ABSTRACT
For molecules composed of light elements, and particularly organic molecules, spin-orbit coupling (HSO) usually is the most important of the small terms in the molecular Hamiltonian operator that mix states of different zero-order spin multiplicity. Hyperfine coupling with magnetic nuclei (HHF) is another, and spin-spin dipolar coupling (HSS) a distant third. The most commonly encountered case is the mixing of zeroorder singlets and triplets, and only rarely does one need to consider the mixing of doublets and quartets. The primary importance of such mixing is that it induces transitions between zero-order states of different spin multiplicity. Radiative transitions of this type are referred to as phosphorescence and spin-forbidden (usually singlet-triplet) absorption, and they are important in spectroscopy and photophysics. Nonradiative transitions of this type are known as intersystem crossing, and they are of particular interest to photophysicists and photochemists.
