Vibrational Relaxation:
In the higher vibrational levels of an excited state, a molecule rapidly loses (in < 10-12 s) its excess vibrational energy through collision along with other molecules and falls to the lowest vibrational level of the excited state. Within this nonradiative mode of relaxation, known as vibrational relaxation, the energy lost is dissipated as heat to the surroundings. In principle, a nonradiative or radiationless deactivation might also lead to the relaxation of the S1 state to S0 state. Therefore, such a relaxation is quite slow.