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Abstract

The Optical Nuclear Polarization (ONP) in molecular crystals is a consequence of the selective population and depopulation of the electronic magnetic sublevels of optically excited triplet states due to symmetry selection rules for the mixing of electronic states by spin orbit coupling. Two ONP-mechanisms have been proposed on this basis: The "relaxation"-mechanism, analogous to the Overhauser-effect, treats the optically produced distribution over the electronic magnetic sublevels as a steady state polarization which is transferred partially to the nuclear spin states via suitable hyperfine relaxation. The "crossing polarization"-mechanism takes into account in addition the mixing of states due to the hyperfine interaction in the electronic intersystem crossing processes. In this paper the latter mechanism is extended to a complete description of the dynamics of population and depopulation of the electronic and nuclear spin levels by including also all relaxation processes among the magnetic sublevels. It is shown that only in the extended form the mechanism can account for the experimentally observed optical nuclear spin alignment in zero external field.