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Mechanism behind photon absorption and photon emission

  1. Jul 26, 2013 #1
    hi all, i learned that only certain orbits were allowed in the atom and that if the electrons occupied any of the orbits in between, that they would no longer be in a resonance orbit (i was taught that the allowed orbits were the electron probability wave in resonance and therefore no EM radiation given off) and emit EM radiation; and then we just learned the fact that the absorption of a photon means that the atom is absorbing energy and this excites the electron up to the energy level corresponding to that "hf" value, and also the de-excitation of an electron leads to the electron giving off energy in the form of a EM wave.

    but how exactly does this work? (obviously i haven't yet taken QM)

    i want to know the mechanisms behind these phenomena

    first off, when a photon of energy "hf" is incident to an electron cloud, but there is no other energy level with the energy difference of "hf", why does this photon still not get absorbed? what is wrong with absorbing the photon and then getting excited into the energy gap "orbital" and then instantly having its energy radiated since it is not in a resonance orbit? it's as if the electron knows that it cannot be in that orbit at all. how is this?

    how are electrons excited and de-excited? do they physically travel across the energy gap? or do they somehow teleport? i've heard that they teleport

    what is the mechanism for photon absorption? how exactly does an electron cloud absorb
    i'm aware that EM waves are created by the acceleration of a charge. If this is true, ie.. if this is the only mechanism for which an EM wave can be created as far as we know, then this implies that the de-excitation of an electron causes the electron to undergo deceleration (rather obviously..); however, in any allowed orbit besides the s-orbit, the electron is accelerating and decelerating continuously, but in resonance. So, if the electron does physically travel across the energy gap, then is the emission of the EM wave solely due to the facts that the electron is 1.) undergoing deceleration and 2.) not in a resonance orbit and therefore it radiates away the energy of the still orbiting electron, as i've learned in my class?
  2. jcsd
  3. Jul 26, 2013 #2


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    That's a semi-classical picture, convenient for describing atomic spectra when you haven't seriously studied QM, but that you'll have to unlearn as part of learning modern QM. Most of the questions that you're asking serve to highlight the difficulties with this semi-classical model.
  4. Jul 26, 2013 #3
    This can happen.

    The energy gap is not a physical location across which electrons travel, and electrons do not teleport.

    Imagine a guitar string that is vibrating at its fundamental frequency. It can also vibrate in a different mode at twice this frequency, and it can vibrate in both modes at the same time in superposition, in which case you will hear both frequencies at once (this is simple classical superposition of two waves, nothing quantum). Now imagine that the guitar string, which is originally vibrating only at its fundamental frequency, gradually starts to vibrate at its higher frequency as well, perhaps because it is being excited by an external source of sound at that frequency. The fundamental mode might then gradually die off, leaving the guitar string vibrating only at the higher frequency.

    This situation forms a fairly precise analogy to an electron transitioning between energy levels, with the following correspondences:

    electron wave function = guitar string
    electron energy = string's frequency
    energy gap between electron energy states = frequency gap between guitar string resonant frequencies
    light source = sound source

    This is true in classical electrodynamics, but in quantum electrodynamics this is not how things work because the concept of "acceleration" is fuzzy or nonexistent. In the quantum theory charged particles can simply emit or absorb photons at any time, subject to conservation of momentum and energy.
  5. Jul 26, 2013 #4


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    That kind of model of an atom is nowhere near the level of abstraction that you'd need to even define what a "photon" actually is, let alone calculate absorption cross-sections or decay lifetimes.

    In first-year chemistry and physics classes they show you nice pictures of hydrogenic orbitals and claim that these are the only states that the orbiting electron can be in. That is a really crude simplification, but it's probably pedagogically necessary at that point. In reality the electron can be in any "mixture" (linear combination) of those states, it's just that in the mixture states the atom does not have a definite value of total energy and the electron density is not time-independent.
  6. Jul 26, 2013 #5
    Good question, welcome to the club....There is no simple universally agreed upon 'one liner' [nor even a paragraph or two] that can answer your question precisely. We would all like to know that but nobody can even describe exactly what an electron 'is'. What we can do is model via mathematics what so far tracks well with experimental observations of that particle behavior.

    Wikipedia has a decent, short explanation:

    http://en.wikipedia.org/wiki/Energy_..._energy_levels [Broken]

    This means from a quantum mechanics viewpoint the probability density of an electron in a nucleus of, say, hydrogen, is just about zero.

    Also try reading this which gives some QM insights.


    For example in the Standard Model of Particle Physics, relativistic quantum electrodynamics [QED which is QM modified via special relativity] describes such interactions as 'instantaneous' among 'point particles'. This approach uses such mathematical approaches as probability amplitudes and propagators...and Feynman diagrams.

    [Are electrons really 'point' particles: There are discussions in these forums offering different perspectives. ]

    Here is one good discussion about energy levels and energy gaps in general:

    Photoelectric effect versus Compton Scattering

    [A key take away from the above discussion is that it is the degrees of freedom of the atom or lattice structure that determines allowed energy levels.]

    For more discussion, search these forums for 'electron orbitals' or 'electron energy levels'

    Regarding the prior post from Duck, which I like, a convenient way to think about 'superposition' [which is a cornerstone of quantum mechanics] is via Fourier transforms. If you haven't studied those yet, a simplified view arises from HS trig: take, say, Sin2x = 2SinxCosx. This says that a sine wave of one frequency [2x] can be decomposed into the product of two other waves of half that frequency [x] ....crazy, but true. And such relationships can be carried out via all the trig identities...lots of different frequencies are allowed in a linear system to combine in limited ways.

    An electron in 'free space' [an ideal] is 'free' to vibrate at all frequencies appropriate to it's energy...and its waveform extends indefinitely] But confine that same electron, as in an atomic orbital, and it is 'bound' to certain energy states [as you described]. The analogy with the guitar strings is that when the ends of a guitar string are bound [fastened firmly and cannot vibrate] only certain vibrations are allowed...depending on how tightly the string is strung...that is energy levels.
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