Dynamic nuclear polarization by photoexcited-triplet electron spins in polycrystalline samples

It is shown that the large electron spin polarization created in the photoexcited triplet species can be transferred to the nuclear spins even in a polycrystalline sample by utilizing a partial area of the extremely broad ESR powder spectrum for cross polarization (CP). The proton polarization 3160 times as large as the thermal equilibrium value was obtained in 0.018 mol% pentacene-doped naphthalene at 100 K in a field of 0.319 T. The influence of the proton spin–lattice relaxation and the sample thickness on the attained polarization is discussed.