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When the atom is emitting electromagnetic wave in a classical view it is an oscillating dipole. If during the emission that atom interacts with another atom via collision, emission is interrupted and that makes the emitted wavepacket shorter in time and having broader spectrum, which is called pressure broadening.

Looking at the emission process in a quantum way, it goes into a superposition (dipole) and emits a photon (that has some spectrum). When the collision occurs during emission and the atom loses the energy it had "prepared" for emitting a photon, what happens with the photon (as it is already "partially emitted")? It has to be either emitted or not, can't have half of the photon, right? Does it go into some superposition of existing and not existing? And how does it relate to spectrum broadening?

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Ok, so I learned the answer to that problem.

As the atoms never truly collide, like ping-pong balls can in a microscopic view, the effect is easily explainable.

During the emission of a photon, when another atom is moving towards the emitting one, electic field of its electron cloud will change the energy levels that emitting atom has. It will now have shifted energy levels compared to the vacuum case.

During such collision approaching and receding of another atom will modify the energy levels to have lower and higher energy than for an atom in a vacuum. So the spectrum of the photon will contain lower and higher frequencies of electromagnetic field.

More classically, oscillating dipole will oscillate a bit differently in a situation that some other electric field source will fly towards it.

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