A new mechanism is proposed for spin-lattice relaxation (SPL) in the phosphorescent triplet state of a molecule that has no inversion point. The mechanism is based on the fact that the internal spin-orbital interaction in a molecule allows the transition between spin sublevels of the triplet induced by the variable crystal field of the lattice.
Recent experiments on phosphorescent-microwave double resonance and the spin echo in triplet excited states of organic molecules at helium temperatures [1-4] have revealed a strong dependence of the spin-lattice interaction on the kind of molecule as well as on precisely which triplet sublevels the relaxation processes proceed. Electron spin-lattice relaxation {SLR) of a triplet molecule inserted in a molecular crystal-carrier has been examined in [5][6][7]; however, in this case the explanation of the SLR mechanism is still far from definite. This is because the crystal field and the spin-orbital interaction (SOI) in molecular systems is much weaker than in ionic crystals. Hence, the SLR mechanism associated with the crystal field vibrations (Van u mechanism [8]) will have little effect. The role of the spin-lattice coupling mechanisms, i.e., the hyperfine and exchange mechanisms [9, 10] which are usually neglected in ionic crystals, will grow correspondingly. Thus, the Van Vleck mechanism is generally neglected in [7] in comparison to the hyperfine mechanism in an examination of SLR in a zero external magnetic field.