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?