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Think of an hydrogen atom with an electron at n=1. What happens when a low energy photon(not enough to excite electron to even to n=2) collides with the electron at the first energy level? Does photon scatter without losing its energy and momentum?

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Scattering in a different direction means that the photon momentum changes. Both energy and momentum of the system are conserved. The scattered photon would transfer some momentum to the atom. So an atom that was initially at rest would also gain some kinetic energy. The photon experiences a recoil shift.

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  • $\begingroup$ Thanks for response, but actually what I ask is, what happens if photon strikes to the first orbital electron of then hydrogen atom. Because photon is low energy, it will not be able to excite electron to n=2 or bigger. So energy and as a result momentum of photon and electron will not change. So what happens? $\endgroup$
    – Salihm
    Mar 8, 2018 at 15:04
  • $\begingroup$ @Salihm I understood your question. The answer is that the atom will recoil. It is that hydrogen atom as a whole that will start to move (if it was at rest to begin with). (A detailed description would need QED, I cannot do that.) $\endgroup$
    – user137289
    Mar 8, 2018 at 15:30
  • $\begingroup$ oh, I didnt know that. So proton and electron as a whole, will gain some kinetic energy, but, electron state will not change. Ok, one more thing, how proton is affected by the collision of electron and photon? $\endgroup$
    – Salihm
    Mar 8, 2018 at 15:45
  • $\begingroup$ @Salihm The detailed mechanism is a problem in QED (quantum electrodynamics), which is really beyond me, as I am a simple experimentalist. But it is probably correct to say that the proton is affected indirectly by the E-field of the incoming radiation. $\endgroup$
    – user137289
    Mar 8, 2018 at 18:02

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