Tuesday, May 12, 2015

Quantum efficiency of Photoelectric effect


What is the typically measured quantum efficiency of photoelectric effect experiments involving ultra violet photons incident upon common metals like copper or aluminum. In case I'm not being clear, what I mean to ask is how many electrons are ejected per photon.



Answer



The number of electrons ejected per photon is around $10^{-6}$ to $10^{-5}$ depending on the metal.


The quantum efficiency for the initial creation of the photoelectron is around 0.1 to 1. This is the process used in photomultiplier tubes, and the high quantum efficiency is responsible for the very high sensitivity of PMTs. The trouble is that the photoelectron is travelling in the same direction as the incident light ray i.e. down into the bulk of the metal. We only see an electron ejected from the metal surface if the initial photoelectron is backscattered, or if it can scatter off other electrons and transfer enough energy to them to allow them to escape the metal surface. This process has a low probability, which is why the overall quantum yield is so low.


Incidentally, photomultiplier tubes use a thin metal film and the initial photoelectron travels right through the film and escapes from the other side of the film.


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