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Question about absorption spectra

  1. Jul 14, 2012 #1
    Been viewing this site for a while now and very much appreciate all the contributors! I can't seem to find an answer to this anywhere so I think I may be misunderstanding the basic ideas of absorption and emission spectra...
    An absorption spectrum, say of our Sun, is as I understand it the result of electrons being promoted to higher energy levels through accepting photons at certain wavelengths, thus giving black lines in a spectrum. However, this energy is released as the electron 'wants' to return to a more stable state with lower energy (which gives an emission spectra if there isn't a light source behind the object). So my question is, how quickly do electrons emit this energy? As I understand it, it should be immediately, but wouldn't that then cancel out the absorption lines? Or is a spectrum only a 'snapshot', with spectra shown in real time having the lines fade in and out?

    Edit: sorry just saw an answer to this towards the bottom of the page! If someone could delete this...
    Last edited: Jul 14, 2012
  2. jcsd
  3. Jul 14, 2012 #2
    Hi, welcome to PF! You make a good observation, it had never occurred to me before. I think you are right in presuming that an electron, after excitation, will sometimes immediately (or after a short delay) undergo de-excitation via the reverse process, i.e. between the same two atomic states, thus re-emitting a photon of the same energy as was absorbed. However, this photon is emitted in a random direction, whereas the white source beam is necessarily directional. Therefore, the beam becomes deficient of photons with energies corresponding to atomic transitions.

    Another factor, although perhaps a minor one: after absorbing a photon, an atom may shed the acquired energy via any of a range of de-excitation paths. The simple case of de-excitation via the reverse process of one-photon excitation is just one of these paths. Others include double- or multiple-photon emission, or some more complex cascade of electronic transitions involving multiple electrons, and there are probably many other paths that I have not even heard of. These processes would not contribute to cancelling the absorption lines. However, I must admit I do not know how the branching ratio typically falls for these exotic de-excitation paths—they might be very rare. Perhaps someone with knowledge can enlighten us.
  4. Jul 14, 2012 #3
    As I understand it, it should be immediately, but wouldn't that then cancel out the absorption lines?
    This is not true at all. The electron can remain in its new orbit for a very long time. For atomic gases under low pressure, it could be microseconds. Then, it may go back to a different lower energy level.
    There are collisions at higher pressures. The electron can be knocked clear of the nucleus. It can be knocked into a lower or higher energy level. It can jump to the other atom.
    If the gas were made of molecules instead of atoms, the electron could relax to a different vibrational or rotational level. Then the molecule could vibrate or rotate.
    The point is that the electron is unlikely to go straight back to its original level. An electron can have a big "hang time". All sorts of thing can happen to the electron other than going back to the original level.
    On the sun, the big thing that alters spectra are the collisions between atoms and ions. I don't think an excited electron has much of a chance of getting home!
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