Optical nuclear polarization of protons in fluorene-d 8,h 2 single crystals

Abstract

The optical nuclear polarization has been studied in single crystals of octadeutero-9,9′-proto-fluorene (fluorene-d 8,h 2). In this case, only two protons with spin I = 1 2 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-h 10. Proton ONP was observed at room temperature as a function of the magnetic field H p as well as of its orientation in all crystalline planes. The maximum ONP is p L ≈ 10 −3, corresponding to a spin temperature of 8 mK in the optimal polarization field H p = 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.