The relaxation of excited molecular states is often followed by the emission of light. Molecular excited states are related to the orbital, vibrational, and rotational levels of the orbital electrons. In addition, the electron spin determines the singlet and triplet states. In the singlet state the spin of an electron is paired, whereas in the triplet state the spin is not paired. When a molecule is excited to an upper singlet energy level, such as S2(Figure 1), it can rapidly (10−10sec) go to the lowest excited state S1without emission of light. From S1the molecule may go to any of the rotational and vibrational levels of the ground electronic state S0. This happens either by fluorescence emission or by nonradiative internal processes, referred to as intersystem crossing. From T1, the molecule can return to S0by nonradiative processes or by a radiative process called phosphorescence. Under suitable conditions S1and T1can also transfer their excitation energy to other molecules.