# How do EM waves cause molecules to rotate, move, and cause electrons to move up energy levels?

Atoms can absorb electromagnetic waves to cause electrons to jump up energy levels, when electrons go down energy levels they release a photon. But how do EM waves cause atoms to rotate, and vibrate?

• It's more for Physics.SE Probably there are some answers on this topic Jul 9 '16 at 21:16
• Ok so I'll ask it in the physics stack exchange. Jul 9 '16 at 21:18
• No, don't do this. It would be cross-posting. If it won't get answers here, it can be migrated. Jul 9 '16 at 21:19
• Actually, this question can be put into physical chemistry domains, as quantum chemistry need to deal with such interaction Jul 9 '16 at 22:25
• Electrons move up energy levels simply because they gain such under the influence of EM force. This process however is unstable and if no further perturbations apply, any excited electrons will tend to go back in their basic state. If you ask why up rather than down - this is due to the Pauli Principle... The vibration of molecules is due to the fact that their bonding resembles Simple Harmonic Oscillator => any perturbation would cause a vibration of some sort. Jul 10 '16 at 9:32

But how do EM waves cause atoms to rotate, and vibrate?

In no way do EM waves cause atoms to rotate and vibrate. Atoms (more precisely, nuclei) in a molecule are always moving, so EM radiation can only change the way they move.

The thing is that within the Born–Oppenheimer approximation an energy state of a molecule is the sum of the electronic, vibrational, and rotational components, $$E = E_{\rm{el}} + E_{\rm{vib}} + E_{\rm{rot}} \, ,$$ and there are accordingly three types of states and energy levels: electronic, vibrational, and rotational. For each and every electronic state there are few vibrational states associated with it and for each and every vibrational state there are few rotational states associated with it.

A transition can happen not only between different electronic states, but also between different vibrational states associated with the same electronic state or between different rotational states associated with the same vibrational state. Different vibrational and rotational states correspond to different ways atoms of a molecule move in space, so vibrational and rotational transitions change the way atoms move.

And, of course, any transition emits or absorbs a photon, the frequency of which is proportional to the difference in energy levels. Vibrational and rotational transitions spectra are studied by rotational–vibrational spectroscopy.

Atoms cannot vibrate or rotate, so I'm guessing you mean molecules as this is in the chemistry section, if not that is your answer.

To explain heaps of spectroscopy is clearly not possible here, so I attempt at some brief explanations. If an atom absorbs a photon it cannot cause it to rotate as such but it can induce orbital angular momentum. By selection rules the orbital angular momentum has to change by 1 unit up or down, thus an s orbital in an H atom, say, is promoted to a p orbital which is a change of 1 unit but s cannot be promoted to d orbital as this is a 2 unit change, and so forth. Emission changes from p to s with the same rules.
In molecules infra red photons can excite vibrational quanta (if the molecule has a permanent or induced dipole). For simplicity consider a diatomic molecule. Upon absorbing a photon the molecule given one extra quantum which causes it to vibrate with a larger relative displacement of the two atoms, i.e. the bond length is unchanged (assuming harmonic motion) but the atoms vibrate with bigger amplitude (using a classical description). Molecules also have whole body rotational motion and we assume that they are free to rotate in the gas phase. They can have zero angular momentum (no rotation) and this increases by +-1 units for rotational motion left or right. Rotational motion is induced by absorbing a photon and converting its unit of angular momentum into motion of the molecule.

(edit: In case you are wondering why vibrational transitions occur that seem not to involve any angular momentum, it is because pure vibrational transitions do not occur, but only transition involving simultaneous vibration and rotation.)