posted on 2016-04-04, 00:00authored bySridhar
A. Lahankar, Jianming Zhang, Timothy K. Minton, Hua Guo, György Lendvay
The
O atom exchange reaction, 16O(3P) + 18O18O(3Σg–) → 16O18O(3Σg–) + 18O(3P), was investigated at a hyperthermal center-of-mass
(c.m.) collision energy (Ecoll) of 86
kcal mol–1, using a crossed-molecular-beams apparatus
and quasiclassical trajectory (QCT) calculations. The inelastically
scattered 16O and reactively scattered 16O18O products were detected with a rotatable mass spectrometer
employing electron-impact ionization. The 16O atoms are
scattered in inelastic collisions in the forward direction relative
to their initial direction of flight, with most of the available energy
partitioned into translation. The 16O18O products
of reactive collisions are mainly formed through impulsive dynamics
and are scattered in the forward as well as sideways directions relative
to the direction of the reagent 16O atoms, with a slight
majority of the available energy partitioned into translation (⟨ET⟩ = 58%) and a significant contribution
to internal degrees of freedom. Excellent agreement was found between
the experimental c.m. angular and translational energy distributions
of the inelastically scattered 16O and reactively scattered 16O18O products and those obtained from QCT calculations,
which were carried out on a ground-state singlet electronic potential
energy surface. The QCT calculations predicted 16O18O products that are both highly rotationally and vibrationally
excited, with j′(16O18O) up to 150 and v′(16O18O) up to 15, respectively. The QCT simulations indicate that the
translational energy distribution of the reactively scattered 16O18O is bimodal, corresponding to two distinct
interaction mechanisms that are dependent on impact parameter: one
at impact parameters below ∼0.5 Å and another in the vicinity
of 1.6 Å. Collisions in the former regime produce 16O18O with internal energy closer to the maximum available
energy while the latter mechanism, involving strong interaction within
the O3 potential well, is responsible for the low-energy
peak of the product translational distribution. The inelastic collisions
also follow two basic impact-parameter-dependent mechanisms. At impact
parameters above 2.1 Å, the 16O atom is reflected
from the outer repulsive wall of the O2 molecule, resulting
in exclusively forward scattering, while collisions at impact parameters
below ∼2 Å access the O3 potential well and
lead to ejection of either an 16O or an 18O
atom. Scattering remains preferentially forward in both cases due
to the large momentum of the attacking 16O atom.