Multiple potential surfaces and movable energy barriers in the chemiluminescent reaction Mn + SnCl4

Abstract

A laser-ablated pulsed beam of Mn atoms, in various long-lived states, has been employed to determine excitation functions for the reactions Mn + SnCl4 → MnCl* ("blue," "green," "red") + SnCl3. The data all show a sharp rise at low energies, followed by an equally rapid fall, but they differ in the initial thresholds and the rate at which σ(ET) approaches zero. Analysis in terms of a multiple line-of-centres model (Levy, Res. Chem. Kinet. 1, 163 (1993)) indicates that each excitation function is unique, and is due to reaction of only one Mn reagent state — most probably the first metastable state, a6DJ. This suggests a strong symmetry control in the entrance channel of the reaction, at least; that is, the Mn atom approaches along the C3 axis of SnCl4, so that the symmetry corresponding to the spatial configuration of the odd d electron is retained. In two cases, "blue" and "red," if not all, the luminescing MnCl* state seems to be produced on a surface that does not directly correlate with the reagents. As collision energy increases in each case, the reaction transition state seems to shift forward into the entrance valley, a result attributed to restricted access to the exit valley.