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The problem

Photocurrent graph for varying frequency

Here in this graph when the potential is 0 photocurrent is different for different frequencies from what I understand frequency shouldn't affect the amount of photocurrent and yet it is different here.

My First guess

I think that because electrons are emitted in all different directions not all of them are able to contribute to the photocurrent and since more frequency gives them more kinetic energy the effect of the potential applied is lesser on them but as more potential is applied all of them are captured.

The second problem

If my guess were true shouldn't higher frequencies produce less photocurrent originally at 0 potential because in that case more potential would be required to capture all the electrons? or the amount of potential applied at which saturation current is reached should be lower for lower frequencies and higher for higher frequencies?

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1 Answer 1

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The photoelectron has less energy than the photon. If W is the "work function", the minimum energy necessary to free an electron from the photocathode, the maximum energy the photoelectron can have is hν-W. However, not all photoelectrons will have this much energy: some of the available energy usually goes into heat and/or secondary electrons.

When the anode potential is negative, that repels electrons back toward the photocathode. If that potential is sufficient to prevent electrons with the maximum energy hν-W from reaching the anode, the photocurrent will be zero. Thus, the point at which the photocurrent curve goes to zero measures hν-W.

If the anode potential is sufficiently positive, all of the photoelectrons emitted by the photocathode will reach the anode, and the photocurrent saturates at a value given by the rate at which the illumination is producing photoelectrons (plus secondaries: details, details ツ). So, the magnitude of the photocurrent is proportional to the photon flux. It does not depend on hν.

In between, you get partial collection: some photoelectrons have enough energy to reach the anode, but not all of them. The rate at which the cathode emits photoelectrons depends on the photon flux, while the fraction that the anode collects depends on . So, the photocurrent depends on both.

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