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In an answer to this SE question, the respondent explains that heating a perfect diamond will not cause it to glow with thermal blackbody radiation. I don't quite follow his explanation. I think it comes down to: there is no mechanism for diamond to generate light in the visible region of the spectrum.

He mentions that interband transitions are well out of the visual range, so there will be no contribution from that.

He mentions that the Debye temperature for diamond is > 2000 K. I presume that the argument here is that optical phonons will be frozen out, too. (But diamond doesn't have infrared-active phonons, does it?)

So is that why hot diamond doesn't glow?

I suppose that if one considers real (not ideal) crystals, imperfections, impurities, and the existence of surfaces lead to the possibility of emission mechanisms, and thus glow. In fact it might be the case that a finite but otherwise perfect crystal might have an extremely faint glow.

Is this basically the reason that hot diamond does not glow? Further elucidation welcome.

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2 Answers 2

This is because of Kirchhoff's law of thermal radiation. The corollary from it is that emissivity of a material is equal to its absorptivity.

As diamond is transparent even at large temperatures, which can be seen in this answer, its absorption coefficient is very low. Thus its thermal radiation in that spectral region is also very low.

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I bet that diamond will glow. you may not have heard of quenching before, but Soviet opticians love to tell the story of Vavilov. he studied luminescence, and before photomultipliers were invented, he used sit in the dark room for hours. the eye would get used to darkness, and become very sensitive. the observer would be able to "detect" a single photon. ok, I'm exaggerating, not a single photon but a few of them at once :) it's fascinating story, they developed quantitative techniques using their eyes.

anyhow, i bet that if you heat up the diamond, you'll see it glow in the dark.

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I fail to see how a question stating an answer as a bet is worthy of an upvote. –  Almo Mar 4 at 22:26
I have sat in the dark to "see" scintillation (which I would normally observe with a PMT) so I understand what you are talking about in principle. But it would be better if you tried to estimate (or find data for) the intensity that you might expect compared to a "real" black body. –  Floris Nov 20 at 20:56

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