Proton Polarization with Naphthalene Crystals by Integrated Solid Effect on Photoexcited Triplet State

We have carried out detailed systematic studies of relaxation effects, which contribute to a significant improvement in proton polarization in a single crystal of naphthalene doped with pentacene. The experiments have been performed in a magnetic field of 3 kG at room temperature or at 77 K and with a pulsed dye laser or a pulsed N-2 laser. We have found that a relaxation rate in the presence of irradiation with visible light (lambda = 595 nm) is much smaller than that with ultraviolet light (lambda = 337 nm) at room temperature, although the absorption rate of visible light is much larger than that of ultraviolet light. By cooling the crystal, the slow motion of the naphthalene molecules, which is a dominant relaxation process, can be suppressed. Surprisingly, we have found that the relaxation time at 77 K in the presence of the dye laser becomes long, but still depends on the laser power. Our investigation has also indicated that both the longer relaxation time and the irradiation with the dye laser lead to a great improvement in the polarization of protons in the bulk of a crystal from 0.5% at room temperature to 32% at 77 K.