What happens when a co2 molecule absorbs an infra red photon?

  1. Its common knowledge that infra red radiation heats up an absorbing solid surface.
    That is,the em radiation energy is converted into translational KE of the particles of the solid.
    What happens in the case of an isolated co2 molecules absorption?
    Can the EM energy be converted into increased molecular KE, and what is the probability of this happening in contrast to simply being re radiated as a photon with the same frequency?
  2. jcsd
  3. turin

    turin 2,326
    Homework Helper

    For a bulk material, there is a virtual continuum of energy states that can absorb the infrared photon. However, for a single molecule (of CO2 or whatever), there are only very discrete energy levels. The photon would need a frequency very near a particular resonance.

    Assuming that the photon has a favorable frequency, various things can happen. It can excite a vibrational mode, it can ionize the molecule, it can be scattered by the molecule. In all cases, the original photon ceases its existence. In the case of scattering, a new photon is born, most likely with a modified frequency.
  4. Gokul43201

    Gokul43201 11,044
    Staff Emeritus
    Science Advisor
    Gold Member

    If an IR photon is absorbed by a CO2 molecule, it is almost certainly exciting a vibrational mode. The molecule then drops back to a ground (vibrational) state by emitting one or more IR photons (within microseconds of the excitation).
  5. This sounds like generally a correct answer, but I have to wonder why the rotational modes are not often mentioned in this context. The bonds are at least slightly polar and the molecule must be bent like H20, surely not linear; so any tumbling motion ought to couple to the e-m field. And unlike vibrations, shouldn't rotations be relativly broad-band, not limited to very exact frequencies?
  6. Gokul43201

    Gokul43201 11,044
    Staff Emeritus
    Science Advisor
    Gold Member

    CO2 is in fact linear, therefore has no dipole moment and hence, not much of a vibrational spectrum. And even for molecules that are polar and couple to an EM field via rotation, excitation of pure rotational modes is typically in the microwave range. In the IR range, the role of the rotational spectrum is that it modifies the vibrational spectrum (adds a fine structure with an additional selection rule to purely vibrational excitations).
  7. Thanks turin,gokul and conway for your helpful replies.

    I gather from what you are saying is that for a gas like co2 at STP then being subjected to a strong beam of infra red radiation for however long will not result in itself in an increase in any molecules translational KE.
    That is the temperate of the gas will not rise!
  8. Oh, right. Because of the double bonds.

    Microwave, huh? I have to wonder about this; isn't the specific heat of common diatomic molecules eg. air etc. pretty much 5/2 R, meaning you have all five modes (translational plus rotational) active? So a polar diatomic molecule like CO should have rotational modes going in the thermal range at room temperature. It seems to me these should interact with the e-m field.
  9. No, because once the vibration is excited it can collide with another molecule and transfer some of its vibrational energy to the KE of the other molecule. So yes, the temperature of the gas rises.
  10. Thanks Conway any chance of putting me in the direction of answer as to the probability of photon energy changing to KE verses simply re-radiating
  11. Although CO2 doesn't have a permanent dipole moment, the permanent dipole moment of a static molecule tells you nothing about its interaction with an infrared photon. If you want to consider a molecules ability to absorb in the IR, then you need to consider how the dipole moment changes as the nuclear coordinates change position. In order to be IR-active, the vibrational mode must involve a change in the permanent dipole moment.

    As a linear molecule, CO2 has 3N-5 = 3(3)-5 = 4 normal vibrational modes. One of these modes is a symmetric stretch, which has a net zero dipole moment at all times; therefore, the symmetric stretch cannot interact with the IR photon, as there is no means to connect the initial vibrational state to the final vibrational state. The symmetric stretch is IR-inactive.

    In contrast, an asymmetric stretch involves a change in permanent dipole moment; therefore it is IR-active.

    Similarly, the two bending modes involve changes in the permanent dipole moments and are both IR-active.

    Carbon dioxide is a strong IR absorber. Indeed, anybody that does IR spectroscopy must background subtract to remove the absorption due to atmospheric CO2 and H2O. If you would like to see the IR absorption spectrum of CO2, then click here.

    If CO2 wasn't a significant IR absorber, then we wouldn't consider it a greenhouse gas.
  12. Is anyone aware if it's possible, using a high intensity laser beam, to change the composition of the air (say over a football field), and either reflect or absord the sun's rays to cool the area below that beam? (Pardon my ignorance, I'm working off of three quarters of physics classes).

    Thanks and best regards,

    BTW - there could be some compensation for someone to come up with a solution, whether its using lasers or any other means.
  13. they have laser cooling ,
  14. Cooling lazers might work. Do you happen to know if they only cool the area they come in contact with, or might they absorb or reflect the sun's rays to be able cool down the area beneath the lazer's rays?
  15. First of all, solids do not have translational kinetic energy. That is why they are solids. The only thermal energy that a solid has is vibrational energy. Infrared light has just the right energy to interact with vibrational energy and so when a solid absorbs infrared light, it causes the solid to go to higher vibrational states which heats the solid. Gases also absorb infrared light, but there are some strict rules from quantum mechanics. First, gases have to be able to vibrate, i.e., they have to be molecular. Any monatomic gas like helium is not IR active. Second, when they vibrate there has to be an oscillating electrical charge distribution (we call it a dipole moment), so that a coupling will occur between the oscillating charge and the electric field component of IR light. So gases like O2 and N2 vibrate, but there is no change in charge distribution when they do. They are not IR active. They do absorb heat, but only through molecular collisions. CO2, on the hand, has vibrational modes that are IR active, so when a CO2 molecule absorbs IR light, the molecule is excited to higher vibrational states. Now, here's the tricky part. The molecule has at least two choices. It could simply de-activate sending the IR photon out (emission). If it does that, there is no thermal consequence. Or, it can collide with gas molecules around it and transfer the vibrational energy to, say, O2 or N2 around them as translational KE, causing them to warm. Of course, this is the greenhouse gas effect. Note that in the greenhouse gas effect it is mainly the air around the CO2 that warms. The CO2 will warm, but at 0.04%, they do not contribute much to the warming.
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?