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How do amorphous objects emit blackbody/thermal radiation when such objects don't have optical phonons?

adam3033
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  • See Luboš Motl's answer to the question I've linked. The crystallinity or otherwise of the material makes litle difference. – John Rennie Oct 03 '15 at 06:07
  • @JohnRennie Thanks. I looked at that page but couldn't find info regarding amorphous solids. What really confused me is, if there aren't ions or dipoles in amorphous solids, then how can there be thermal radiation? You would need the molecules to each have a dipole, correct? – adam3033 Oct 26 '15 at 23:20
  • Hi Adam. The radiation is mainly from collective oscillations in the electron density. Remember that in solids the electrons are delocalised into bands, and it is oscillations in this diffuse mass of electrons that causes the radiation. If you consider an element in solid form it doesn't have any molecules and therefore can't have molecular dipoles, but such solids emit BB radiation just like everything else. Amorphous solids have a band structure that isn't vastly different from crystalline solids, ust less well defined. – John Rennie Oct 27 '15 at 06:06
  • @JohnRennie thanks so much for your helpful response. And, I came across something you were saying on another thread: 'Typically thermal vibrations in whatever material you're looking at result in accelerated electrons and oscillating dipoles within the material, and both generate the electromagnetic waves.' -- when you mention accelerated electrons, are you referring to oscillations in the electron density? – adam3033 Oct 27 '15 at 07:18
  • When I say oscillating dipoles within the material I am referring to oscillations in the electron density. These oscillations cause oscillating dipoles in the electron gas that makes up the energy bands. The dipoles aren't due to individual molecules. – John Rennie Oct 27 '15 at 07:23
  • @JohnRennie thank you for clearing that up. Finally, what would 'accelerated electrons' be in reference to? (This was from http://physics.stackexchange.com/questions/71503/what-causes-a-black-body-radiation-curve-to-be-continuous) – adam3033 Oct 27 '15 at 07:40
  • Any electron that accelerates radiates EM. Oscillating dipoles are a special case of acceleration. There will be more general fluctuations in the electron density that are well described as oscillating dipoles but that still radiate. The phrase accelerated electrons is just meant to capture the more ill defined fluctuations. – John Rennie Oct 27 '15 at 07:42
  • @JohnRennie so it wouldn't be best to use the term 'acceleration' to describe the more general fluctuations in the electron density? – adam3033 Oct 27 '15 at 08:10
  • I'm not sure how much mileage there is in arguing about the exact phrasing. The physics behind the creation of black body radiation should be clear. – John Rennie Oct 27 '15 at 08:19
  • I hear ya. I guess when you said oscillating dipoles are a special case of acceleration, I took that as including the general fluctuations. And then meaning the I'll defined fluctuations were another type of acceleration. – adam3033 Oct 27 '15 at 08:32
  • @JohnRennie -- let me just say thank you, again, for taking the time to help me. And please don't think I was trying to debate any issue here. I am just trying to get a good grasp of some concepts that I've not been able to get clear answers on in the past. I just want to leave you with the following question, though, to know if I'm way off base or not. Are the oscillating dipoles specifically the periodic motion of positive and negative ions within the molecules or technically something else? – adam3033 Oct 27 '15 at 18:37
  • I must admit that I've never thought of exactly what the dipoles look like. Both the nuclei and the electrons will be vibrating, so the motion of both nuclei and electrons will contribute. – John Rennie Oct 28 '15 at 06:02
  • @JohnRennie I've been studying up more on blackbody radiation. From my understanding, if a solid is made up of just molecules (say billions of molecules), then it would be the molecular dipole of each molecule, in the solid, causing blackbody radiation in the infrared range. Now, this makes me wonder b/c you'd said typically any material results in oscillating dipoles but not of individual molecules. You seemed to be referencing, perhaps, another type of dipole? If so, did you mean dipoles in the valance band? And for metals, dipoles in the conduction band? And both bands can emit infrared? – adam3033 Feb 24 '16 at 02:46
  • @JohnRennie just not understanding how there can be oscillations in electron density, i.e. surface dipole, for non metals. With non metals, there aren't delocalised electrons in the conduction band. And you'd made clear that this type of dipole (oscillations) were in reference to whatever material you're looking at. If you could clarify this it would help tremendously. – adam3033 Feb 28 '16 at 07:48
  • In a solid the electrons are delocalised into bands, and lattice vibrations (propagation of phonons if you prefer that description) mean the density of the nuclei oscillates relative to the density of the electrons. This inevitably causes transient oscillating dipoles. The bands may be partially filled in a metal or completely filled in an insulator, but the principle remains the same. There will also be vibrations specific to molecules in the solid, but these will normally be non-black body i.e. they will create distinct lines/bands superimposed on the black body spectrum. – John Rennie Feb 28 '16 at 08:08
  • I understand what you're saying. I'm just confused regarding non-metallic solids and how they emit blackbody (infrared). For non metals, would it be the electrons in the valance band that are the transient oscillating dipoles (due to there not being electrons in the conduction band for non metals)? – adam3033 Feb 28 '16 at 09:00
  • Yes. In principle the electrons in lower energy bands will contribute as well, though in practice I suspect that with decreasing energy the electrons become too localised to contribute much. – John Rennie Feb 28 '16 at 09:03
  • So inner core states involving orbitals in shells closer to the nucleus probably not really much contribution (at best, minimal) and with the valance electrons contributing a bit more. For liquids and very dense gases, are there bands with dipoles emiting, or is it just the molecular dipoles that would be emiting an amount of blackbody? – adam3033 Feb 28 '16 at 09:22
  • Dilute gases (e.g. air at STP) don't emit BB radiation just the sharp emission lines. As you increase the density at some point you'll get a BB like spectrum, but offhand I don't know where that point is. – John Rennie Feb 28 '16 at 10:14
  • That would definitely make sense. Let me leave you with one other question. When speaking of oscillations in electron density, exactly what type of oscillating motions are taking place with the electrons themselves? – adam3033 Feb 28 '16 at 10:37
  • In the conduction band I see the electrons as having the room to randomly oscillate. It was more the valance band I was thing of -- how the electrons have the freedom to oscillate... – adam3033 Feb 28 '16 at 11:51
  • The electrons oscillate relative to the nuclei, so even if the electrons can't change their momentum (because the band is full) the vibrations of the nuclei will create oscillating dipoles. – John Rennie Feb 28 '16 at 11:53
  • I know you said that even if the electrons can't change their momentum (due to the band being full) the vibrations of the nuclei will create the oscillating dipoles. Just trying to figure out in what manner the electrons would oscillate though. Of course there is orbital angular momentum, but I don't think that's what you're referring to.. – adam3033 Feb 28 '16 at 15:05
  • I understand you when you say that both the nucleus and electrons in each of the atoms are oscillating relative to one another, but just don't understand in what type of manner. I know electrons can jump levels and fall, can be in a superposition, i.e. Rayleigh scattering, be described as having a non-classical vibration when in their orbitals, move freely in the conductive band, but this other type of oscillation eludes me. Is there any possible way to describe these oscillations other than just being relative to one another? – adam3033 Feb 29 '16 at 15:37
  • You really need to raise a new question if you want to go into the details of what exactly happens, though if you're looking for a specific mechanism I suspect you will be disappointed. Any random fluctuations in charge density can be expanded as a set of multipoles and are likely to have a non-zero dipole component, and that dipole component will radiate. – John Rennie Feb 29 '16 at 16:40
  • These random motions of the electrons...could they be due to, perhaps, the periodic motion of the lattice. In other words, when the atoms shift back and forth in the lattice, or atoms move in molecules as part of some type of normal mode, it causes the electrons to fluctuate while they're moving with the atom, with the orbitals shifting ever so slightly? Similar to a car driving up and down the same street in a seemingly straight line but with a bit of swerving along each way? – adam3033 Feb 29 '16 at 17:58
  • Hey John...I just asked the question separately as you'd suggested. I can't seem to get a grasp on this. Again, my biggest concern was regarding non-metallic solids with just atoms or ions and how there can be oscillations in electron density in the valence band or shells if full. I was thinking maybe you were referring to the stretching of the atomic or ionic bonds and those being the oscillations that produce radiation? Here's the link: http://physics.stackexchange.com/questions/242657/in-non-metallic-solids-w-just-atoms-or-ions-no-molecules-are-bonds-vibratio density in the valence – adam3033 Mar 10 '16 at 20:30

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