Electron Light Absorption

1. Jan 19, 2006

Intuitive

Question.

Does a stream of Electrons emitted in a Hard Vacuum Diode absorb light?

If so what is the absorption Frequency?

This Question is intend for only mean free Electrons in a Hard Vacuum.

I have been wanting to emit some different LASER Frequencies into a Vacuum Diode and see if the Electrons absorb light in the visible spectrum.

The only thing I have to play with is a 50K Fly back Transformer, a Hard Vacuum Diode and a lack of Variable LASER frequency, I need more broad band frequencies to test.

2. Jan 20, 2006

Meir Achuz

At optical wavelengths, the scattering of light by electrons is well described by the Thomson cross-section
$$\sigma=\frac{8\pi}{3}\frac{\alpha^2}{m^2}$$,
in units where hbar and c=1.

My guess is that this is much too small to be observed for the electrons in a vacuum diode. The electron density is just too small.

Last edited: Jan 20, 2006
3. Jan 20, 2006

Gokul43201

Staff Emeritus
A strictly "free" electron can not absorb a photon. If you write out the energy and momentum conservation equations, you'll find there's three equations in two unknowns - a situation where you do not, in general, have a solution.

This does not preclude the possibility of free electron scattering (as pointed out above). While this scattering cross-section is small, it is not below detection limits in certain cases - notably for cosmologists who study redshifts in photons penetrating hot plasmas.

4. Jan 20, 2006

ZapperZ

Staff Emeritus
This question really is a bit unclear to me. Having read the responses so far, I think we all are interpreting this in the same way.

As Gokul has pointed out, at the QM scale, a photon cannot be absorbed by a free electron. However, if you go beyond that scale into where classical EM and mechanics apply, a bunch of electrons in an external field with long-enough wavelength, then the situation is a bit different. Here, what you have is nothing more than electrons in an oscillating E-field. When this situation occurs, then yes, the electron DOES absorbs energy from the field and get accelerated along the E-field direction till it reverses.

This is essentially what goes on in a particle accelerator using RF cavities (i.e. not using DC fields). If one monitors the RF field strength in the cavity when there's no electron beam, and then look at the RF field again when the beam is passing by being accelerated, one notices a dip in the RF field when there's an electron beam being accelerated. This is what is known as "beam loading". The electron beam is pulling energy out of the RF field for acceleration.

Of course, the physics and mechanics is a lot more complicated than what I've described. But essentially, this is the technique most particle accelerators use, especially at the various synchrotron centers around the world to accelerate their electron beams.

Zz.