Radiative Deactivation:

While the molecule in the excited state (S₁) relaxes down to the lowest vibrational level it might emit a photon and come down to the electronic ground state (S₀). This procedure is known as fluorescence and takes about 10^-9 s.  Another radiative relaxation process could arise from the excited molecule which had crossed over to the triplet excited state through intersystem crossing and has relaxed to the vibrational ground state in the triplet excited state. Within such a case the molecule emits a photon and comes down to a vibrational mode of the electronic ground state, S₀. This phenomenon is known as phosphorescence.   Since the transition from a triplet state to a singlet state is theoretically forbidden, it does not take place readily. Therefore, while the fluorescence emission can take place within 10^-9 - 10^-6 seconds, a transition from an excited triplet state to the ground state in that case of phosphorescence needs at least 10^-4 seconds and may take as long as 10² seconds.