Nuclear magnetic relaxation of3He gas. I. Pure3He

Longitudinal relaxation timesT1 have been measured in3He gas, using pulsed NMR, for number densities between 3 × 1023 and 6 × 1025 spins m−3 and temperatures between 0.6 and 15 K. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements ofT1 give information about the surface phases. A cryogenic wall coating of solid molecular hydrogen was found to delay the formation of a3He monolayer on cooling, andT1 measurements were consistent with a binding energy of ∼13 K for a3He atom to a hydrogen surface. At temperatures below ∼2 K a completed3He monolayer forms on the H2 coating. No variation of the areal density of monolayer completion with bulk number density at fixed temperature could be observed and the completed3He monolayer is thought to be a dense fluid. Baking the Pyrex sample cells under vacuum and using an rf discharge in3He gas to clean the walls before sealing in the sample gas were found to increase the observed T1's by up to three orders of magnitude. Once a3He monolayer has formed on the H2 surface in these cleaned, sealed cells, the dipolar interaction between adsorbed spins is thought to be the dominant source of longitudinal relaxation. The data are consistent with a dipolar relaxation model with a correlation time of ∼2 × 10−9 sec. This time is long compared to the value of 10−11 or 10−12 sec in the 3D fluid. This suggests that if the surface phase is a 2D fluid and the dipolar mechanism is indeed the dominant one, then the atoms in the 2D fluid are less mobile than in three dimensions. This is consistent with recent susceptibility measurements.