NMR Study of3He Adsorbed in Zeolite Pores

Abstract

The NMR properties of 3He adsorbed in the pores of MCM-41 zeolite have been studied in the temperature range 1.4 to 15 K, at pulsed NMR frequencies of 1.66 and 3.26 MHz. At a coverage x=0.84 monolayer, the linewidth 1/T*2 scales approximately linearly with the magnitude of the static magnetic field, and T*2 increases linearly with increasing temperature with an extrapolated low temperature limit of order 80 µs. However T2 is significantly longer, increasing from 0.7 µs at the lowest temperatures investigated in a way suggesting thermally activated motion. We attribute T*2 to static field variations between pores arising from paramagnetism of the zeolite and the random orientation of the pores. On the other hand T1 increases monotonically with decreasing temperature to around 80 ms at 1.6 K, indicating that the correlation time of the local magnetic field fluctuations τc>ω−10. A detailed temperature dependence of T1 and T2 has been carried out at a coverage of 0.32 monolayer. Here a minimum in T1 is observed at 11 K for a Larmor frequency of 3.26 MHz corresponding to a correlation time τc of 5×10−8 s. T1 increases by around three orders of magnitude on cooling to 1.8 K. At this temperature T1 decreases significantly with increasing coverage while T2 shows a very much weaker coverage dependence.