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

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The surfaces of platinum clusters in the supercages of NaY zeolite have been studied by ' Xe NMR over the temperature range 80 -293 K. The sensitivity of this weakly perturbing technique makes it possible to reveal the heterogeneity of the electronic structure of the particle surface. The temperature dependence of the ' Xe spin-lattice relaxation indicates that the surface sites with the highest adsorption energy possess metallic character.

NMR studies of xenon gas dissolved in the liquid crystal ZLI 1132 confined to submicron cylindrical cavities are reported. Spectra taken as a function of temperature yield a clear indication of the nematic to isotropic phase transition of the confined liquid crystals. In the nematic phase at 21 "C, the resonance line of dissolved 129Xe exhibits a chemical shift anisotropy of 15 ppm due to a random distribution of director axes in the plane perpendicular to the long axis of the cylinder. The anisotropy and temperature dependence of the confined system are compared to control experiments that use the bulk liquid crystal. The quadrupolar splitting observed in the I3'Xe NMR spectrum of the confined liquid crystalline solution of xenon gas is slightly greater than that found in the bulk. Two-dimensional exchange NMR demonstrates that the xenon atoms probe different average liquid crystal directors within a single cavity on a 20 ms time scale and that interpore exchange occurs on a time scale of 400 ms. The exchange data indicate that changes in the orientation of the director within individual cavities occur on a length scale of about 2 pm. IntroductionLiquid crystals are technologically important due to their widespread use in displays (LCDs) and their potential applicability in nonlinear optical devices.'-3 Liquid crystalline phases exhibit long-range molecular orientational order, in contrast to normal liquids which lack positional and orientational order. Devices based on liquid crystals are able to exploit the anisotropy of the medium for the control of alignment and switching of the director axis formed by the liquid crystalline phases. Characterization of the orientational order and the factors which govern it are therefore of considerable scientific and commercial interest.Nuclear magnetic resonance (NMR) is useful for studying liquid crystals4 due to the sensitivity of the NMR spectrum to orientational order. Anisotropic interactions such as chemical shift anisotropy, quadrupolar interactions, and magnetic dipolar couplings make it possible to measure the degree of orientational order. However, spectra can be extremely complicated and intractable for abundant spins in multiple sites, e.g., protons in the liquid crystal molecules. A common simplifying approach is to study small probe molecules dissolved in the liquid crystal or to use liquid crystal molecules which have been isotopically labeled at specific sites. For solute molecules within a liquid crystalline environment, the ordering of the solute by dispersive and steric forces can be described by an order tensor that relates the average alignment of the solute molecular frame to the liquid crystal director frame.5 A particularly simple example is the case of an atom dissolved in a liquid crystal solvent; in this case the principal axis system of the probe is completely determined by the liquid crystal director field. The use of xenon as a microscopic probe of various materials has rapidly developed since the pioneering work in the early

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