XE-129 AND XE-131 NMR OF XENON ON SILICA SURFACES AT 77 K

Little is known about Xe-129 NMR spectral features and spin-lattice re laxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (less than or equal to 77 K), where exchange with gas-phase atoms is not a significant complica tion. We report Xe-129 NMR experiments at 9.4 T that provide such info rmation for xenon adsorbed onto the hydroxylated surface of a number o f microporous silica samples at 77 K. A convenient design for these cr yogenic experiments is described. Dynamics of surface-adsorbed xenon a toms on the time scale of seconds can be observed by Xe-129 NMR hole-b urning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the Xe-129 NMR chemical shift s. The peak maxima occur in the region ca. 180-316 ppm, considerably d ownfield of Xe-129 shifts previously reported on surfaces at higher te mperatures, and closer to the shift of xenon bulk solid (316.4 +/- 1 p pm). The Xe-129 spin-lattice relaxation times T-1 range over five orde rs of magnitude; possible explanations for both nonexponential relaxat ion behavior and extremely short T-1 values (35 ms) are discussed. Pre liminary Xe-131 and H-1 NMR results are presented, as well as a method for greatly increasing the sensitivity of Xe-129 NMR detection at low temperatures by using closely-spaced trains of rf pulses.