}\]

\[\tilde{\nu}=\frac{1}{\lambda}=\frac{1}{2 \pi c} \sqrt{\frac{k}{\mu}}=54494 \mathrm{~m}^{-1}=545 \mathrm{~cm}^{-1}\]