129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Abstract

Little is known about129Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report129Xe 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 by129Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the129Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of129Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The129Xe spin-lattice relaxation timesT1 range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT1 values (35 ms) are discussed. Preliminary131Xe and1H NMR results are presented, as well as a method for greatly increasing the sensitivity of129Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.