Hey guys,
this is a concept i have either forgotten or disregarded as being obvious but i dont undersntand why, for photoelectric emission, an electron has to gain enough kinetic energy from a single photon only in order to escape the surface or material it is on. LIke say for example, an electron accepts a photon and gains kinetic energy but not enough to escape, what if it gets another low energy photon, wouldnt the kinetic energy be increased until the point it has enough energy to escape? We spoke about this is my secondary school days but i dont recall our teachers ever explaining why this is the case.

 

In fact, one can observe the photoelectric effect from the simultaneous absorption of two photons, neither of which have sufficient energy alone to produce it. See:
http://www.bu.edu/qil/pdf/JAP-100-023521-2006.pdf

The simple explanation why it is not much more commonly observed is because the absorption of the photons has to be virtually simultaneous. Since only a small proportion of electrons on the surface are actually displaced by a beam of light, the probability that two photons would hit the same atom/electron is quite low. Very intense (e.g, focused laser) light is necessary to get significant two-photon emissions. Alternatively, if the absorption of light and emission of the electron is slowed, then the probabilty of two-photon photoelectric emission is also increased. John

Cheers for the response, that helps alot! One more question though, in a led, a certain threshold voltage has to be achieved to allow electrons to flow across the depletion region. What i am not sure of, is if the voltage across the region is incresed, as the electrons jump between atoms, wouldnt their kinetic energies increase due to the potential difference? And when this electron recombines with a hole in the p junction, howcome the photon it emits isnt a higher energy photon? Basically why is it that recombination of higher energy electrons which have more energy than the work function for that material just produces phonons and not higher energy photons?

 

You can get a small emitted light frequency sweep in a LED by varying the current. Chemists use this as a sensitive probe in experiments. Things like finding peaks in fluorescence and such.

 

Oh... ok. You see the thing i was confusing here was i thought that as the electron transistions between holes, i thought that the kinetic energy it receives as a result of the potential difference. My lecturer was explaining to me however that the electrons dont get extra kinetic energy as they travel between holes... actually i still dont entirely understand why this is the case... even if the voltage is increased slightly between the ends of the junction, wouldnt the electrons gain some more kinetic energy as a result of travelling through the stronger field? Even if this gain of energy is slight, wouldnt this extra energy increase the energy of the emitted photon when it recombines with the next hole? I know the answer to this question is know but i am still unsure as to why this is the case. Any kind learned souls out there care to take a stab at this?