Interpretation of level-anticrossing phenomena in phosphorescence and optical nuclear polarization in organic crystals

Abstract

Level-anticrossing (LAC) phenomena in electronic states of molecules in organic crystals are described with a density matrix formalism. A condition for the observability of LAC signals in phosphorescence and ONP is derived. The validity of these predictions is tested at quadrupole satellite positions in the LAC spectra. As a consequence of the full density matrix treatment, off-diagonal damping terms Γ mn = T 2 -1(mn) are introduced. These turned out to be essential for the interpretation of the vanishing of experimentally observed LAC signals at higher order satellite positions. If T 2 equals the lifetime τ of the triplet state, earlier predictions based on kinetic arguments evolve as a limiting case of the present theory. The quadrupole satellite signals can be described properly by the density matrix formalism and there is no need for the introduction of an additional cross-relaxation mechanism.