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 relaxation 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 complication. We report Xe-129 NMR experiments at 9.4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by Xe-129 NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the Xe-129 NMR chemical shifts. The peak maxima occur in the region ca. 180-316 ppm, considerably downfield of Xe-129 shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4 +/- 1 ppm). The Xe-129 spin-lattice relaxation times T-1 range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely short T-1 values (35 ms) are discussed. Preliminary 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.