Dynamics of NH3 trapped in N2 matrix: Tunneling, motional narrowing, and vibrational relaxation

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Abstract
A quantitative interpretation of the features observed in the high-resolution infrared profile and in the vibrational relaxation of the umbrella ν2 mode of ammonia trapped in a nitrogen matrix is given through the coupled dynamical study of the molecule and the crystal. The motional narrowing that occurs in the quadruplet structure of the vibration–inversion spectrum, as temperature rises, is due to the closing of the inversion levels of the two isotopic ammonia species (A2 and E) by dynamical coupling with the ambient solid. Indeed, this coupling is responsible for the tunneling slowdown of the ammonia frequency due to the tendency for the bath to localize the molecule in a given inversion state. The temperature behavior of the experimental profile in the range 5.5–25 K is well explained on the basis of inversion–libration relaxations assisted by one and two bulk phonon–libron processes. Moreover, the fast vibrational relaxation is also interpreted, within our approach, as being assisted by the ammonia orientational modes. The calculated relaxation time (0.05 ns) agrees with the experimental one (0.07 ≤τ≤ 2ns).