Can one photon emit two electrons? My question refers to the photoelectric effect. I have heard that it is possible, that for one photon, two electrons leave the irradiated metal. Is this correct and by which process can this be explained?
My thought was, that this could be possible, if two electrons are in the same state, but this is forbidden by Pauli's principle.
 A: It is possible and it has already been observed. In the following article they suggest both single and double step emission mechanisms: 

Two Electron Photoemission in SolidsR. Herrmann, S. Samarin, H. Schwabe, and J. Kirschner Phys. Rev. Lett. 81, 2148 – Published 7 September 1998

The single step emission process is because of correlletions of the two electrons that will be emitted. Or as they say: 

If we
  imagine the process of photoionization as interaction between one
  photon and one electron, it is obvious that the simultaneous emission
  of a second electron requires a direct interaction between both
  electrons.

In the double step processes the photon is absorbed by one electron which transfers part of its energy to the second electron. Two examples from the article are: 

As an example for a double step process, we consider the excitation of
  a photoelectron from a core level and the subsequent filling of the
  core vacancy by a valence electron, leading to the emission of a
  second valence electron via an Auger mechanism. These processes are
  well known, and have been widely studied in various solids [8]. As a
  further double step scenario for a two-electron photoemission, we
  suggest the combination of a single photoionization in the valence
  band and a subsequent collision between the photoelectron and a second
  band electron leading to the emission of both electrons.

Note that the first mechanism is already used in surface analysis methods like AES. 
As a last note I would like to point out that when working with metals (and solids in general) you should always work within the framework of "Band theory for solids"!
A: No because according to particle nature of light there is a collision between an electron and photon. Thus one photon can emit only one electron the rest energy get converted to overcome collisions by other metal atoms and kinetic energy.
