Chemiluminescence in the Reaction of Mn Atoms with CF4

Abstract

Translational excitation functions have been determined for production of several MnF* statesb5Π, c5Σ+, d5Π, and (most probably) e5Σ+in the reaction of a laser-ablated beam of Mn atoms with gaseous CF4. Although all observed channels show high initial thresholds, ∼200−300 kJ mol-1, reaction appears to be due to excited Mn atoms rather than the ground state, a6S. The reagent species appears to be either the first or third metastable level, a6DJ or a4DJ. Analysis of the energy dependences, in terms of a multiple line-of-centers model [Levy, Res. Chem. Kinet. 1993, 1, 163], indicates that at relatively low energies, a common process is responsible for b5Π and c5Σ+ formation, involving a ∼14% forward shift in reaction transition state as collision energy increases. Quite separate processes, without transition state shifts, lead to production of MnF*(d5Π) and of MnF*(e5Σ+)/“blue” emission at relatively low energies and to enhanced c5Σ+ production at high energies. It is possible that enhanced production of MnF*(e5Σ+) and perhaps the d5Π state from ∼650−700 kJ mol-1 derives from the depletion of MnF*(b5Π, c5Σ+). Despite the undoubted negative CF4 electron affinity, it seems likely that avoided ionic−covalent curve crossings at least play a role in the b5Π/c5Σ+ production channel.