Understanding the Raman Spectra of Metals

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Discussion Overview

The discussion revolves around the absence of Raman peaks in pure metals and the underlying reasons for this phenomenon. Participants explore the relationship between electromagnetic wave penetration and the polarizability of metals, as well as comparisons with materials like diamond and graphene that exhibit Raman activity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why pure metals do not exhibit Raman peaks, suggesting that the inability of visible light to penetrate metals may be a factor.
  • Another participant explains that for a material to be Raman active, it must have vibrations that change its polarizability, noting that metals are approximated as infinitely polarizable, which complicates this process.
  • A different participant expresses confusion over the contrast between metals and materials like diamond and graphene, which do show Raman peaks, and questions whether the penetration of visible light into metals is a contributing factor.
  • One participant proposes that Raman peaks in metals might be observable using UV light that exceeds the plasma frequency.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the reasons behind the absence of Raman peaks in metals, and multiple competing views regarding the role of light penetration and polarizability remain present.

Contextual Notes

There are limitations regarding the assumptions made about the polarizability of metals and the conditions under which Raman activity may occur in small clusters or thin films, which remain unresolved.

jrobin
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Hi all,

I'm just wondering why pure metals, whatever kinds, do not have Raman peaks. Because the visible light cannot penetrate the metals? If so, what kind of EM wave can penetrate metals? Just need some key words for further information. Of course, I would appreciate if you would explain more for me. Thank you!
 
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jrobin said:
Hi all,

I'm just wondering why pure metals, whatever kinds, do not have Raman peaks. Because the visible light cannot penetrate the metals? If so, what kind of EM wave can penetrate metals? Just need some key words for further information. Of course, I would appreciate if you would explain more for me. Thank you!

In order for a species to be Raman active, it must have vibrations (or rotations) that result in a change in the polarizability of the molecule or material. Since metals are infinitely polarizable (to a good approximation), it is hard to see how a vibration of the atoms in the crystal lattice could cause a change in the polarizability. Of course this description is only approximate ... it may be that small clusters of metal atoms, or perhaps even thin films, do show Raman activity to some extent.

I am not certain that this explanation is completely accurate, but I do think it is qualitatively correct with respect to your question.
 
First of all, thank you so much SpectraCat for the kind discussion.

I was thinking of it in the similar way in the first place, however, it kind of confused me when I saw that diamond and graphene (all composed by "atom" with specific orientations in crystal) actually have Raman peaks.

Would it be possible that it's because the visible light actually is hard to penetrate the metals?
 
I would guess that you can observe Raman peaks in metals using UV light above the plasma frequency.
 

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