Optical nuclear polarisation (ONP) in anthracene doped with phenazine in the temperature range 1,4...300 K

Optical nuclear polarisation (ONP) of proton spins in anthracene doped with 1,000 ppm phenazine has been measured as a function of the external magnetic field Hsub(p) and its orientation with respect to the crystalline axes for the temperatures: 300, 78, 4.2 and 1.4 K. At 300 K characteristic features in magnetic fields up to 150 G are similar to results observed in doped fluorene crystals. They are interpreted as due to ONP as a consequence of level anti-crossing (LAC) in the triplet state of guest-host complexes. The zero-field splitting tensor is determined. The most prominent feature at all temperatures is the field dependence of ONP when the magnetic field is oriented along the projection of the long molecular in-plane axes of the anthracene molecules. The excitable triplet states in the doped anthracene crystal are discussed in terms of their qualification as ONP-active states. (orig./GSC)