Anisotropic Spin–Lattice and Spin–Spin Relaxations in Hydrogen Molecules Trapped in Non-Spherical Nanocavities

We report on ¹H NMR measurements of spin–lattice and spin–spin relaxation times in hydrogen molecules confined in nanocavities of the a-Si–H thin films. We found that the ¹H spin–spin relaxation time T₂ and the spin–lattice relaxation times T₁ and T_1ρ in the laboratory and rotating frames, respectively, exhibit anisotropic behavior as functions of the angle between the film growth direction and the applied magnetic field. This effect is caused by the dipole–dipole interaction of nuclear spins of hydrogen molecules experiencing restricted diffusion in ellipsoid-like nanocavities. The experimental results are analyzed within the framework of the previously developed theory. The analysis allows determining the distribution of nanocavities over orientations in the film under study. Similar phenomena can occur in various materials containing nanocavities and in nanoporous compounds of various origins in which molecular diffusion occurs and to which the above approach is applicable.