A process in which a singlet excited electronic state makes a transition to a triplet excited state at the point where the potential energy curves for the excited singlet and triplet states cross. This transition is forbidden in the absence of spin-orbit coupling but occurs in the presence of spin-orbit coupling. A triplet formed in this way is frequently in an excited vibrational state. This excited triplet state can reach its lowest vibrational state by collisions with other molecules. The transition from this state to the singlet state is forbidden in the absence of spin-orbit coupling but allowed when there is spin-orbit coupling. This gives rise to the slow emision of electromagnetic radiation known as phosphorescence.