Do atoms recoil when emitting a photon?

[vanhees71]

How is that consistent with the Blackbody spectrum if an atom in its ground state can move around?

Let's argue within non-relativistic QT for simplicity (and it's entirely justified for not too high ##Z##). Due to translation invariance the total momentum of an atom is conserved, i.e., the center-of-mass motion separates from the relative motion of the electrons and the atomic nucleus. The bound energy eigenstates of the latter define the intrinsic states of the atom. Indpendently from this the atom can move with any momentum relative to your (arbitrarily chosen) reference frame.

If you know have the atom in some excited state you can always Galilei-boost to its rest frame, i.e., to the frame, where the total momentum of this atom is 0. Now it will at some random time relax to a lower state by spontaneously emitting a photon (due to the vacuum fluctuations of the em. field). This photon has a momentum ##\vec{p}_{\gamma}=\hbar \vec{k}##, and since momentum is conserved (due to spatial translation invariance of the closed system consisting of the atom + (quantized) radiation field) the total momentum of the atom after emission of the photon must be ##\vec{P}_{\text{atom}}=-\vec{p}_{\gamma}##.

This should be in any textbook on atomic physics. I've no time to look for a specific reference though ;-)).

 

[Frabjous]

I do read your responses. The problem I think is that I am too critical and I do not just accept something. And you are so to speak textbook correct. But since I doubt textbooks before accepting something, my questions might sound stupid.

Thanks for the exchange. I will think about it.

You need to apply some of that criticality to yourself. You are taking a hodge podge of facts and are trying to paste them together. If you glue an apple to an orange, it does not mean that an “orple“ is a useful concept.

 

[berkeman]

I doubt textbooks before accepting something

How is that learning strategy working out for you so far? Maybe more importantly, how is it working out for your teachers and tutors (like us)?

 

[Chas Tennis]

The text Optics, 3rd Ed by E. Hecht (1998) has the answer to the OP question. Page 51, Figure 3.16. Yes, atoms recoil. I don't see a reference.

The experiment shown is of
1) Source of atoms.
2) Through 1st aperture to form directed atoms.
3) Excitation is added.
4) Through 2nd aperture forming beam of excited atoms.
5) Later atoms radiate and the beam spreads.

Search for that unidentified experiment and similar experiments.