High Field Cross Polarization NMR from Laser Polarized Xenon to a Polymer Surface

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Surface-selective characterization of materials with NMR has been quite useful in the few cases where sufficient sensitivity and selectivity have been achieved.1 In this communication we report the use of laser-polarized xenon as the source of magnetization for a high-field cross polarization experiment, obtaining surfaceselective magnetization transfer. Gas-phase xenon with nuclear spin polarization several orders of magnitude higher than thermal Boltzmann levels in a high magnetic field can be produced using optically pumped rubidium vapor according to the pioneering work of Happer and co-workers.1 2 The angular momentum of circularly polarized laser light is transferred, via the rubidium electron spins, to the (slowly relaxing) xenon nuclear spin system. We have previously used xenon, with a large polarization enhancement («10 000), as a probe of low surface area materials.3 More recently, using thermal mixing in low field,4 polarization has been transferred between xenon isotopes5 and from xenon to 13C02 occluded in solid xenon.6 The approach of low-field mixing has the disadvantages of nonselective magnetization transfer, typically very short spin-lattice relaxation times in low field, and the necessity of rapid field switching or mechanical transport of the sample to high field for detection. High-field cross polarization methods should be advantageous in allowing nuclear spin selectivity in the transfer step as well as circumventing relaxation and transport problems. In the present work, contact between laser-polarized xenon and surface spins was achieved in high field by Hartmann-Hahn matching of the energy levels in the rotating frame with direct NMR detection of the polarized species.7-8 Proton spins are observed due to their abundance at the surface and the dominant dipolar interactions with adsorbed xenon.

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Journal of the American Chemical Society





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