CHEMILUMINESCENCE IN THE REACTION OF MN ATOMS WITH SICL4

Chemiluminescence in four different MnCl band systems - e(5) Sigma(+) --> a(5) Sigma(+) (''blue''), d(5)II --> a(5) Sigma(+) (''green''), c( 5) Sigma(+) --> a(5) Sigma(+) (''red''), and b(5)II --> a(5) Sigma(+) (''infrared'') - has been observed from the interaction of a laser-abl ated beam of Mn atoms with gaseous SiCl4. The initial translational th resholds range from similar to 150 to similar to 300 kJ mol(-1), incre asing in the order e(5) Sigma(+) < c(5) Sigma(+) similar to d(5)II < b (5)II. Analysis of the measured excitation functions, in terms of a mu ltiple line-of-centers model [Levy, M. R. Res. Chem. Kinetics 1993, 1, 163], shows that, with the exception of a high-energy c(5) Sigma(+) c hannel contribution to d(5)II production and possibly a joint c(5) Sig ma(+)/d(5)II production process at low energies, all the reactions tak e place on distinct potential surfaces. Although, on energetic grounds , the b(5)II and c(5) Sigma(+) channels could derive from reaction of ground-state Mn(a(6)S) atoms, the first excited state, a(6)D(J), is th e most likely reagent species in all cases, indicating substantial exc ess barriers. In contrast to the corresponding SnCl4 reactions, only t he c(5) Sigma(+) excitation function reveals a shift forward in transi tion-state location with increasing collision energy. The results have been rationalized in terms of a hierarchy of ionic-covalent curve cro ssings at short internuclear distances.