Proton relaxation by tunnelling in partly deuterated ammonium salts

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Abstract
Proton relaxation in partly deuterated (NH4)2SnCl6 and NH4ClO4 at 4.2K is very much faster than in the non-deuterated salts. The short T1 is 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 Omega 0= Omega tunnel and 2 Omega 0= Omega tunnel. 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 CH3 and CH2D compounds.