POLARIZING PROTON SPINS BY ELECTRON-SPIN LOCKING OF PHOTO-EXCITED TRIPLET-STATE MOLECULES

The transfer of the electron spin polarization from photo-excited trip let state molecules to proton spins has been studied in single crystal s of fluorene doped with 2000 ppm acridine in three isotopically diffe rent forms. From spin-locking experiments it appears that the proton s pins on the acridine guest molecule play a minor role in this process and that the evolution of the nuclear spin polarization is determined by the second moment of the dipolar interaction with the proton spins on neighbouring fluorene molecules. In contrast to recent reports we d o not observe fast Rabi oscillations in the polarization transfer proc ess. This is explained by the fact that the magnetic field in our expe riments is large, resulting in a weak contact between the nuclear Zeem an reservoir and the electron dipole-dipole reservoir.