Presented at the Fifth International Symposium on Magnetic Resonance, Bombay, India, January, 1974

The optical nuclear polarization has been studied in single crystals of octadeutero-9,9′-proto-fluorene (fluorene-d8,h2). In this case, only two protons with spin per molecule interact with the excited triplet states of spin S = 1. From the proton NMR data it is concluded that the crystalline proton positions agree with those predicted for the known crystal structure of fluorene-h10. Proton ONP was observed at room temperature as a function of the magnetic field Hp as well as of its orientation in all crystalline planes. The maximum ONP is pL ≈ 10−3, corresponding to a spin temperature of 8 mK in the optimal polarization field Hp = 80 G. The quantum efficiency, i.e., the number of polarized proton spins per absorbed light quantum, is about 10−2. The basic experimental results can be understood with a general ONP mechanism which is reviewed briefly. Some striking properties of the field and orientation dependence are tentatively related to particular cross-relaxation processes, which occur if the transition frequencies between different pairs of the electron and nuclear spin sublevels are so close that they overlap within their respective linewidth