Proton relaxation by tunnelling in partly deuterated ammonium salts

Proton relaxation in partly deuterated (NH4)2SnCl6and NH4ClO4at 4.2K is very much faster than in the non-deuterated salts. The short T1is caused by tunnelling, which for asymmetric NH3D+allows a two-step energy transfer process: Spins to Tunnel states to Lattice. The spin-tunnel transfer is resonant at the level crossings Omega0= Omegatunneland 2 Omega0= Omegatunnel. In symmetric NH4+the Pauli principle requires simultaneous change of spins, tunnel and phonon states, which can only happen through higher-order transitions with small probabilities. A similar relaxation difference is predicted between CH3and CH2D compounds.