# Photoelectric effect, very high frequencies and lower current

Discussion in 'Physics' started by Robin Mitchell, May 31, 2016.

1. ### Robin Mitchell Thread Starter Well-Known Member

Oct 25, 2009
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Hi all,

Quick question, if you increase the frequency of incoming photons in a photoelectric effect setup would the current rate eventually reduce? My idea is that because the wavelength of the incoming photons becomes much smaller than the size of the atoms and the space between shells the photons would just pass straight through (like in an X-Ray machine)?

All the best,
Robin

2. ### Papabravo Expert

Feb 24, 2006
10,164
1,797
Can you put some numbers on what you think the order of magnitude is for the distances and frequencies/wavelengths involved? One thing I know is that as frequencies go up so does the attenuation in any medium you can think of from the vacuum of space to transmission lines to antennas.

3. ### Robin Mitchell Thread Starter Well-Known Member

Oct 25, 2009
734
200
I dont have numbers (I know thats really bad), but I got into a convo with someone who asked me why the current goes down as the frequency goes up (after saturation).
I said that I dont know the real answer but my theory was as thus:

1. As frequency gets higher, so does the energy of individual photons
2. Absorption coefficient is inversely proportional to energy
3. Photoelectric effect is directly proportional to Absorption coefficient (unless high energies are involved which then brings in Compton and pair production)
4. Therefore photoelectric effect is inversely proportional to frequency after a certain value which is far beyond the work function

4. ### crutschow Expert

Mar 14, 2008
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The vacuum of space??

5. ### Papabravo Expert

Feb 24, 2006
10,164
1,797
So here are some numbers from my experience:
1. Useful RF propagation is about 1 km @405 GHz (740 μm)
2. Visible light propagation is about 500' @475 THz (631 nm)
The scale of an atom is on the order of 0.1 to 0.5 nm. You are talking about something like

https://en.wikipedia.org/wiki/Very-high-energy_gamma_ray

Which are at even higher frequencies and energies. They don't appear to be stopped or scattered by very much. Since photons do not appear to have a charge they are not going to respond to a nucleus of charged particles. They do have sufficient energy to knock electrons out of their shells. These electrons will give up that energy, as visible or infrared light, when they return to a lower energy level. IIRC there is nothing about the photoelectric effect that imposes a limit on photon energy and what happens in a solid material when it experiences an impact from high energy photons.

I haven't really considered Compton scattering or pair production in quite a while so I'll have to do some reading.

6. ### Papabravo Expert

Feb 24, 2006
10,164
1,797
Would you prefer the vacuum of "interstellar space", which actually still has some stuff in it.

7. ### Wendy Moderator

Mar 24, 2008
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I am interested in the "tuned nano antenna" concept that was floating around a decade or so. Supposedly the efficiency for a specific frequency of light would be through the roof. While I suspect it will get there eventually, we're not there yet.

8. ### Papabravo Expert

Feb 24, 2006
10,164
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How were they fabricated?

9. ### Wendy Moderator

Mar 24, 2008
20,766
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As I understand it they weren't. It was in concept stage, but the technology is getting better all the time. You make an antenna array, each with a built in diode. If they were transparent at other frequencies it might amount to something, but I am waiting and seeing (a common practice with vaporware).

Basically ultra fine wafer fab fabrication.

Last edited: May 31, 2016
10. ### BR-549 Well-Known Member

Sep 22, 2013
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"I dont have numbers (I know thats really bad), but I got into a convo with someone who asked me why the current goes down as the frequency goes up (after saturation)."

This effect has nothing to do with frequency. The photo detector simply ran out of charge.

Are you varying the frequency of the light source?

Aug 27, 2009
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12. ### Robin Mitchell Thread Starter Well-Known Member

Oct 25, 2009
734
200
I am varying the frequency of the incoming photons but please understand that I am not performing a real experiment. My brother used an online app that allows you to vary the different aspects of the setup and when he turned the frequency right up the current dropped.