Why do pure metals not show Raman peaks?

[jrobin]

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!

 

[SpectraCat]

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.

 

[jrobin]

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?

 

[DrDu]

I would guess that you can observe Raman peaks in metals using UV light above the plasma frequency.