3 ms·

Quite interesting - the details get ever more complex as you dig deeper. So the incoming photons interact with the oscillations of the free electrons (i.e. "pla

Quite interesting - the details get ever more complex as you dig deeper. So the incoming photons interact with the oscillations of the free electrons (i.e. "plasmons"), causing new EM waves to be emitted. I take it that this interaction between photons and plasmons is equivalent to the changing of energy states of electrons and the release of photons as electrons fall to lower states. So in non-metals, does the band gap between the valence and conduction bands get crossed by electrons excited by incoming photons? You said a black cloth is able to absorb and emit virtually all visible light, but as infrared, but how do band gaps work there? I guess this is too advanced for a casual conversation with a layman like me, so thanks for the explanations. In the end, maybe it just boils down to "free electrons reflect light because that is what they do".

Goety 5y ago
>So in non-metals, does the band gap between the valence and conduction bands get crossed by electrons excited by incoming photons? It is an active area of research and they started exploring 'metallic ammonia' last year. recent paper looking at metallic ammonia using synchrotron x-ray PES in a refrigerated liquid microjet setup: https://science.sciencemag.org/content/368/6495/1086 https://science.sciencemag.org/content/368/6495/1086 They are currently known as 'metastable' nanostructures exhibiting plasmonic effects similar to noble nanoparticles.
- creamyhorror 5y ago
  Hmm, I was referring to typical non-metals rather than ones with excess electrons added, but thanks for the glimpse into current research!
pas 5y ago
I'm far from an expert either! The important thing is that the sea of electrons in metals are able to interact with EM waves coherently, so phase is conserved, so the classic ray optics happens, an that's why we can see things in a mirror, while in case of a white non-mirror surface we have the absorption-emission, and in that case the emitted photons are going everywhere, so we just see a big homogeneous surface. > I take it that this interaction between photons and plasmons is equivalent to the changing of energy states of electrons and the release of photons as electrons fall to lower states. Yes, except "better", because it keeps the phase. > band gap So there's the optical band gap and the electric. (The electric is required for charge transport, so it's sometimes also called transport gap.) So a cloth is an insulator, in an insulator the band gap is pretty big. The incoming photon does not lead to transport, but it leads to optical excitation. And then an emission of a photon with a lower energy. In some instances the relatively high energy of the incoming photon gets converted through various radiationless processes into other forms of energy. For example molecular rotation and vibration (usually photons in the infrared and microwave range do this to matter). These also lead to at least one quantum number change [as every interaction in a quantum theory must], but in case of UV or visible light the electron getting excited and jumping to a higher atomic orbital shell is in an atom that's bound in a molecule. The incoming EM field wave is not high energy enough to "penetrate into" the atom, it interacts with the whole sum of the EM field of the molecule. (And the energy basically gets transferred into the molecular bond: https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy#Principle_of_ultraviolet%E2%80%93visible_absorption https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_sp... ) See also: https://en.wikipedia.org/wiki/Hot_band https://en.wikipedia.org/wiki/Hot_band
- creamyhorror 5y ago
  Thank you for the reply and for highlighting the importance of phase being preserved!

Goety 5y ago

>So in non-metals, does the band gap between the valence and conduction bands get crossed by electrons excited by incoming photons? It is an active area of research and they started exploring 'metallic ammonia' last year. recent paper looking at metallic ammonia using synchrotron x-ray PES in a refrigerated liquid microjet setup: https://science.sciencemag.org/content/368/6495/1086 https://science.sciencemag.org/content/368/6495/1086 They are currently known as 'metastable' nanostructures exhibiting plasmonic effects similar to noble nanoparticles.

creamyhorror 5y ago
Hmm, I was referring to typical non-metals rather than ones with excess electrons added, but thanks for the glimpse into current research!

pas 5y ago

I'm far from an expert either! The important thing is that the sea of electrons in metals are able to interact with EM waves coherently, so phase is conserved, so the classic ray optics happens, an that's why we can see things in a mirror, while in case of a white non-mirror surface we have the absorption-emission, and in that case the emitted photons are going everywhere, so we just see a big homogeneous surface. > I take it that this interaction between photons and plasmons is equivalent to the changing of energy states of electrons and the release of photons as electrons fall to lower states. Yes, except "better", because it keeps the phase. > band gap So there's the optical band gap and the electric. (The electric is required for charge transport, so it's sometimes also called transport gap.) So a cloth is an insulator, in an insulator the band gap is pretty big. The incoming photon does not lead to transport, but it leads to optical excitation. And then an emission of a photon with a lower energy. In some instances the relatively high energy of the incoming photon gets converted through various radiationless processes into other forms of energy. For example molecular rotation and vibration (usually photons in the infrared and microwave range do this to matter). These also lead to at least one quantum number change [as every interaction in a quantum theory must], but in case of UV or visible light the electron getting excited and jumping to a higher atomic orbital shell is in an atom that's bound in a molecule. The incoming EM field wave is not high energy enough to "penetrate into" the atom, it interacts with the whole sum of the EM field of the molecule. (And the energy basically gets transferred into the molecular bond: https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy#Principle_of_ultraviolet%E2%80%93visible_absorption https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_sp... ) See also: https://en.wikipedia.org/wiki/Hot_band https://en.wikipedia.org/wiki/Hot_band

creamyhorror 5y ago
Thank you for the reply and for highlighting the importance of phase being preserved!