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.