Quantum Diffusion of HD Impurities in Solid Hydrogen

The NMR linewidth of HD impurities in solid para hydrogen has been measured to be less than the rigid lattice value. Motion due to quantum diffusion, similar to the exchange of atoms in dense hcp 3He, of the HD molecules is believed to be the cause of the narrowing of the linewidth. This motion is severely restricted for ortho-H2 because of their quadrupole-quadrupole interactions with ortho neighbors. Pulsed NMR experiments designed to measure the motional effects on NMR spin echos are discussed. Pressure and temperature effects on the HD motion are also examined.