Nonexponential proton relaxation for tunneling of NH+4 in the rotating frame

Proton spin-lattice relaxation times T₁ and T_1θ have been measured in NH₄IO₄ between 150 K and 50 K. The relaxation in the rotating frame was strongly nonexponential between 70 K and 53 K, which supports the tunneling assisted relaxation model of NH⁺₄ in the rotating frame. The tunneling frequency was determined to be $\text{Λ}{}_0\text{2Π}\text{=1.5 MHz}$.