The process of exchange of energy between a photon and an electron only occur after a specific energy called work-function of the material. Thus, the energy transferred is quantised due to the fact that the electron can reside only in quantised orbits.

But, why doesn't the process happen in steps? I mean the electron getting excited to a higher orbit and before the electron gets de-excited another photon gives it energy to escape from the metal. Is it theoretically possible?


Interactions between electrons in excited states and photons can happen. The probability will of course depend on the time that electrons spend on the excited level. You will probably need to have a metastable (molecular) orbital in order to have some chances to extract electrons with a double interaction.

However there is another interesting process which involves a similar mechanics. In this case the interaction of the right energy photon with an excited electron results into the electron being dropped to the fundamental level and two identical photons being emitted. It is called stimulated emission and it is the basic of lasers.

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