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Absorption Spectra

  1. Sep 13, 2007 #1
    Hello everyone,

    I'm new to this forum and in fact have never used a forum before so this is a bit new for me. I'm a high school student from NZ, and some of the course I'm doing has flummoxed -not in that I can't rattle off the generic answer, I just don't quite understand. The main area in this is that we just started the idea of absorption spectrum, and basically my problem is: Why can an electron only absorb a specific energy from a photon to move up an energy level? I understand it obviously can't absorb less, but I don't see why it can't just absorb more, and then re-emit the excess so as to fall back down. If anyone could spread some light on this it would be greatly appreciated, and sorry if I don't reply on time as unfortunately have homework. If anyone has the time to answer some more questions on some general quantum theory, that would be neat too:

    How does the electron actually know what energy to absorb....I'm assuming it doesn't absorb the energy, escape orbit and then realize it doesn't have enough energy and fall back down...

    Also why doesn't the electron spiral into the nucleus...I've read explanations about the orbitals being stable, but surely they can't be so stable as to overcome electrostatic force of attraction (well they can obviously in real life, that a statement of my opinion there).

    And this is related to the above question...why does an accelerating charge emit radiation?

    Also related, and last one I think; why are there then many stable orbitals. I have read somewhere that it is to do with angular momentum being conserved, but don't understand this concept... and somewhere else i read it is because the orbital it is like a standing wave with electrons wavelength (Oh is this like the de Broglie wavelength of electron and sets up standing waves with n number of waves so each orbital has a radius of an integer multiple of 0.5 times the electron wavelength. Wow I'll be chuffed if that ones right).

    Hmmm i think i'll leave it at that for now, and as i haven't done this before i'm not really sure whether its appropriate to have a whole lot of questions in one, or make each a separate thread.

    Anyway thanks so much if you have the time to answer even one,

  2. jcsd
  3. Sep 13, 2007 #2
    for a simple analogy, think about a person on a swing. they swing back and forth with some frequency.

    if you are standing behind them, you can deliver energy to the person swinging as long as you push with the same frequency that they are swinging. we call this "resonance".

    so, given the energy spacing between electron states we know that E=hv and so there is a characteristic frequency at which the electron will be resonant with energy imparted via radiation.
  4. Sep 13, 2007 #3


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    Actually, the theory is based on the *observation* that absorption only occurs in specific discrete (i.e., quantized--hence "quantum") peaks. So, the question of "why" in some sense is a metaphysical one. On the other hand, if you are asking "why" within the formalizism of quantum theory this is the case, then the answer is that the theory has been set up this way. Of course, that's a historical perspective, so then one can still ask the question. "If I start from the schrodinger equation for an atom (say hydrogen) and put it in an electric field, will absorption occur only at discrete frequencies?" The answer is "yes. And the frequencies predicted by the schrodinger equation agree with experiment."

    more or less than what? if we consider a hydrogen atom initially in its ground state, then there is a minimum frequency that can be absorbed, but also there *are* higher frequencies too. For example, there could be a 1s to 3p transition, which would show up as absorption at a higher frequency than a 1s to 2p transition. In both cases there will also be subsequent *emmision* of a photon as the atom returns to its ground state.

    For example: 1s --> absorb at large frequency--> 3p --> emit at small frequency --> 2s (metastable)

    Or: 1s --> absorb --> 2p --> emit at same frequency --> 1s (stable)

  5. Sep 14, 2007 #4

    Ah to quetzalcoatl9, I think i get what you mean here, and actually thought about that myself today, in that electrons are standing waves with a frequency so to move up/down energy level they must resonate.

    To, olgranpappy i see what you mean in that quantum mechanics is based on this assumption, and that it does not prove it...it annoys me when things happen like that. And for the more or less what, i meant energy from a photon. I know that it can absorb more energy to simply move to a higher energy level, but i meant why can't it absorb energy from a photon that is in between two energy levels, so that it does not have the exact energy levels...i.e. why couldn't it absorb say 10.5eV, as opposed to the 10.2 needed to move from low energy (-13.6) to next energy level (-3.4). It could use the 10.2eV of the 10.5eV to get to higher energy level, and then emit radiation for the additional 0.3eV. This is were I'm confused...

  6. Sep 18, 2007 #5
    Hi Kcondo,
    An atom in basic energy state absorbs a photon only when there is resonance, i.e the frequency or photon equals to resonant frequency. That condition explains why an atom can not absorb photon with higher or lower energy, because, a photon with higher or lower energy has frequency also higher or lower frequency (E=hv), i.e differs from the resonant frequency, therefore the resonance does not happen, synonyming with no absorption happens.
    Best regards.
  7. Sep 18, 2007 #6
    Quantization was an observation that was not possible to explain given the newtonian physics. But when it is assumed that a system has a wave function, and solving for it given the hamiltonian, the mathematical solution appears to have a set (or more) of quantum numbers which shape a set of possible wave functions. And this really agrees with the observed phenomena.
    This should not be as strange as it seems. After all, if you accept that matter consists on atoms, you must accept that the thinnest line you can get is one atom thick, and above that is a line two atoms thick, and so on. You cannot expect something 3/2 atoms thick.
  8. Sep 19, 2007 #7
    Thanks vivesdn and baolkvn, I think i get the idea of fixed energy levels and about resonance now...its such an elegant answer! And i think i can safely say that this builds upon my somewhat lacking knowledge of quantum mechanics, and thanks so much again,

  9. Sep 19, 2007 #8
    u're welcom kcodon.
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