1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Radiative energy

  1. Mar 3, 2009 #1
    I'm currently in a debate elsewhere. I contend that the physical process of absorbing/emitting blackbody type radiation in a continous spectrum is strictly a function of temperature, mitigated by characteristic of the substance called emissivity; and the absorption/emission of infrared radiation into a molecule's vibration and rotation energy is strictly a function of the physical makeup of a gas molecule (center of mass, bond length and strength, etc.) mitigated a bit by temperature ala the Boltzmann constant (and other quantum stats). And the physical processes are not the same. Generally meaning that the highly quantized molecular energy stores are not functioning as a blackbody with a very pecular highly discontinous emissivity of zero throughout the spectrum except a couple or so extremely narrow "bands" where it is "1".

    What's anyone's thoughts? Know of a web accessible text or paper that discusses this specifically?

    PS I know Planck radiation theory is used to help analyze atmospheric warming ala IR absorption, but that, IMO, is simply a (very helpful) construct.
     
  2. jcsd
  3. Mar 3, 2009 #2

    alxm

    User Avatar
    Science Advisor

    So, in other words, you think the Boltzmann distribution is wrong?

    My thought is that you need a textbook on statistical thermodynamics.
     
  4. Mar 4, 2009 #3

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    It's tought to tell what your question is, it seems you are having trouble reconciling discrete and continuous models.
     
  5. Mar 4, 2009 #4
    alxm, I'm don't see how the Boltzmann distribution relates to the quantized discrete absorption. Taking just one (of the only 3 or 4) bands (lines) and ignoring relatively insignificant doppler and pressure "band" spreading, the CO2 molecule absorbs energy at precisely and only roughly 15um which equates to an explicit and precise energy ala hf. I'm missing the Boltzmann distribution tie-in. Or what?

    Andy Resnick, yes, it's hard to ask. I'm not specifically trying to reconcile any models. As a matter of fact one can (and usually does) use the Planck blackbody model to analyze discrete molecular absorption by the appropriate manipulation of parameters. I'm asking about the physical mechanism, model constructs not withstanding, of the two emit/absorb processes. For example, you heat any body and you get Planck blackbody-type radiation: continuous spectrum, power a function of the temperature of the body, etc. My contention (but a question here) is that the mechanism of absorbing and emitting discrete photons into and out of the intramolecular vibration and rotation modes is different: has no direct dependence on temperature, is no where near a continuous spectrum, e.g. But even so, these are just overt characteristics. I'm also asking if the physics process itself is different: exactly what the molecules, particles, ions, electrons are doing in each case.
     
  6. Mar 5, 2009 #5

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    Well that's not true- if you heat an arbitrary body, you do not *have* to get a backbody distribution of emitted radiation. Heating a gas, for example. Or a dichroic mirror. Or any real materal, actually.

    For molecules and other materials that have spatially extended (bound) electrons, the absorption and emission profiles do not consist of sharp spectral peaks- look up any absorption profile of any dye molecule.

    In the IR and further out- think microwave and NMR- the probed states involve very delocalized electrons (collective modes of vibration) and so there is no real contradiction with the need to quantize the electromagnetic field and the spectral response of a molecule. Once one of the giant wobbly molecules gets excited, the absorbed energy can dissipate among many modes, leading to a red-shift in the emitted energy. The emission spectrum is then seen as a statistical ensemble of many emission events.

    Does that help?
     
  7. Mar 5, 2009 #6
    Andy Resnick, I think you are saying my original assertion is not correct. I still have to disagree -- at least in part. You can/do get planck radiation from a gas (cosmic background radiation being a prominent example??); I don't think a filter ala dichroic mirror affects my assertion -- one can do lots of things to a blackbody/planck radiation to affect what finally gets through, but that doesn't alter the blackbody radiation per se. But, I'm not well versed on the spatially extended (bound) electrons piece, so I have to go research that in terms of what you say. Thanks for your interest.
     
  8. Mar 6, 2009 #7

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    The CMB is not from a heated gas. Heating a dichroic mirror (i.e. a material with spectrally varying transmittance) will likewise not provide a blackbody spectrum when heated.
     
  9. Mar 7, 2009 #8
    I'm half understanding. I understanding the emissivity is wavelength dependent and you can get gaps or reduced intensity areas in what otherwise is a "continuous" spectrum. I don't think this refutes my original question. Which, maybe put another way, is what precisely physical is going on that generates black body type radiation, of whatever less than a smooth radiation curve?

    Where s does the CMB come from? What is it that's at 2 (or whatever) degrees K and presumably generating a near perfect Planck curve?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook