Deuteron spin alignment: A probe for studying ultraslow motions in solids and solid polymers

Abstract

Deuteron spin alignment offers a new possibility to investigate extremely slow rotational motions in solids and solid polymers. A convenient theoretical description of the creation and detection of spin alignment by application of the Jeener–Broekaert pulse sequence is given for both static and slowly time dependent quadrupole coupling, as well as for spin–lattice relaxation of spin alignment. It is shown that the NMR signal following spin alignment yields a correlation function of the time dependent quadrupole coupling. This correlation function is evaluated explicitly for a deuteron on the corner of a regular tetrahedron undergoing tetrahedral jumps. Various applications of deuteron spin alignment are demonstrated experimentally, e.g., its use to obtain undistorted deuteron line shapes, from which the orientational distribution of partially ordered solids or solid polymers may be determined. In solid polyethylene it is shown that not only the deuterons in the crystalline regions but those in the mobile amorphous regions as well give rise to deuteron spin alignment. Tetrahedral jumps are investigated through deuteron echoes of a single crystal of hexamethylenetetramine. It is shown that the type as well as the time scale of rotational motions with correlation times 1 ms≲τ≲100s are obtained from the quadrupole coupling correlation function provided by deuteron spin alignment.