A type of spectroscopy based on the fluorescence of molecules (see luminescence). When a molecule absorbs a photon, this causes a transition from the electronic ground state to a vibrational state of an excited electronic state. The molecule then loses energy when it collides with other molecules, resulting in it going to the lowest vibrational state of the excited electronic state. The molecule finally emits a photon as it drops down to a vibrational energy level of the ground state. The pattern of photons emitted enables the pattern of energy levels of the molecule to be determined.