Nuclear spin orientation via pulsed EPR and optically cooled triplet electron spins

Abstract

In this paper we discuss a method to achieve high nuclear polarizations at room temperature using optically “cooled” triplet spins and pulsed EPR techniques. We present the results of experiments performed on a single crystal of naphthalene doped with pentacene guest molecules. By using pulsed laser excitation we populate the pentacene molecules almost exclusively in one sublevel of the lowest, metastable triplet state. Then by applying pulsed microwave fields we create an efficient transfer of polarization, via the Hartmann-Hahn condition for cross-relaxation, between the electron spins and the proton spins. Further we ensure that all electron spins in the inhomogeneously broadened EPR line participate in the transfer process. The result of our experiments shows that an unusually high proton spin polarization can be achieved and that the transfer occurs in a time determined by the dipolar interaction between the electron spins of the pentacene guests and the proton spins on the surrounding naphthalene molecules. © 1992 IUPAC