# Where does the momentum go when atom absorb a photon? [duplicate]

Imagine an electron around an atom absorbs a photon and becomes excited, it has now jumped to a higher orbital. At this point in time, where does the momentum of the photon goes?

• Can we reopen this question because I'm having difficulty following the Q&A written by Emilio? – user6760 Dec 27 '18 at 11:19
• What level of rigor were you looking for? – bRost03 Dec 27 '18 at 15:42

Emilio's answer (How does one account for the momentum of an absorbed photon?) is great but quite technical. The simplified version is some of the momentum is accounted for by the different linear momentum associated with different orbitals. From the Bohr model we have $$p_n=\dfrac{\hbar}{a_0 n}$$ where $$n$$ is the principal quantum number which changes when an atom is excited. The rest of the momentum causes the atom itself to recoil. This is the mechanism whereby a photon absorption can excite a phonon (it kicks the atom which sets the lattice vibrating).
• The recoil affects the linear momentum of the whole atom, not just the electron(s) relative to the nucleus. It might be easier to see this by looking at the reverse process. If an atom emits a photon in the X direction, the recoil forces the atom to move in the -X direction. The atom's velocity is very tiny though, around $10^{-9}$ the speed associated with its thermal motion at room temperature. – PM 2Ring Dec 27 '18 at 17:05