The idea that an electronic transition
occurs much faster than nuclear
motion. For example, if you hit a molecule
with a photon
, driving it into the excited state
, the electrons will rearrange their distribution in the orbitals without the nuclei
having a chance to rearrange themselves. The resulting system is often not in equilibrium, and depending on the lifetime of the excited state, the nuclei may then rearrange (slightly) to accomodate the new electron distribution.