STRONG TRANSIENT EFFECTS OF NONSTATIONARY DIFFUSION ON EXCIMER FORMATION - TEST OF THE CONCEPT OF CONVOLUTION KINETICS

The concept of convolution kinetics (CK) remains valid if rate coefficients depend on time, where rate equations (RE) fail. Excimer kinetics are without strong spurious influences, thus being convenient for a crucial test of CK. With strong excimer dissociation, the expectedly small effect of nonstationary diffusion becomes a large one, thus making the solution of integral equations in CK necessary. The rules of CK are given, and the most novel and uncommon part of the procedure is rederived. With time-independent coefficients, CK give the same result as RE. Some simpler applications, where CK are straightforward, are sketched. The problem as given in the title is initially solved correctly with CK. The integral equations can be solved by Laplace transformation and the back transformation is accomplished via series expansion. As far as convolutions cannot be performed in closed form, a Simpson procedure maintaining the general dependence on kinetic parameters is applied. On the basis of Smoluchowski's approach, and assuming reasonable values of disposable experimental parameters, considerable deviations from results of the RE procedure are predicted by CK with high viscosity and reasonably small reaction radius even at low dissociation rate; with strong dissociation, however, such deviations happen even at low viscosity. Both these influences mutually enhance when combined. As a first experimental test, the time dependence of pyrene excimer on flash excitation was measured with η = 0.152 P, T = 334 K, and 0.05 mol/L (about the half value concentration). The curve shape, readily explained by CK with a reaction radius 2.5 × 10-10 m, is in disagreement with the RE calculation. Smaller deviations between the CK curve and the experimental one may be explained by insufficient validity of Smoluchowski's approach, but mistakes from not applying CK are much more significant and must not be attributed to the former. © 1990 American Chemical Society.