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Name: Scott
Status: other
Age: 20s
Location: N/A
Country: N/A
Date: 1999 

I've been trying to help students understand where light, or electromagnetic radiation (EMR) in general, comes from. I've told them that in classical electrodynamics the fundamental source of EMR is charge acceleration. However, I went on to tell them that in quantum mechanics EMR emission events are simply coupled to the transition of an electron from a high energy state to a lower one, and that the energy that the electron has lost is exactly equal to the energy emitted as radiation. The students went on to ask if the mechanism of the emission event involved the electron undergoing an acceleration or deceleration as it transits from the higher energy state to the lower one? I wasn't sure? Do we know the mechanism of emission events? What is different about the electron before versus after it has emitted a photon - only it's energy? Or, is there something else like its velocity?

Hard question! The great ones are always simple and hard.

Well, the easy part is what is different about the electron before and after photon emission. Clearly something must balance the photon's momentum, and since the atom retains the electron, it must be the atom as a whole that does this.

The hard part is reconciling the acceleration idea with the quantum idea. I don't know how to do this. One of my professors said you could think of the electron oscillating between the higher and lower states as the photon is being emitted, but he didn't elaborate on this, and if you follow it through you get into trouble: We know that photons can be thought of as having length because interference experiments have shown that more monochromatic photons interfere with each other even though their amplitudes have been split between paths of different lengths. Given this, the picture of electrons bouncing between high and low energy states raises a question about the state of the electron after half of the photon has been emitted. What if the atom is disturbed during the emission time?

I think what it comes down to is that Q.M. doesn't address the question of precisely where an electron bound in an atom is at any particular time, and the time during photoemission is no exception.

In the most satisfying mental picture I can think of, the electron drops from a high-energy state to a low one, and the atom as a whole rings for some time afterward -- for a time that depends on how precisely the transition energy is determined. I can't make the picture rigorous, and I can see flaws in it (e.g., the energy width of the initial state should affect the ringing time, but I can't see how it would accomplish this), but it's better than the picture of a single electron doing the radiating, and it's testable. Other properties of the atom should be affected during this time.

Tim Mooney

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