• Question: If a photon is just a packet of energy, why can an electron only absorb one of them in the photoelectric effect?

    Asked by r3vilo to Andrew, Daniel, Hayley, Natalia, Peta on 16 Nov 2011.
    • Photo: Andrew Cairns

      Andrew Cairns answered on 13 Nov 2011:

      I’m so very NOT an expert in this area – I may be able to understand it if I thought about it long enough (! – a very long time!) so I think the others in this area might be better qualified to answer!

    • Photo: Daniel Scully

      Daniel Scully answered on 14 Nov 2011:

      When the electron absorbs a first photon it gets kicked off it’s atom.
      It might leave the metal then, but even if it didn’t electrons can’t absorb photons on their own. They need another particle to be around (like the nucleus). And it’s not in an atom any more.

      [ An electron can’t absorb a photon on its own because energy and momentum have to be conserved, and on its own the electron can’t do both at the same time ]

    • Photo: Peta Foster

      Peta Foster answered on 16 Nov 2011:

      An electron can absorb multiple photons… it is a process called multi-photon ionisation… but the beam of light needs to be very intense to see this… you can do it with my laser 🙂

      In the classroom you would use a low intensity beam of light and so you need each photon to be energetic enough to liberate the electron on its own… however you could have a laser like mine which is infra-red (and has a much lower energy per photon than the UV light you commonly use with the photoelectric effect) and then you would see that you do get electrons absorbing multiple photons and getting ionised.

      When the light field gets even more intense still you get something called barrier-suppression ionisation where the light field overwhelms the field felt by the electron and everything in the field is ionised. 😀

    • Photo: Hayley Smith

      Hayley Smith answered on 16 Nov 2011:

      This isn’t an answer, more of a comment (but I don’t seem to have a comment box) so I’m just going to say that that’s pretty cool Peta – I wasn’t aware of that!

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