Ammonium tunnel levels and spin-rotational wavefunctions in (NH4)2S2O8

Proton spin-lattice relaxation in a single crystal of (NH4)2S2O8 was studied as a function of resonance frequency at various constant temperatures between 4.2 and 30 K. Two T1 minima were found, one at 8.6 MHz and the other at 4.3 MHz. They are related to the splitting between the lowest T level and the A level, equal to 8.6 MHz nearly independently of temperature below 25 K. Together with the large tunnel splitting of 269 MHz, determined earlier by Clough et al. (Chem. Phys. 152, 343 (1991)) our results define the spin-rotational wavefunction of the lowest T level very accurately, although those of the two higher T levels remain largely undetermined.