DIPOLAR INTERACTIONS BETWEEN NH3 MOLECULES TRAPPED IN SOLID ARGON .1.KINETICS OF THE NUCLEAR-SPIN SPECIES CONVERSION

The rovibrational nu2 and nu4 absorption spectra of NH3 trapped in sol id argon at low temperatures are re-examined. They provide a quantitat ive basis to study the spin species conversion, which occurs in the ma trix 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 mon itored by recording the integrated absorption of the aQ (1,1) ''M'' tr iplet, at different times after cooling down. A decay law of the (v=0, a, J=K=1) level population, on the form exp-(alphat)1/2, is found and this shows that intermolecular proton-proton interactions, of magneti c dipolar type, dominate the conversion process in the concentration r ange 1/200-1/1000. This interaction can be characterized by a distance dependent rate constant k(r)=kappa r-6 , with kappa=1800 s-1 angstrom 6.