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

It is shown that the large electron spin polarization created in the photoe xcited triplet species can be transferred to the nuclear spins even in a po lycrystalline 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 o f the proton spin-lattice relaxation and the sample thickness on the attain ed polarization is discussed. (C) 2001 Elsevier Science B.V. All rights res erved.