Why does Raman shift increase as temperature decreases?

In summary, the conversation discusses the question of why stokes/anti-stokes shifts increase in magnitude at lower temperatures and speculates about potential explanations, such as anharmonicity in the lattice. The speaker also mentions their inability to provide a definitive answer due to the lack of an answer key.
  • #1
gomboc
39
0
This was a quick short answer question from a previous final exam I'm studying from - the prof didn't provide an answer key.

I can't think of any physical reason why this should happen. I can explain why the stokes/anti-stokes intensities vary with temperature, and why the linewidths vary with temperature, but I can't explain why the stokes/anti-stokes shifts actually increase in magnitude at lower temperatures.

I can't think of any good reason why the zone center optical phonons would have higher energy at lower temperatures...
 
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  • #2
unless there is some kind of anharmonicity in the lattice that increases with decreasing temperature.
 

FAQ: Why does Raman shift increase as temperature decreases?

1. What is a Raman shift?

A Raman shift refers to the change in frequency of light that is scattered by a molecule. It is caused by the interaction between the light and the vibrational modes of the molecule. The Raman shift is typically measured in units of wavenumbers (cm^-1).

2. How does temperature affect Raman shift?

As temperature decreases, the Raman shift increases. This is because at lower temperatures, the molecules have less thermal energy and are therefore vibrating at a slower rate. This results in a larger difference between the incident light frequency and the scattered light frequency, leading to a higher Raman shift.

3. What is the physical explanation for the relationship between Raman shift and temperature?

The increase in Raman shift with decreasing temperature can be explained by the Boltzmann distribution. This distribution describes how the population of molecules is distributed among different vibrational energy levels at a given temperature. At lower temperatures, there are fewer molecules in higher vibrational energy levels, resulting in a larger difference between the incident light frequency and the scattered light frequency.

4. Is the relationship between Raman shift and temperature the same for all molecules?

No, the relationship between Raman shift and temperature can vary for different molecules. This is because different molecules have different vibrational modes and energy levels, which can affect how temperature affects their Raman shift. Additionally, the strength of the interaction between the light and the molecules, known as the Raman cross-section, can also play a role in the relationship between Raman shift and temperature.

5. What are the practical applications of studying the relationship between Raman shift and temperature?

Studying the relationship between Raman shift and temperature can provide valuable information about the vibrational properties of molecules. This information can be used in various fields such as chemistry, materials science, and biophysics. For example, Raman spectroscopy, which relies on the Raman effect, is a powerful analytical tool used for chemical identification and analysis of materials. Understanding how temperature affects Raman shift can also aid in the development of temperature-sensitive sensors and devices.

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