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Apparently, the answer to this is that the saturation current is reached when all the ejected electrons are able to reach the anode. But if I keep the cathode illuminated with light of frequency above the threshold frequency, wouldn't more electrons reach the anode in a given time interval if I increase the voltage? Since the electrons are replenished in the cathode due to the circuit, electrons should continue to be ejected and the current should increase without bounds as I keep on increasing the voltage.

Does this have something to do with the experimental procedure? Like, is the cathode irradiated with pulses and not continuously illuminated?

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Akshit
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1 Answers1

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In a typical photoemission experiment one electron is emitted for every $10^5$ to $10^6$ photons that strike the metal surface. This ratio is called the quantum efficiency and its exact value depends on the metal. However in any one experiment the ratio can generally be taken as constant.

If you have a constant light intensity then the number of photons per second hitting the surface is constant, so the number of electrons emitted per second is constant. Let's call this number $N$. As you increase the voltage you capture a bigger and bigger fraction of these $N$ electrons, but the limit is when you capture them all in which case the current will be:

$$ I_\text{sat} = N\,e $$

because if the charge of each electron is $e$ and we have $N$ of them per second the total charge per second is $N\,e$.

This is the saturation current. The current can't get any bigger than this because $N$ is determined by the light intensity not the applied voltage.

John Rennie
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