Association of CN in an Expansion Wave

Abstract

The expansion wave generated by the simultaneous arrival of the ``contact'' surface and the reflected rarefaction wave was used for a direct kinetic study of a condensation process, the association of CN radicals generated by shocking C2N2/Ar mixtures to about 2000°K in the reflected-shock region. During the quenching-expansion phase the concentration of CN radicals decreases due to the concurrent chemical association and the lowering in total density. The CN concentration was followed spectrophotometrically at 3883 Å. The results were reduced in three ways: (A) unidirectional association to C2N2 in the presence of a third body, (B) relaxation to the local equilibrium via a three-body process, and (C) bimolecular reaction with C2N2 and CmNm polymers (formed during the heating process) to produce higher polymers. The data show that at the onset of quenching, the removal of CN was sluggish. This is attributed to failure to attain equilibrium in the preparation of the sample. After ≈200 μsec the data were best fitted by the third mechanism. This conclusion is in agreement with results obtained by another method and is supported by a reaction scheme which accounts for the formation and decay of CN in the presence of CmNm species. For a 2% initial concentration of C2N2, an average polymer number of ≈10 was indicated.