Different mechanisms of nonradiative relaxation:

The nonradiative relaxation refers to the deactivation of the molecule within the excited state without emission of radiation. This may occur through any of the three mechanisms viz., vibrational relaxation, internal conversion and inter-system crossing.

Within vibrational relaxation, the molecules rapidly lose their excess vibrational energy through collision with other molecules and the energy is dissipated as heat to the surroundings.

Within internal conversion the molecule is transferred from an excited singlet state to other singlet state of the similar energy.

Within intersystem crossing a molecule is transferred from an excited singlet state to vibrational stages of a triplet state (or vice-versa).