<P> In molecular spectroscopy, a Jablonski diagram is a diagram that illustrates the electronic states of a molecule and the transitions between them . The states are arranged vertically by energy and grouped horizontally by spin multiplicity . Nonradiative transitions are indicated by squiggly arrows and radiative transitions by straight arrows . The vibrational ground states of each electronic state are indicated with thick lines, the higher vibrational states with thinner lines . The diagram is named after the Polish physicist Aleksander Jabłoński . </P> <P> Radiative transitions involve the absorption, if the transition occurs to a higher energy level, or the emission, in the reverse case, of a photon . Nonradiative transitions arise through several different mechanisms, all differently labeled in the diagram . Relaxation of the excited state to its lowest vibrational level is called vibrational relaxation . This process involves the dissipation of energy from the molecule to its surroundings, and thus it cannot occur for isolated molecules . A second type of nonradiative transition is internal conversion (IC), which occurs when a vibrational state of an electronically excited state can couple to a vibrational state of a lower electronic state . A third type is intersystem crossing (ISC); this is a transition to a state with a different spin multiplicity . In molecules with large spin - orbit coupling, intersystem crossing is much more important than in molecules that exhibit only small spin - orbit coupling . This type of nonradiative transition can give rise to phosphorescence . </P>

Radiative and nonradiative pathway of excited molecular system