Why do Photons make matter hotter?

Discussion in 'Chemistry' started by Captain Kremmen, Jun 15, 2013.

  1. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    This is a question which has occurred to me while trying to clear up someone else's misunderstanding.
    When a photon is absorbed by an atom or molecule, it excites an electron to jump to a higher orbit.
    But why would that make the particle move faster?

    Seems counter-intuitive. I would have imagined the opposite.
    That when the volume of an atom became smaller, it would move faster.
     
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  3. Janus58 Valued Senior Member

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    Remember that photons have momentum as well, and and this momentum is transferred to the atom/molecule during the interaction. Also, the frequency of the photon determines the level interaction. For instance infrared acts on the molecular level and causes vibration, while microwaves cause torsion and rotation. Ultraviolet raises electron levels. X-rays cause ionization and Compton scattering.
     
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  5. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Thanks.
    That looks like a really good answer to my question.
    It has raised me to a new level of understanding.
    Has anybody got any objections, clarifications, or additions to that answer?
     
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  7. exchemist Valued Senior Member

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    I think it's also worth remembering that photons come in a wide range of frequencies, some of which (UV, visible) are absorbed by electronic transitions and some (IR, microwave) are absorbed by exciting molecular vibrations and rotations, which give rise to heating effects directly. Also, once molecules are excited in one mode, the energy is passed around to other modes of excitation in other molecules by the random collisions between them. This process fairly quickly redistributes the energy into the Boltzmann statistical distribution among the allowed energy modes in the ensemble of molecules, which allows a temperature to be defined.

    There are circumstances in which absorption of photons does not immediately get redistributed in this way. This unstable or metastable condition is called a "population inversion" and is the basis of the laser for example.
     
  8. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    The idea of packets of energy being used to boost electrons to higher orbits is quite easy to comprehend.
    I wonder though, how this would work in regard to vibration.
    Would the atoms begin to vibrate in phase with the wavelength of the light wave?
    Is light still acting as quantum particles.
     
  9. exchemist Valued Senior Member

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    More or less yes. I'm rusty on the details but as I recall it there needs to be change in dipole of the molecule arising from the excitation, in order for the electric field of the radiation to couple with it. You can imagine with rotation that the molecule can spin in phase with the sinusoidal field of the radiation. A vibration it just is a harmonic oscillation, either longitudinal or torsional, depending on the mode being excited, so again this can couple with the field. But yes, both vibrational and rotational states are quantised, as can be seen in IR spectra. The rotational states appear as "fine structure" in the IR spectrum: you have bands of absorption, rather than narrow lines, which on close inspection are made up of a series of fine lines, each one due to individual rotational modes. Or you can excite them directly with microwaves, i.e. very low energy (long wavelength) photons.
     
  10. exchemist Valued Senior Member

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    But now you make me think of the quantum mechanical process by which an excited electronic atomic or molecular state can lose energy via collisions between atoms or molecules. I don't think I recall how this works....I'll have to look it up, unless another reader can supply the answer.
     
  11. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    This is a quote from a yahoo answer to the question of how microwaves heat food:

    Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwaves. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion.
    http://answers.yahoo.com/question/index?qid=20071009114907AA2iYxO

    Nice answer.
     
  12. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Yes, how does it maintain the quantum effect?
     
  13. exchemist Valued Senior Member

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    Yes it is, though it is put in terms of classical physics, so it misses the bit about quantisation. You asked about the radiation being absorbed in quanta, i.e. photons, which it is. The thing this article does not say is that molecules cannot spin at any rate they like but only at a series of fixed speeds, that are determined by the characteristics of the molecule, not by the radiation they encounter. Thus if you illuminate molecules of a substance with a spectrum of microwaves, the molecules absorb at certain frequencies only, producing spectral lines due to rotation, just as you get a line spectrum by excitation of electrons in an atom.

    This idea, that even something as simple as the spin rate of a molecule can only take discrete values, is fairly counterintuitive and would not be predicted by the classical explanation above.
     
  14. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Would the same thing occur if the collision of a moving atom set a stationery atom into motion?
    Ie, that it would be set into motion at a set speed?
    I would imagine so, from the other cases.
     
  15. Aqueous Id flat Earth skeptic Valued Senior Member

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    Keep in mind that heat transfer via photons is heat radiation. It's not the transfer of a photon's velocity (which is light speed) but rather the photon's energy - which is not even kinetic energy. It's frequency energy. And though we think of hot stuff as more energetic than, say, visible light, a photon of green light carries more energy than a photon of infrared because it's higher in frequency.

    For simplicity you may want to start by visualizing some food being kept warm under an infrared lamp. The lamp emits several hundred watts, which is a zillion photons per second in the 300-400 THz band. The energy conveyed in each photon is absorbed by the electrons in the water molecules in the food. The electrons rise and fall as they absorb photons and re-radiate them. The wavelength of the radiation, around 700 nm, may seem small, but it's much larger than the distance between water molecules. The molecules in water are polar; they are affected by the electromagnetic disturbance created by this IR wave that is crashing through them, and which is reinforced by internal re-radiation. The molecules are whipped around, and the tension on their molecular bonds comes into sync with the radiation, which further reinforces the re-radiation of IR band waves. The food may become so hot you can't touch the plate, and you can feel the IR radiation it's producing long after it's removed from the lamp. This tells you the internal resonance and re-radiation is remarkably persistent. Think of an extremely long echo that takes the energy from your voice and sustains an audible trace 15 or 20 minutes later.

    In other materials the phenomenon has different manifestations. In metals the atoms or molecules are arranged in lattices - like, say, vast stacks of sugar cubes - which also exhibit a natural resonance. In glass similar crystals of silicon are formed, but now the stacks of sugar cubes are broken into stacks of random size and orientation. But these still resonate fairly well. In gases when energy is added the molecules are relatively free to pick up kinetic energy (which may have been the case you had in mind) and at the macro scale this manifests as an increase of both temperature and pressure.

    Obviously all materials do not resonate the same, and the ones that do reject the IR waves exhibit the properties of good insulators.
     
  16. exchemist Valued Senior Member

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    Yes. Rotation in molecules is quantised so, regardless of the means of excitation, only the states permitted by quantum theory can be populated.

    P.S. see separate reply re the outstanding issue of how electronically excited states give up their energy to heat. I have at least a partial answer for you.
     
  17. exchemist Valued Senior Member

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    I looked it up and it comes back to me, vaguely. In a molecule, as opposed to an atom, an electronic state consists not of a single energy level but a whole "ladder" of energy vibrational energy sub levels within the electronic state. Intuitively you can imagine an electron in a chemical bond being excited to a higher molecular orbital: well, the energy of vibration in that bond can take a lot of quantised values too, depending on how hard it is vibrating, i.e. how "hot" the molecule is. The total energy in the bond is the sum of the electronic energy level plus the vibrational energy. Generally an electron excitation will take place to a vibrational level some way up from the lowest one. If so, the molecule can lose energy by clattering down the "ladder" of vibrational sub-levels, which it does by collision with other molecules, thereby heating them up. Once it is in the bottom vibrational level of the excited electronic state, it must undergo a transition back to the electronic ground state to lose the rest. There are radiationless processes, called Internal Conversion and InterSystem Crossing that permit this. This takes the molecule across, without losing energy, to a highly vibrationally excited level within the ground state. So the molecule has stepped across from the bottom of the ladder that led down to the first floor and onto a point on a new ladder that leads down to the ground. Then it can clatter down this new ladder, via more collisions, heating as it does so.

    I do not as yet have the explanation of how exactly these IC and ISC processes occur. The electron is changing to a new orbital without losing energy, but that means the wavefunction has to change and I'm not sure how that is treated in QM. But I'm trying to find out.
     
  18. exchemist Valued Senior Member

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    To close this out, here is a description of the role of Internal Conversion in aiding the transformation of visible/UV photons into heat:
    http://en.wikipedia.org/wiki/Internal_conversion_(chemistry)

    It does not however explain the bit I was really looking for, which is what the QM process is for a radiationless change of the MO wavefunction from the excited to the ground state. Since no (antisymmetric) photon is emitted, one might think the selection rules for this process would differ from those governing normal radiative transitions. But it's all too long ago for me to be able to remember, so unless some reader can help I'm afraid I've now come to a halt on this issue.
     
  19. arauca Banned Banned

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    I am not sure on how all this classic QM. fit in a real world because if your reactants are in a solvent the solvent have an affect on the system , if you have a catalyst and a pure system it have also an affect , so the QM explanation in my view does not fit very well. At the end of a reaction is the close contact of molecules . In other words " no contact no action.
     
  20. exchemist Valued Senior Member

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    No, this theory applies equally in solution. The cascade down the ladder of vibrational states will occur faster than in the gas phase, due to the higher frequency of molecular collisions, that's all. Look up things such as the quenching of fluorescence, for example, here:http://link.springer.com/chapter/10.1007/978-0-387-46312-4_8#page-1

    It is clear this is discussing processes in solution.
     
  21. arauca Banned Banned

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    Here you go, " collision " that is the final word, That means the energy from the photon have to come all the way down to put the system of molecules in motion and now because of collision from reactants a possible rearrangement of molecules will take place ( product are formed ) due to electronegativity ,
     
  22. exchemist Valued Senior Member

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    Er, yes, we've been talking about collisions from the start of this thread.

    But I don't know why you are suddenly talking about reactions and electronegativity: all we were discussing is how photons in the visible or UV, which excite electronic transitions rather than molecular translation, rotation or vibration, can nevertheless transfer their energy into heat. This requires neither reactions nor electronegativity.
     
  23. arauca Banned Banned

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    I agree about the continuous cascading transfer were eventually ends up in kinetic energy of molecules . The reaction because of collision will take preferentially at places in the molecule were high electronegativity reside in the functional group., I suppose you will agree in this example ( primary alcohol and tertiary alcohol ) or depending on the attached on a benzene ring so it will lead to a different position on the ring.. I believe also if you have a specific wavelength you don't have to cascade from electronic excitation and cascade down to heat you can react in a cold system . example Uv or visual frequency polymerization . In other words not all system have the same mechanism
     

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