Polarizing proton spins by electron-spin locking of photo-excited triplet state molecules

Abstract The transfer of the electron spin polarization from photo-excited triplet state molecules to proton spins has been studied in single crystals of fluorene doped with 2000 ppm acridine in three isotopically different forms. From spin-locking experiments it appears that the proton spins on the acridine guest molecules 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 do not observe fast Rabi oscillations in the polarization transfer process. This is explained by the fact that the magnetic field in our experiments is large, resulting in a weak contact between the nuclear Zeeman reservoir and the electron dipole-dipole reservoir.