When an atom or molecule absorbs energy, some of its electrons can be shifted to higher orbits. This is called an excited state. By studying how this energy is absorbed and how it is released as the atom or molecule decays back to the ground state, insights into the structure and bonding can be gained.
Most excited states exist for short times, on the order of a nanosecond. There are several ways that the decay back to the ground state can occur. A molecule can be put into an excited state by absorption of a photon and then fall apart (photodissociation). An excited molecule can emit a photon to return to the ground state via a fluorescence or phosphorescence process. There are even modes of decay where no photon is emitted (radiationless decay).
The atoms that make up a molecule are vibrating. These vibrations can influence the electrons and create numerous vibrational energy levels within an electronic level. Probing these vibrational sublevels with various techniques can also help to reveal the structure and environment of the molecules.