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EMR
Name: Scott
Status: other
Age: 20s
Location: N/A
Country: N/A
Date: 1999
Question:
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?
Replies:
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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