MULTIPLE POTENTIAL SURFACES AND MOVABLE ENERGY BARRIERS IN THE CHEMILUMINESCENT REACTION MN+SNCL4

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 f all, but they differ in the initial thresholds and the rate at which s igma(E(T)) approaches zero. Analysis in terms of a multiple line-of-ce ntres model (Levy, Res. Chem. Kinet. 1, 163 (1993)) indicates that eac h excitation function is unique, and is due to reaction of only one Mn reagent state - most probably the first metastable state, a(6)D(J). T his suggests a strong symmetry control in the entrance channel of the reaction, at least; that is, the Mn atom approaches along the C-3 axis of SnCl4, so that the symmetry corresponding to the spatial configura tion of the odd d electron is retained. In two cases, ''blue'' and ''r ed,'' if not all, the luminescing MnCl state seems to be produced on a surface that does not directly correlate with the reagents. As colli sion energy increases in each case, the reaction transition state seem s to shift forward into the entrance valley, a result attributed to re stricted access to the exit valley.