In-class Questions: Raman Spectroscopy

I begin this section by indicating that Raman spectroscopy is a way to probe vibrations of molecules. I also describe that, whereas IR spectroscopy could probe vibrations that caused a change in the overall molecular dipole, Raman spectroscopy probes vibrations where a change in the polarizability of the molecule occurs.

What does polarizability mean? How do things like bond strength and bond length affect the polarizability?

From prior experience, the groups can usually determine the correct answers.

Before getting into virtual states and the use of visible light, I give them the next question.

1. Consider the molecular vibrations of carbon dioxide and determine whether or not they are Raman active.

We have already examined carbon dioxide in the unit on IR spectroscopy so they already know the vibrations.

Consider each bond in a particular vibration and determine whether the polarizability of that bond decreases, increases or stays the same for the symmetric and asymmetric stretches.

They can usually reason through the changes in polarizability of the bonds as you lengthen or shorten them in the symmetrical and asymmetrical stretches.

Considering what happens for each bond, are the symmetrical and asymmetrical stretch Raman active?

They can usually reason out that the symmetric stretch will be Raman active but that the effects in the asymmetric stretch cancel each other out such that it is Raman inactive.

Does the polarizability of each bond change in the bending vibration?

They usually realize that it won’t change and can rationalize that the binding vibration is Raman inactive.

Compare the activity of the vibrations in the IR and Raman mode.

They reflect back and see that they are completely complementary – those that are IR active are Raman inactive and vice versa.

I then explain that the technique will use a visible light source and explain the idea of a virtual state. I also describe the processes that lead to the origin of Stokes and anti-Stokes lines.

2. Which set of lines, Stokes or anti-Stokes, is weaker?

We have talked often enough earlier in the course about the populations of energy levels so the groups tend to readily appreciate that the anti-Stokes lines will be weaker.

3. What effect would raising the temperature have on the intensity of Stokes and anti-Stokes lines?

They can rationalize out that increasing the temperature raises the population of vibrationally excited molecules such that the intensity of the anti-Stokes lines will increase.

4. What would be the ideal source to use for measuring Raman spectra?

What general criteria have been important in all spectroscopic measurements?

They can usually come up with resolution and sensitivity. I remind them that Raman scatter is a weak process. They realize that a high powered laser will provide better sensitivity than a continuum source.

Is a laser better than a continuum source when trying to measure the frequency of the Raman bands?

They already know that lasers are highly monochromatic and realize that this is another reason to use a laser.

5. The molecule carbon tetrachloride (CCl₄) has three Raman-active absorptions that occur at 218, 314 and 459 cm^-1 away from the laser line. Draw a representation of the Raman spectrum of CCl₄ that includes both the Stokes and anti-Stokes lines.

I first point out that there will be Rayleigh scatter at the laser line and draw a representation of the Rayleigh line in the spectrum. I also indicate the higher and lower energy side of the spectrum relative to the Rayleigh line.

Draw the placement of the Stokes and anti-Stokes lines of CCl₄.

I remind them to look back at the energy level diagram as they think through where to place each specific line. Usually they can rationalize out within their group that the placement of the Stokes and anti-Stokes lines from the Rayleigh scatter ought to be mirror images of each other.

6. Why do the anti-Stokes lines of carbon tetrachloride have the following order of intensity: 218 > 314 > 459 cm^-1?

I suggest that they consider the energy level diagram and think about the transitions that correspond to each of these. They can rationalize that the line for the 218 cm^-1 band originates by forming a virtual state from the most populated vibrational level and the band for 459 cm^-1 from the least populated vibrational level, which explains the relative intensities.