Infrasound said:
So, if i am understanding this correctly, heat could cause electrons to become excited through the energetic collisions. I remember an experiment with burning different chemicals in a flame and seeing different colors emitted. Is is possible that electrons are knocked out by the collisions, then decaying back, emitting the brilliant pure colors?
Heat is a curious problem all its own. In general, if one builds a box out of any type of atoms whatsoever -- just so long as they are able to withstand the heat applied without destruction -- they will begin to glow dull red, then orange, then yellow, and finally brilliant white as the temperature is raised. The phenomena is called blackbody radiation and the most notable characteristic of it is that the spectrum is continuous.
When you burn things in a flame -- oxygen attacks the item being burned and causes the electrons in the elemental substance to switch from one kind of bond to another. For example, hydrogen atoms either H2 or hydrocarbon are first broken by the heat -- and then oxygen initiates a decay of the excited electron state releasing a photon. Individual atoms, then, emit their characteristic spectrum lines associated with the energy eigenstates and molecular/atomic orbitals. Adding NaCl salt to an alcohol flame, for example, will generate brilliant yellow light. (As a bonus it turns out to be monochromatic and can be used to do Michaelson-Morley, Farby-Parrot, Newtons rings, and other wavelength/coherence sensitive optical experiments -- eg: the way it was done before the invention of the laser...) The yellow light is the most prominent line from sodium and happens to be in the eye's most sensitive color area. (AKA. One of the reasons sodium vapor lamps are so effective electrically as streetlamps. )
On a spectrogram, light from an evenly heated solid object will have continuous spectra whereas atomic transitions from vaporized atoms or diatomic molecules tend to have distinct spectral lines.
The sun is a rather curious light source in that the intense pressure appears to favor the blackbody emission characteristic of continuous spectra -- and yet one finds darker points in this spectrum at discontinuous points (eg: hydrogen's H
α frequency of ~650nm -- Lyman, Balmer, and other emission lines);these normally emitting points are also where the likelihood of energy being adsorbed is enhanced. Presumably, the thermal agitation of the sun is so high that the likelihood of an electron falling to the ground state is less likely than a thermal disturbance causing random wavelength light to be emitted. ( I haven't analyzed it, so I am speaking in general only and the specific mechanism for this inversion of behavior admittedly might be subtly different. )
Light frequency changing materials tend, excluding the sun, to be harmonic oriented. For example, in lasers one may up-convert or down convert the laser frequency to double or half of it's original frequency (/wavelength in free space). The actual mechanism I am not certain of, but the sample of literature I have read on the subject while building/repairing lasers for a client suggests that it is a bulk property of the crystal due to the change in speed with which EM waves propagate in a solid as opposed to individual excited electrons rising and falling in energy states. Specifically, it is nonlinear behavior induced by the structure of the bonds in the crystal which cause a reflected wave to have an EM field which adds to the original in such a way as to double or halve it's frequency. Again, the actual mathematics I have not worked out.
Finally, there is the anomalous case of phosphorescent materials. Again, these are not individual atoms -- but rather crystalline structures with defects (like "pits") in them from impurities typically. For whatever reason, electrons in an excited state will fall a very short energy distance when becoming trapped near one of these impurities. The net effect is that an electron stays in an excited state for a very long time -- often on the order of seconds to hours -- and the original waveshape of when the exciting photon was adsorbed is long since diffused into obscurity. The emitted light is a function of the energy states associated with the trap and not the original site of where the electron became excited in the first place ( which is typically a different location in any event ).
As a kind of eye opener -- consider the case of a television tube (not a LCD). On the screen phosphorescent material (not containing phosphorus usually ... in spite of the greeny recycle people's fears... often it is zinc oxide / sulfide doped with copper, and other *metals* as impurities -- ZNO2 is the same stuff found in UV sunburn blocker lotions with the white color, and baby butt ointments... sulfur is used to dust roses and in garden situations... it isn't particularly toxic by itself and tends to make stable/inert chemicals in soil if not burnt. )
The excited energy for the glowing dots on a television tube are electrons not belonging to the material in the first place, but imported from a gun a short distance away. The screen is robbed of a few electrons by a high positive voltage supply so that when these "excited" electrons (eg: moving) arrive they must "fall" in energy in order to be trapped by the phosphors on the screen -- the net result is that the material on the screen glows. The design challenge for engineers is to pick materials which don't hold these charges for long periods of time like phosphorus based rock does -- rather in order to do motion pictures, one wants the lifetime to be close to the refresh time of the screen. eg: 1/60th of a second is typical -- so 16.666ms or so. Anything longer than that causes blurring of motion.
What is interesting is that chemical bonds so drastically affect excited electron lifetime and how far is can "wander" before falling back down to a basic state of lower energy.
Hmmm...
Some Japanese / Korean import tubes from many years ago. eg: amber only text monitor screens, were notorious for glowing long after being turned off. I wonder if the idea that all television tubes have phosphorus in the screen might have come from cheap manufacturing processes not seen in many years...
There is one other way I would expect a different color to be emitted from an atom/molecule than the original wavelength -- and that is when the original wavelength has energy output equivalent to a sum of more than one excited state. The atom will likely adsorb such a photon -- but there are many ways which the light can decay back down since it is free to eject one of those excited states at a time rather than all at once. In that case, one adsorbed photon becomes many emitted photons of lesser energy.Hope this helps some...
--Andrew.
