What is Radiationless processes?

Vibrational relaxation brings an excited molecule to the lowest vibrational level of its electronic state by collisions, in picoseconds. Internal conversion is a radiationless transition between states of the same multiplicity ($S_2 \to S_1$, $S_1 \to S_0$); intersystem crossing one between states of different multiplicity ($S_1 \to T_1$, $T_1 \to S_0$).