We are taught that in photoelectric effect if the frequency of light is lower than the threshold, then no matter how long a metal is exposed to it there won't be any ejection of electrons.

This made me consider a thought experiment. If we shoot two photons at an isolated atom, having energies equal to half of the ionisation enthalpy of the atom and hence a sum which is equal to the ionisation enthalpy of the atom. Now, in case they hit the atom at the same exact instant, would there be any photoelectric effect?

If no, then why? Why no Photo E Effect when some low frequencies are exposured for more than 20-30 hrs?

I felt like the explanation is that since electrons take energies at discrete amounts, taking some energy from one photon and the rest from the others won't work. However, hitting the atom at the same instant should actually let it happen.

My little thought experiment might be a little vague. Thus, My direct question is why PEE starts only at a particle frequency? Why increasing the intensity of light of lower wavelengths don't work?

  • $\begingroup$ It is not very meaningful to talk about shooting photons at the same time at the same atom. But yes, at high light intensities there are non-linear effects. And there is for example resonant 2-photon photoemission via image-potential states. $\endgroup$
    – user137289
    Jul 10, 2020 at 12:41
  • $\begingroup$ No mine is a little different. The two electron absorption was only a solution that I thought of but I still don't understand why exposure of low frequencies don't constitute photoelectric effect $\endgroup$ Jul 10, 2020 at 18:42

1 Answer 1


The answer is 'yes, sort of.' The approach isn't necessarily to get two identical wavelength photons to be absorbed simultaneously, but rather to have one photon "bump up" an electron to a higher orbital, then the second photon kicks the electron free before it can reradiate back to ground state.

In fact, 2-photon absorption is an effect used for passive Q-switching in some laser designs. In this setup, rather than ionization occurring, the material changes its absorptivity quite dramatically when the electron is pushed up to a certain high orbital. In the ground state, the optical transmissivity is low, but once the incident photon density reaches a critical level, 2-photon absorption happens, the material's transmissivity goes close to 1.0 .

  • $\begingroup$ And multi-photon photoelectric effect is known, and mentioned in answers to other questions here. $\endgroup$
    – Jon Custer
    Jul 10, 2020 at 12:15
  • $\begingroup$ So why there's no photoelectric effect in the case of low frequency light?(even after exposing for a long time) $\endgroup$ Jul 10, 2020 at 13:09
  • $\begingroup$ @uddhavsaikia because the lifetime of the intermediate state is very short, so unless "lots" of photons show up at essentially the same time, 2-photon absorption won't happen. $\endgroup$ Jul 10, 2020 at 13:50
  • $\begingroup$ @JonCuster if you can dig up a link or two to multiphoton, I'll put in a close vote. $\endgroup$ Jul 10, 2020 at 13:50
  • $\begingroup$ @CarlWitthoft - found duplicate. $\endgroup$
    – Jon Custer
    Jul 10, 2020 at 13:53

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