Why are photons emitted when electrons drop energy levels

Main Question or Discussion Point

Hi everyone,

So I'm doing some research on fluorescence and I'm puzzled. I understand that running a current through fluorescent gasses like neon cause the valence electrons to jump up an energy level, and I understand that photons are emitted when they fall back down into their default energy level. What I don't understand is WHY these photons are emitted.

I'm told by friends not to ask such questions, and that there is no answer to this yet. Is this true? Does anyone out there have any theories?

I'm fascinated how the electric current and the energy level jump/decay can produce a photon. But I don't understand the mechanisms.
 

Answers and Replies

1,341
3
It really comes from an understanding of electromagnetism. However just on the basis of energy conservation you could see that something has to be emitted to conserve energy.
 
K^2
Science Advisor
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Consider classical picture. It's completely wrong, but it can work as a starting point. An electron is orbiting a nucleus. It has an electric charge. As the electron goes around, the net charge is oscillating around the nucleus. An oscillating charge produces an electromagnetic wave. Hence, a photon is emitted.

Of course, things aren't nearly as simple, since electron isn't really orbiting the nucleus. Still, it has a charge, and in general, an angular momentum. The electron will therefore still interact with electromagnetic field. Deriving correct expressions here is complicated, and finding even numerical solutions is very complex, but you end up seeing the same thing in the end. An oscillating electric dipole gives rise to electromagnetic wave.
 
I get that the electron oscillating around the nucleus creates an electromagnetic wave, but if this alone is what emitted the photon, wouldn't photons be emitted constantly due to all electrons constantly orbiting around their nuclei?

What is it about the decay, what is it about the electron jumping back down to its "resting" energy level that emits the photon?

And why are photons emitted in the presence of electromagnetic waves? There are lots of electromagnetic waves that don't emit photons, right?
 
K^2
Science Advisor
2,468
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In order for the photon to be emitted, the energy of the electron needs to change. If the electron cannot decrease its energy because all lower energy levels are occupied, it cannot emit anything.

Yes, it sounds a bit counterintuitive from perspective of classical mechanics, but this is quantum mechanics for you.
 
sophiecentaur
Science Advisor
Gold Member
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The link to quantum mechanics is to consider the electron, bound to an atom, as a sort of standing wave. The standing wave needs to have a whole number of nodes in order to be sustained (like on a string). In that model, it can't just dribble away energy and spiral inwards. It can only lose or gain energy if the jump corresponds to the difference in energy between two 'permitted', energy states (standing wave modes).
 
4,222
1
The question is about Spontaneous Emission, isn't it? Can any one supply the cause of spontaneous emission?
 
68
0
Note that an electron or proton or any ionized particle will also emit radiation(photons), say being spun at relativistic speeds in a particle accelerator and slowed down. 'Synchotron radiation' 'Bremsstrahlung radiation' if I got that right.
 
sophiecentaur
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The question is about Spontaneous Emission, isn't it? Can any one supply the cause of spontaneous emission?
Following on from post #6. If an electron is in an energy state higher than its lowest (base) state then it is not stable. There is a probability of it falling into a lower state. when it does, it emits the characteristic frequency of em. Understanding what actually governs this probability is more 'second / third year' work on QM.
 
sophiecentaur
Science Advisor
Gold Member
23,858
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Note that an electron or proton or any ionized particle will also emit radiation(photons), say being spun at relativistic speeds in a particle accelerator and slowed down. 'Synchotron radiation' 'Bremsstrahlung radiation' if I got that right.
In those cases, there is a continuum of possible energy states for the electron to exist in so you can get a continuum of possible transitions and, hence, emitted frequencies. An electron can gradually lose / transfer its energy and keep emitting em under those conditions. The Hydrogen Atom model is not a relevant explanation for all phenomena.
 
346
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Popular books usually explain it using the twisted table cloth. To change things locally you need wrinkles where it joins up to the rest of the expanse. The whole idea is that the conservation of electric charge requires gauge bosons to exist, and these are our photons. It's not easily explained in a sentence! But taking it to an extreme, I might try a Haiku:

[pre]
Since charge is local
For symmetry to prevail
Light will force itself
[/pre]
 

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