Threshold thermal/microwave emissions from hot objects

[CopperCone]

So things have a blackbody spectrum they emit. If you get some stuff really hot it will make terahertz even, so you seem to have a bell curve with a peak that moves around based on object temperature. Like a glowbar is kind of a weak THZ source despite being broad band.

Does size of the radiator come into play at all? I was thinking about smokescreen that the military uses, and they sometimes use little wire strands and shit they glow hot to mess with thermal imaging. If you heat tiny particles, say of something hard to melt and vaporize, like tungesten alloys, which are seperated in a gas or vacuum, what exactly happens? Does it have any effect? I typically think of blackbodies with large objects.. but when you get down to nanoscales its just a handful of atoms so I am curious what happens.

Also does shape effect it at all? Like having little bars, balls, etc

like take this alloy
http://www.dailymail.co.uk/sciencetech/article-3178616/Have-scientists-invented-real-life-adamantium-New-alloy-highest-melting-point-known-substance-4-126-C.html

do you get some kind of plasma conductor around the surface of it if its so hot because of the vapor pressure? What if its like laser heated in a super field that wisks away a plasma surface quickly to reduce its effective size, but then it becomes some kind of conductive tear drop shape thing?

[tomato]

So things have a blackbody spectrum they emit. If you get some stuff really hot it will make terahertz even, so you seem to have a bell curve with a peak that moves around based on object temperature. Like a glowbar is kind of a weak THZ source despite being broad band.

The blackbody radiation from an object at liquid nitrogen temperature (77K) peaks at about 8 THz. Even at liquid Helium temperature (4K), there is blackbody radiation in the THz range.

[CopperCone]

does it depend on particle size at all? i guess like, what is the minimum number of atoms you need for it to work and have temperature as a solid? How do spread out gasses emit?

I know with gasses you get emission lines, like measuring a star, but what about some kind of small particle thats not quite an atom under vacuum. Do they have kind of unique signatures?

[ejeffrey]

Yes, size matters.  Sub-wavelength particles don't emit well.  That is why e.g., clean burning flames are blue and hydrogen flames are basically invisible.  The gas molecules in a hydrogen flame are plenty hot but they are too small to effectively emit visible light.  A propane torch has small amounts of carbon soot since propane is a hydrocarbon.  However, the soot particles are very small and don't effectively emit black body radiation.  Because blue is shorter wavelength than red, the flame emits a small amount of blue light. A similarly hot solid object would look white and be much brighter as the blue would be joined by green and red and all wavelengths would be much more strongly radiated.  A rich or poorly mixed flame burns cooler and generates larger soot particles.  These larger soot particles can emit the entire visible spectrum, and will be various colors depending on the temperature and composition.

[CopperCone]

so what do they emit? I mean does it just stop, go to a different band, etc?

Does soot have some kind of weird spectrum like emiting extreme UV or something that does not propagate in air?

I kinda imagine it like little antennas, where a big thing is a broadband fat monopole, while the little particles start to have a very high Q, but since its electrically nonconductive I don't know whats going on. But they don't really resonate, or at least if there is some kind of resonance like a comb then there is frequency shifting or mixing based on some kind of atom properties that are nonlinear or geometrically related (I don't see a clear pattern for atomic emission spectrum by mass/number of protons really.. at least I never noticed or read about it, but I would not expect one because the atom structure changes three dimensionally or something

[CopperCone]

What is the technical term for this? Gasses have absorbation/emission lines, but what is for nano particle emission spectrums? Like if you were interested in a simulation/calculation.

[CopperCone]

Also, large things seem to have blackbody emissions, while rare gases are based on atomic composition. Where do nanoparticles fit in? Will nanoparticles of different materials do different things (some kind of inbetween?)

Like, pure low pass filter based on size or composition too? I imagine eventually at some small scale there will be some kind of in between?

It seems as you increase gas densiry you decrease q of emissions but this says nothibg of solids

[ejeffrey]

It is basically an antenna problem.  Small particles make poor antennas for long wavelengths.

Being small suppresses long wavelength emission.  It doesn't dramatically affect short wavelengths.  So a flame will emit some UV if it is hot enough, but not more than a larger object would.  The Planck law for black body radiation is the maximum rate of thermal emission for a body at a given temperature at each wavelength.  Many things emit less than that, which is what gives them color.

Dilute gases are an extreme example of this.  They can only effectively couple to certain wavelengths.  If they are in thermal equilibrium they are still limited by the black-body radiation limit, but in addition they can only absorb or emit certain wavelengths.  As you increase the density, the emission lines broaden, and eventually become a a continuum -- i.e., it will behave similar to a typical solid or liquid.

[CopperCone]

hard to think about an antenna from a hot ceramic but I get it now kinda, so as you change size maximum thermal emission changes which is why things don't make disproportionate amounts of energy in limited bands, but with small particles the emission should be greater because they are small, but overall less because the medium is less dense, and the definition of blackbody requires some particular grouping, and you get radiation transfer between them rather then conduction since they are a bunch of free particles so its kind ablocking itself?

I kind of gather that it has to do with general density, because there are reflections in the medium and stuff between the atoms (seems really complicated if you get into it), let me see if i can find the web page that got into this,

https://www.quora.com/Why-does-a-gas-discharged-under-low-pressure-give-non-continuous-spectra-whereas-it-gives-continuous-spectra-under-high-pressure
"
However, as you increase the pressure of a gas, the atoms start bumping into one another more frequently. If two atoms collide while an electron in one of them is in the middle of emitting a photon, the emission process is cut short and the lifetime δt
is reduced. Consequently, due to the uncertainty relation above, the spread of frequencies δf of the emitted photons increases. This phenomenon is called pressure broadening of spectral lines. If the pressure is high enough, and the collisions are sufficiently frequent, the bands in the above picture start to blend into one another and you go from a discrete spectrum to a continuous one:"

I think that especially with complicated molecules things can get really funny here based on how that mechanism works.

[ejeffrey]

Its easier to think about a metal.  Obviously metals aren't great "black" bodies, but they are a lot simpler to think about.  The metal is full of electrons and nucleii with a net charge of zero.  If the metal is "hot", the free electrons are moving about randomly.  Sometimes there will be an excess of charge in one place and deficiency in another just due to random fluctuations.  That charge distribution has a dipole moment, creates an electric field, and can radiate EM radiation at the target wavelength.  In insulators, it is still possible to have charge fluctuations at very small scales (several atoms) as the "bound" electrons still have some ability to move.  On the other hand, insulators tend to be transparent to microwaves and RF because there are no free electrons.

Those charge density fluctuations exist on every length and time scale and can therefore radiate at any frequency, except that they can't be bigger than the object itself.

[T3sl4co1l]

What is the technical term for this? Gasses have absorbation/emission lines, but what is for nano particle emission spectrums? Like if you were interested in a simulation/calculation.

Nanodots fluoresce at intermediate energies.  This might be relevant for soot particles, which are conductive carbon with various sizes; the sizes being inconsistent, you get a wide spectrum instead of a sharp (molecular emission) peak.

Clean flames are colored with CH. and CH2: radicals, by the way.  It doesn't much matter what the fuel is, as long as it burns clean; otherwise, C2 (yellow) and soot emission occurs.  You are much more likely to have a dirty flame with higher C:H ratio and heavier hydrocarbons, of course.

Being small suppresses long wavelength emission.  It doesn't dramatically affect short wavelengths.

Careful -- if we're still talking quantum systems here, then it doesn't matter much.  NMR works, after all.   Purely a quantum effect, of course it's very hard to see outside of large systems at low temperature and high field.

And antennas don't need to be physically large, if they are high Q and not directional.

Tim

[pwlps]

Careful -- if we're still talking quantum systems here, then it doesn't matter much.  NMR works, after all.   Purely a quantum effect, of course it's very hard to see outside of large systems at low temperature and high field.

NMR works with near field, not radiated field, antenna considerations are useless here.
Also, the quantum effects are not necessarily observed: for a non-interacting spin 1/2 (like in MRI applications) the classical EM description is strictly equivalent to the quantum one and sufficient.