Raman spectroscopy

Question 1

What is molecular fingerprint?

Answer 1

Generally speaking, vibrational and rotational motions are unique for every molecule. The uniqueness to molecules are in analogous to fingerprint identification of people hence the term molecular fingerprint.

Question 2

What does Raman spectroscopy measures?

Answer 2

• Scattered light
• There are three types of scattered lights: Rayleigh scattering, Stokes scattering, and anti-stokes scattering. Rayleigh scattering is elastic scattering where there is no energy exchange between the incident light and the molecule. Stokes scattering happens when there is an energy absorption from the incident light, while anti-stokes scattering happens when the molecule emits energy to the incident light. Thus, Stokes scattering results in a red shift, while anti-stokes scattering results in a blue shift. (Figure 1) Stokes and Anti-Stokes scattering are called Raman scattering which can provide the vibration/rotation information
• The intensity of Rayleigh scattering is around 107 times that of Stokes scattering. [4]According to the Boltzmann distribution, anti-Stokes is weaker than Stokes scattering. Thus, the main difficulty of Raman spectroscopy is to detect the Raman scattering by filtering out the strong Rayleigh scattering.

Question 3

Label the schematic diagram

Answer 3

Question 4

How to reduce the intensity of Rayleigh scattering?

Answer 4

In order to reduce the intensity of the Rayleigh scattering, multiple monochromators are applied to selectively transmit the needed wave range. An alternative way is to use Rayleigh filters. There are many types of Rayleigh filters. One common way to filter the Rayleigh light is by interference.

Question 5

What are the advantages of Raman or IR?

Answer 5

• Raman Spectroscopy can be used in aqueous solutions (while water can absorb the infrared light strongly and affect the IR spectrum).
• Because of the different selection rules, vibrations inactive in IR spectroscopy may be seen in Raman spectroscopy. This helps to complement IR spectroscopy.
• There is no destruction to the sample in Raman Spectroscopy. In IR spectroscopy, samples need to disperse in transparent matrix. For example grind the sample in solid KBr. In RS, no such destructions are needed.
• Glass vials can be used in RS (this should only work in the visible region. If in UV region, glass is not applicable because it can strongly absorb light too.)
• Raman Spectroscopy needs relative short time. So we can do Raman Spectroscopy detection very quickly.

Question 6

What is the difference between IR and Raman, due to its resolving power?

Answer 6

Because of the weakness of Raman scattering, the resolving power of a Raman spectrometer is much higher than an IR specctrometer. A resolution of 105 is needed in Raman while 103 is sufficient in IR. [5] In order to achieve high resolving power, prisms, grating spectrometers or interferometers are applied in Raman instruments.