Photon Interaction

[Little Bang]

Since I have not found any articles that fully describe the interaction of a photon with an electron I'll tell you what I think it is and you can correct me.

A photon of the correct frequency constructively interfers with the electron wave of an atom (probably the valence electron) putting the electron in a higher energy state. This situation for some unknown reason to me is unstable resulting in the electron dropping back to the lower energy state giving up a photon with the same energy as the original photon.

[sebbysteiny]

Your right that an electron with the right frequency excites the electron into a higher orbit. Although my Quantum mechanics is a little hazy I'll try and explain.

 

This works because the electromagnetic wave makes the electron oscillate in the wave. During the 'transition', the electron oscillates between one energy state and another. It is thus in a quantum superposition of both energy states. By the time the photon as passed the electron, the photon will either be absorbed and the electron will be in the higher energy state, or it will pass through and the electron will remain in the lower state.

 

The reason the photon must be of the correct frequency is that the electron simply cannot occupy an electron state other than the allowed quantum. Thus, it cannot respond to EM waves of a different frequency to that which is required to create that quantum superposition.

 

The system is unstable for a similar reason as it would be if one thinks classically. Classically, the electron would be pushed towards the nucleus releasing energy. However, instead of being pushed, there is a probability of changing states. The greater the energy difference between one state and another, the greater the probability of that electron changing back to the ground state. This probability is entirely dependant upon the atom concerned.

 

In a constant temperature, EM waves are absorbed and released will be equal. Thus, one gets the hydrogen line spectrum from hydrogen observed in space.

[Mercedes Benzene]

If I remember correctly, when a photon interacts with an electron, the De Broglie wavelength of the electron changes to match whatever the frequency of the photon is...

 

I'm a little rusty on my quantum mechanics, and sebbysteiny's explanation is undoubtedly more thorough than mine...

but there's my 2 cents worth.

[Jay-qu]

If I remember correctly, when a photon interacts with an electron, the De Broglie wavelength of the electron changes to match whatever the frequency of the photon is...

 

No I dont think it matches it, like sebbysteiny said, the photon can only be absorbed if it has the energy corresponding to the difference in the 2 energy levels that the electron is changing between. This is because the circumference of the orbit must be an integer multiple of the DB wavelength of the electron, hence it must go up in energy in discrete amounts. The energy of a photon is hf, this is the amount that gets added to the electrons energy.

[Mercedes Benzene]

oh. :love:

What would I do without you Jay-qu?

[Qfwfq]

The Eisberg-Resnik textbook includes a quite detailed description of the transitory situation with semiclassical descriptions that match up magnificently with the quantum selection rules.

 

Basically, the linear superposition of the two energy eigenstates has a time evolution in which the distribution varies between the two pure ones at the beat frequency (which is the frequency difference). Therefore the charge distribution (e times probability distribution) is varying at the same frequency and in some cases this acts as a good antenna, in other cases as a poor one.

[Farsight]

How about if a photon interacts with a free electron anybody?

[sanctus]

How about if a photon interacts with a free electron anybody?

then you get to field theory or more specifically QED. In simple words (anyway the complicated but right version I don't know it without opening a book...).

When the photon and the electron "meet" then you can imagine that they create a virtual particle (for example an electron) which carries all the 4-momentum (which is the energy and the spatial momentum) of the photon and the original electron and then disintegrates into another pair of electron-photon with energy so that the total energy and momentum is conserved.

[CraigD]

How about if a photon interacts with a free electron anybody?

My understanding is that a free electron can absorb a photon of any frequency. For this reason, it’s believed that that sufficiently dense ion-containing “clouds” has a probability approaching 1 of absorbing any photon that enters it.

 

Examples of such clouds include

• The interiors of stars, where it’s believed that, on average, a given single photon takes 100,000s of years to escape into space (see http:// http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980414a.html)
  
• Conditions that are believed to have existed throughout the universe from about 1 second to 300,000 years after the Big Bang, when it was so improbable for any photon to travel a substantial distance without interacting with a fundamental (eg: electron) or composite (eg: nuclear proton or neutron) particle that the universe was effectively glowing and opaque.
  

[Qfwfq]

My understanding is that a free electron can absorb a photon of any frequency.

Yes, except for what Sanctus said. A photon must be emitted, in order to have a "good" state (not the virtual particle). The emitted photon typically has a different energy and, of course, the so does the electron.

 

This is Compton scattering. It is, however, also studied with more basic QM although field theory is necessary for a proper analysis.

[Farsight]

I've picked from somewhere that a free electron doesn't absorb a photon, but instead there's an elastic collision. Only the tied electron absorbs the photon, which basically stretches the "tether" between it and the rest of the atom.

 

Can anybody correct or improve this very basic layman's concept?

[Qfwfq]

Elastic collision means that the same kind and number of particles emerge as went in, without a change of "internal" state. They only change each others energy-momentum. The atom, otoh, will be in a state of higher energy, having absorbed that of the photon. In this state, however, the atom is prone to emit a photon of that energy and return to the previous state, but this may occur some time later.

[CraigD]

No, I think.

My understanding is that a free electron can absorb a photon of any frequency.

Yes, except for what Sanctus said. A photon must be emitted, in order to have a "good" state (not the virtual particle). The emitted photon typically has a different energy and, of course, the so does the electron.

For an electron confined to discrete energies by it’s physical confinement in an atom’s orbitals, I agree. However, for a “free” or “ionic” electron, which doesn’t interact with particles in an atomic nucleus or other “hard walls”, the electron is free to have any energy.

 

This “anything’s good” state lasts only until the electron is captured by a nucleus. Then, the electron must have a “good” state permitted by its new atomic orbital, and may need to emit photons to do so. As it settles into various permitted energies, it may emit several photons of different but well-defined energies.

 

This is, to my understanding, one of the reasons why a plasma (matter where electrons aren’t confined in atoms) glows, as “free” electrons are captured.

This is Compton scattering. A cloud of nothing but electrons (which I suspect is rarely or never found) wouldn’t, I think glow.

A quick glance at a text on Compton scattering confirms that it’s only associated with electrons in atoms.

[sebbysteiny]

I think Popular's is the best explanation. The collision is an elastic collision in the sense that no energy is lost during it. The photon cannot be absorbed because that would violate conservation of energy and / or momentum (or conservation of momenergy / 4 momentum).

 

There might be quasi particles produced but this state violates conservation laws and so can only last something in order of h (ie very short indeed) and this can happen in all collisions (and even in empty space). This also only explains the mechanism of the scattering and not the properties of the particles being scattered.

 

A suggestion the electron could absorb the photon is wrong. Thus, the best description of the scattering is simply an elastic collision between photon and electron.

 

For a “free” or “ionic” electron, which doesn’t interact with particles in an atomic nucleus or other “hard walls”, the electron is free to have any energy.

 

This is almost right. A "free" election does not have electron states similar to atomic orbitals but the electron still has quantised states which means, since the electron is a fermion, no two electrons can occupy the same state. Thus, "free" conducting electrons must have energies at the fermi energy (ie the lowest energy state that has not been occupied).

[Qfwfq]

the electron is free to have any energy.

But not any energy-momentum. A single [math]\small A^{\mu}\gamma_\mu[/math] vertex can't conserve energy-momentum, the photon being massless. My memories about the other objection are a bit vague, so I'll look in Itzykson-Zuber when I get home, but I remember it as being that the lack of a mass other than the electron's makes it necessary for there being another vertex, as I said, but the effect also occurs for orbitals when the photon energy is large and it's more like the electron being free. Certainly the Feynman diagram has just the one fermionic line, with the two photonic ones meeting it at each vertex, no atom necessary.