Vibrational relaxation is an important stage in releasing energy to allow FRET to occur. This is because Fret can only occur with electrons in the S1 excited state - electrons at the S2 excited state must loose energy to the S1 level.
Vibrational relaxation releases thermal energy into the environment, converting from potential energy. Therefore, in photosystems, the fate of the S2 excited state is to the S1 excited state, and then transfer to the reaction centre by Fret. ****