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Absorbtion Emission

  1. Jul 7, 2010 #1
    I seem to have some trouble finding a definite answer by searching the following:

    It is my understanding that when an electron is excited to a higher energy state, it will spontaneously decay back and emit a photon.

    My questions are:

    Is the wavelength of light required to excite an electron going to be the same wavelength given off as it decays back? Sort of like a particular freq. for a particular jump going either way?

    Also, is light the only type of energy that can cause an electron to reach an excited state, it seems that maybe one could be "forced" out simply by a collision or even a bond forming/breaking.

    I apologize for being such a lay person here in this section, but I was never taught this in school, and am trying to educate myself. Any help would be greatly appreciated.
  2. jcsd
  3. Jul 7, 2010 #2
    Re: Absorbtion/Emission

    I asked some of these same questions a few days ago. Spontaneous emission may be a misnomer; as I was told to use Fermi's golden rule to compute the time an electron would stay in an excited orbit. Perhaps someone else will correct that notion if I was told wrong -- I have no way to verify it either.

    As to "other ways" of electrons being excited -- there surely are. In a laser, often one molecule is excited by a photon or even by a chemical reaction -- and then the excited molecule will transfer its energy to another molecule nearby during a physical collision. The energy levels of a helium neon laser, for example, are very close between the different molecules -- and so an electron has to loose a tiny bit of energy in order to be able to excite the complementary molecule's electron. This tiny difference in energy is insufficient to excite an electron into another orbit and so must either be dissipated as long wavelength radiation or as kinetic energy of the molecules (heat). I am not sure, but I believe that is known as a phonon rather than a photon for it is a near-field effect (collision/vibration).

    The wavelength of the exciting photon and the wavelength of the dropping electron are different in the helium neon laser. If a photon excites an atom, and the same atom later emits a photon -- generally there is going to be a symmetry in wavelength. ( A strong magnetic field applied during the excited time could change the emission wavelength, but some type of change is generally required. ).
    I would expect that any excitation of the nucleus is as reversible as the excitation of the electron, so that under normal circumstances -- a single atom is very likely to both adsorb and emit identical frequencies of light.
  4. Jul 7, 2010 #3


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    Re: Absorbtion/Emission

    No, it can be lower. If you wear a white shirt under a UV light and it fluoresces purple, for instance.

    Well a chemical bond forming/breaking is by definition an electronic transition. But yes, a collision/vibrational transition can cause an electronic transition. This is unusual though, because electronic states are a few orders of magnitude larger than typical vibrational energies.
  5. Jul 8, 2010 #4
    Re: Absorbtion/Emission

    But the color given back by the shirt would be in the emission spectrum of the atoms of the materials dye, correct?
  6. Jul 8, 2010 #5
    Re: Absorbtion/Emission

    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?
  7. Jul 8, 2010 #6
    Re: Absorbtion/Emission

    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 Hα 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.

    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...
    Last edited: Jul 8, 2010
  8. Jul 8, 2010 #7
    Re: Absorbtion/Emission

    Andrew - I think that you have basically described a phenomena related to what I envisioned above with phosphorescence. I also found some verification in your sun example for another phenomena. Thanks.
  9. Jul 9, 2010 #8


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    Re: Absorbtion/Emission

    A single molecule can phosphoresce. It's got no intrinsic relationship to 'impurities'. Your own description isn't even consistent - if they 'fall' how are they excited?

    Phosphorescence occurs when an electron is excited to a state from which transition back to the ground state is "forbidden", such as a triplet-singlet transition. This makes the lifetime of the excited state exceedingly long, since it cannot occur by a simple single-photon process.

    And if you don't know how phosphorescence occurs why are you trying to explain it to someone else?

    The phosphors in a TV tube are fluorescent, not phosphorescent.
  10. Jul 9, 2010 #9


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    Re: Absorbtion/Emission

    Molecules, not atoms.

  11. Jul 9, 2010 #10
    Re: Absorbtion/Emission

    The description is consistent. In a crystalline structure the electron is excited means that it gains energy beyond its normal lowest state -- whether or not that is the ground-most state is irrelevant.
    Conservation of energy means that an electron must gain energy before it can loose it -- conventional naming of reactions (phosphorescent -- merely "light-bearing" φοτος-φερω Greek, luminescent, fluorescent, chemolumine... ) distinguish vaguely the time that a material traps energy and re-emits it based on characteristics of the process, and the typical method of energy transfer/trapping -- for example: phosphorescent is a longer term trapping/excited state than fluorescent. The distinction is qualitative in common usage for the mechanisms don't have to be different in that particular case. The same is true of ionic bonds vs. covalent -- the difference is arbitrated by an additional definition that people in different parts of the industry, or a particular wise-*** physicist, might reasonably disagree on based on a typical connotation (not denotation) of the word in their sphere of discourse.

    Alex, I have only seen a few of your posts -- but they all share the same characteristic: you seem to post when you can provide a put down. I suppose you don't explain things because you might not know everything? or do you "know everything" but just don't like to share it with the world because they don't meet your pricing standard. Is it *safer* to only be negative?

    I choose to share what knowledge I have, even if imperfect. As I already warned the readers that my statements aren't from the perspective of an in-depth mathematical/formal analysis point of view -- I am not bothered in the slightest that I may have inadvertently botched some subtle distinctions. I will note, that you did not offer a positive explanation voluntarily -- and I find that a shame. It left it to me who supposedly knows less to help someone out instead of bash them down.

    The description I gave would correctly describe at least one form of phosphorescence. It isn't a larger and more common confusion with chemoluminescence...

    That is simply not true. The materials in a television tube and *even* a fluorescent tube show a mixture of fluorescent and phosphorescent properties. Classifying a material as one or the other is arbitrary in many respects -- I will simply state that industrially one does not bother to control the purity of the chemical structure of the product to meet an idealized definition of what it is -- and usually for cost reasons; small imperfections lead to undesired results linguistically.

    In the semiconductor industry a "trap" is a place where a fermion is annihilated or moving charges are pinned such as to interfere with conduction/&or diffusion. Typically a trap is caused by an explicit impurity -- that is an atom NOT like the ones found in the immediate vicinity.
    However, traps can also be simple crystalline defects that have nothing to do with a physical impurity -- rather it is a semantic impurity of structure; a crystalline screw dislocation, structural change, or other mechanical deformation from the norm can act the same as an impurity.

    That you may have an exception to the general statements I have made is no surprise. However, I note that you have taken absolutely no pains to describe anything about them other than to say they are single molecules -- an in your face comment, obviously -- so your statement has really taught me nothing for you didn't even bother to give examples which might be examined.

    I would be delighted to add to my knowledge an example of a *PURE* diatomic molecule which fluoresces -- I don't have one to quote off the top of my head, although a careful reading of my statements would reveal that I haven't ruled the possibility out -- but only spoken of what I believe to be "typical".


    As an electrical engineer I look at these materials from the perspective and language of semiconductor engineering. The usage shown on wikipedia is consistent with the usage in my field -- eg: the one that designs and makes television tubes. Transition metal dopants are the typical impurity found in these phosphors -- even if the effect is arguably "fluorescent".

    So if I have struck another nerve associated with the difference in our disciplines; oh well.
    Best wishes.

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