Chemiluminescence in the reaction of Mn atoms with SiCl4

Chemiluminescence in four different MnCl band systems ↔ e5Σ+ → a5Σ+ ("blue"), d5Π → a5Σ+ ("green"), c5Σ+ → a5Σ+ ("red"), and b5Π → a5Σ+ ("infrared") ↔ has been observed from the interaction of a laser-ablated beam of Mn atoms with gaseous SiCl4. The initial translational thresholds range from ∼150 to ∼300 kJ mol-1, increasing in the order e5Σ+ <c5Σ+ ∼ d5Π <b5Π. Analysis of the measured excitation functions, in terms of a multiple line-of-centers model [Levy, M. R. Res. Chem. Kinetics 1993, 1, 163], shows that, with the exception of a high-energy c5Σ+ channel contribution to d5Π production and possibly a joint c5Σ+/d5Π production process at low energies, all the reactions take place on distinct potential surfaces. Although, on energetic grounds, the b5Π and c5Σ+ channels could derive from reaction of ground-state Mn(a6S) atoms, the first excited state, a6DJ, is the most likely reagent species in all cases, indicating substantial excess barriers. In contrast to the corresponding SnCl4 reactions, only the c5Σ+ excitation function reveals a shift forward in transition-state location with increasing collision energy. The results have been rationalized in terms of a hierarchy of ionic-covalent curve crossings at short internuclear distances.