Split from Spectroscopy Discussion Thread

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Discussion Overview

The discussion revolves around the principles and mechanisms of Raman spectroscopy, particularly focusing on how molecular vibrations and rotations are excited during scattering when using a visible laser. Participants explore the nature of interactions involved and the resulting spectral outputs in the infrared and microwave regions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how vibrations or rotations can be excited during scattering in Raman spectroscopy, suggesting a lack of understanding regarding the interaction that induces these levels.
  • Another participant references a basic spectroscope design from Pauling's General Chemistry, expressing curiosity about the potential results from using a telescope with a prism and ruled grating, indicating a desire to understand practical applications.
  • A participant clarifies that in Raman spectroscopy, a molecule is excited to a virtual state and that the difference in energy levels measured results in spectral lines in the infrared region, while the excitation occurs in the visible range.
  • There is a mention of previous discussions on the topic, indicating that some aspects have been addressed elsewhere, but further clarification is sought regarding the absorption of rotations and vibrations in the Raman spectra.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and curiosity about Raman spectroscopy, with some seeking clarification on specific mechanisms while others provide insights. No consensus is reached on the fundamental questions posed, and multiple viewpoints are presented.

Contextual Notes

Some participants acknowledge that their understanding may be limited or that they are revisiting concepts from earlier studies, which may affect their contributions. The discussion includes references to prior knowledge and assumptions that may not be fully articulated.

photon79
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A laser in visible is used in Raman spectroscopy and the resultant spectrum is in IR and Microwave region(which means molecular vibrations and rotations are excited), what type of interaction is this that induces these levels? ( I cannot think more than..."To be Raman active molecules should posses anisotropic polarizability".)my question is how vibration or rotation can be excited during the scattering? (As the source used is visible, I think some basic understanding is missing from my side)
 
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THis is all very foreign to me and several semesters away, but...

reading through Pauling's General Chemistry, there is a sketch of a simple spectrocope.

It is simply a network of a telescope positioned towards a prism at an angle respective to a collimator. The objective is to use a light source to shine through the collimator, refract it into the prism to allow the light to be seen by the observer looking through the telescope. It also states that a ruled grating can be substituted for diffraction.

I have a telescope but no prism and collimator. But if I were to buy a collimator, prism, and a ruled grating, what kind of results what I get back using this system, what could I possibly achieve?
 
Last edited:
Just a note that I've moved these posts from the Spectroscopy Discussion Thread. The original post in that thread seemed to get eaten by the PF Gremlins, and as a result, the discussion drifted off topic. These are questions worthy of their own thread.
 
photon79 said:
A laser in visible is used in Raman spectroscopy and the resultant spectrum is in IR and Microwave region...
Note : This question has been answered in other places, like here. Photon79 is aware of this.

photon79 said:
Now its much clear to me.
My question was "How can we absrove rotations and/or vibrations in Raman spectra where we use visible source?"
Here what happens is molecule is excited to a virtual state (as Gokul said)say 20,000 Cm-1 which is in visible and scattered at lower vibrational say, 19,000 Cm-1 which is also in visible , what we measure in Raman spectra is difference in these levels 1,000 Cm-1 which is in IR. This is the basis and we get spectral lines in visible but we take the difference which will be in Microwave or IR.

More info and any currections are appriciated.

So [Moonbear], you could probably delete the OP here and leave the thread with plastic photon's question. <and then delete this post too>
 

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