How does an electron lose kinetic energy near an atom's nucleus?

[MartinV05]

How does an electron that passes near an atom (near, but yet not that close to the nucleus) lose kinetic energy and releases an x-ray photon with energy equal to the amount of the lost energy. Shouldn't this interaction be an elastic one? I am not talking about an interaction with any of the atom's electrons, just passing by it and succumbing to the force field of the atom's nucleus. To be more precise I'm talking about the animation starting from 1:13 in this video: I hope someone replies soon :)

 

[JesseC]

As the video says, its a process known as Bremsstrahlung which is German for 'breaking radiation'. You'd need to study classical electrodynamics to understand exactly why and how this happens.

But to put it shortly, when you accelerate or decelerate a charge, it emits radiation. This is the way nature works.

The positive charge of the nucleus causes the electron to take a curved path due to the electric forces between them. Anything which is moving in a curved path (classically) is being accelerated/decelerated by some force. Thus it is impossible for electrons to move in a curved path without emitting radiation.

This is the principle behind a synchrotron if you've ever heard of that? Accelerate electrons around a large ring and tap off the X-rays which are produced as they move to use in experiments.

 

[MartinV05]

I am aware of everything you said. I'm confused because I studied the Frank-Herz experiment, and the elastic collisions happening in there made me think. Because it says, an electron passes through an atom, and if it doesn't have enough energy to shoot the target electron from the first level away no energy is lost (elastic collision).

 

[JesseC]

The difference I think is that in the Frank-Hertz experiment the electrons typically only possesses a few, to 10s of eV of kinetic energy to begin with. Thus the scattering is effectively elastic, except at certain discrete energies according to electronic transitions in the atoms.

An X-ray tube accelerates electrons to thousands of eV, so that's about 3 orders of magnitude more kinetic energy than the Frank-Hertz experiments. These high energy electrons plow straight through the electron cloud and interact with the nucleus directly, which has an extremely strong electric field, thus the scattering is inelastic.

 

[MartinV05]

That may be true, but isn't it more logical that an less-energetic electron is more likely to interact with the nucleus than the more-energetic one (momentum vector addition, because more energy=more speed).

 

[Drakkith]

That may be true, but isn't it more logical that an less-energetic electron is more likely to interact with the nucleus than the more-energetic one (momentum vector addition, because more energy=more speed).

A low speed electron will not get close to the nucleus of an atom because of the electron cloud around it. It does not posess enough speed to "break through" the negatively charged cloud. At least, that's what I think JesseC is saying.

 

[JesseC]

I think you're underestimating the sheer scale of the atom compared to the nucleus. It is absolutely TINY in comparison with the electron orbital radii. At least 5 orders of magnitude smaller.

What would a slow moving electron see as it approached an atom? It would see a vast wall (cloud) of negative electrons all repelling it away. It would interact with/scatter off the cloud of electrons way before it even had a chance of getting near the nucleus.

It would take a high energy electron to penetrate the atoms electron cloud and come into range of the nucleus which is sitting like a pea in the middle of a football stadium.

 

[MartinV05]

I guess you have a point there, all these examples with Hydrogen atoms have altered my perception of the atom structure.

 

[JesseC]

Well I hope that's cleared some things up at least!

 

[MartinV05]

So in that video I posted in the first post, it shows an electron penetrating the electron cloud barier? Because it seems like it is close to the atom, but outside the atom complex.

 

[JesseC]

For scattering off the nucleus to occur the electron must have penetrated through the electron cloud.

I would urge you not to take 2D, not to scale, classical diagrams/animations too literally. They are a useful tool, and certainly help in understanding the principles but the reality of the situation is nothing like that animation. To be honest we can only understand it fully using mathematics, and trying to visualise what's actually going on is really difficult because its all quantum mechanics at that scale. Quantum mechanics is nothing like what happens at scales we're used to so we have little conceptual basis for understanding it.

 

[MartinV05]

An isolated system like the one in the animation is only possible in "mathematically" optimized environment. You are right on that too but, if we don't even try to visualise it's hard to even imagine what is going on. Plus I am not a big fan of quantum mechanics. Very very impractical for me. Or maybe it is because of these badly written books, we'll see in the future. And thanks a lot for this!

 

[Drakkith]

So in that video I posted in the first post, it shows an electron penetrating the electron cloud barier? Because it seems like it is close to the atom, but outside the atom complex.

That is merely a visual representation of what is going on. The nucleus is MUCH MUCH smaller in relation to the size of the orbitals. Plus the energy of the electrons in that video are high enough to penetrate the electron cloud barrier and get close to the nucleus.

You are right on that too but, if we don't even try to visualise it's hard to even imagine what is going on. Plus I am not a big fan of quantum mechanics. Very very impractical for me. Or maybe it is because of these badly written books, we'll see in the future. And thanks a lot for this!

There's no problem with visualizing, it is nearly required! And this doesn't require quantum mechanics to explain either. The actual size of a nucleus was measured many years ago and I believe can be explained without quantum physics. (I think at least)