Dipolar interactions between NH3 molecules trapped in solid argon. II. Line narrowing during nuclear spin species conversion

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Abstract
A spectacular line narrowing as a function of time is observed in the ν2 and ν4 rovibrational absorption spectra of NH3 trapped in argon matrices, at T=5 K. This effect has been predicted on the basis of a fundamental difference between the two nuclear spin symmetry species; molecules belonging to the spin 1/2 species (J=K=1 rotational states) exhibit dipolar interactions, leading to an inhomogeneous broadening of the lines, whereas spin 3/2 species (J=K=0) are unaffected by this interaction. The system presents some analogy with the quadrupolar orientational glass phase exhibited by N2/Ar dilute solid solutions, and also by solid hydrogen. However, here, after a fast cooling, the concentration of ‘‘active’’ (spin 1/2) species decreases with time as the nuclear spin species conversion proceeds. Concentration effects can then be probed spectroscopically within a few hours, inside the same sample. A quantitative study of the perturbation is performed, using a simple model to calculate the perturbed line profiles. This analysis shows that, as the concentration decreases, NH3 molecules undergo a transition from a dipolar behavior to a nondipolar one.