Dipolar interactions between NH3 molecules trapped in solid argon. I. Kinetics of the nuclear spin species conversion

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Abstract
The rovibrational ν2 and ν4 absorption spectra of NH3 trapped in solid argon at low temperatures are re‐examined. They provide a quantitative basis to study the spin species conversion, which occurs in the matrix at T=5 K, after a fast cooling down from 30 K where Boltzmann’s equilibrium has been ensured. The kinetics for this conversion is monitored by recording the integrated absorption of the aQ(1,1) ‘‘M’’ triplet, at different times after cooling down. A decay law of the (v=0, a, J=K=1) level population, on the form exp−(αt)1/2, is found and this shows that intermolecular proton–proton interactions, of magnetic dipolar type, dominate the conversion process in the concentration range 1/200–1/1000. This interaction can be characterized by a distance dependent rate constant kr=κ r−6, with κ=1800 s−1 Å6.