Radiationless Transitions in Small Molecules: Interstate Cascading in Matrix-Isolated CN

Radiationless relaxation following excitation of high levels of the $A{}_{}{}^2{}_{}{}^{}\Pi$ manifold in CN is shown to proceed via an interstate cascading mechanism involving the $A{}_{}{}^2{}_{}{}^{}\Pi$ and $X{}_{}{}^2{}_{}{}^{}\Sigma$ vibrational levels. The direct vibrational relaxation within the $X{}_{}{}^2{}_{}{}^{}\Sigma$ state is shown to be slower by at least 2-3 orders of magnitude. Experimental results and a theoretical discussion of these phenomena are presented.