Q & A: vibronic spectra

Is photon emission possible without electrons changing energy levels? Does molecular vibrational transition and consequent emission of infrared radiation involve electrons changing energy level? In wikipedia, about vibronic transitions it says "Most processes leading to the absorption and emission of visible light, are due to vibronic transitions. This may be contrasted to pure electronic transitions which occur in atoms and lead to sharp monochromatic lines (e.g. in a sodium vapor lamp) or pure vibrational transitions which only absorb or emit infrared light.". Does this mean infrared radiation is emitted without electrons playing a direct part? What about changing in molecular rotational energies? Also, I understand reflection is not a radiation absorption-emission phenomena, so can you explain what actually happens to the photons? Same thing for radiation scattering.
- João (age 30)
Lisbon, Portugal

Just to make sure other readers are caught up, let's mention first that vibronic transitions are ones in which the electron state changes and the vibrational state also changes. That broadens the otherwise sharp electronic transitions, since a variety of low-energy vibrational states are available.

I think you're asking about the vibrational transitions. These involve a change of the vibrational state of the whole molecule while the electrons remain in the state of lowest possible energy consistent with the vibrational state. If the negative electrons and positive nuclei moved exactly together (to use a classical-sounding description)  in the vibrations then there would be no dipole coupling to the electric field. However, in actual vibrations they move a little bit differently, so that give the coupling to the electric field, including dipole coupling except when it's forbidden by symmetry. (A breathing-mode vibration of methane would be an example of such symmetry.)

There's a nice wikipedia article describing the symmetry constraints on rotational spectra: http://en.wikipedia.org/wiki/Rotational_spectroscopy. Electrons of course play an essential role in these transitions, since it's how they're distributed about the nuclei that accounts for the dipole moments of the molecules. That isn't quite the same role as they play in electronic excitations.

As for reflection and scattering, reflection is really just a special case of scattering. Classically, you can picture all the forms of scattering as simply solutions of the wave equation in a medium in which the index of refraction varies with position. For a photon picture, you could say that the energy eigenstates of the coupled electromagnetic-electronic Hamiltonian are not momentum eigenstates (the Hamiltonian doesn't commute with the momentum operator because one assumes that the electrons are influenced by fixed-position nuclei) so a state of definite energy doesn't just keep going in a fixed direction. The point is that in these circumstances if by "photon" you still mean a state with definite energy it's already something that changes direction, so no absorption-emission process with time delay etc. is involved.

Mike W.

(published on 02/06/2012)