Reactions
on stepped surfaces are relevant to heterogeneous catalysis,
in which a reaction often takes place at the edges of nanoparticles
where the edges resemble steps on single-crystal stepped surfaces.
Previous results on H2 + Cu(211) showed that, in this system,
steps do not enhance the reactivity and raised the question of whether
this effect could be, in any way, related to the neglect of quantum
dynamical effects in the theory. To investigate this, we present full
quantum dynamical molecular beam simulations of sticking of H2 on Cu(211), in which all important rovibrational states populated
in a molecular beam experiment are taken into account. We find that
the reaction of H2 with Cu(211) is very well described
with quasi-classical dynamics when simulating molecular beam sticking
experiments, in which averaging takes place over a large number of
rovibrational states and over translational energy distributions.
Our results show that the stepped Cu(211) surface is distinct from
its component Cu(111) terraces and Cu(100) steps and cannot be described
as a combination of its component parts with respect to the reaction
dynamics when considering the orientational dependence. Specifically,
we present evidence that, at translational energies close to the reaction
threshold, vibrationally excited molecules show a negative rotational
quadrupole alignment parameter on Cu(211), which is not found on Cu(111)
and Cu(100). The effect arises because these molecules react with
a site-specific reaction mechanism at the step, that is, inelastic
rotational enhancement, which is only effective for molecules with
a small absolute value of the magnetic rotation quantum number. From
a comparison to recent associative desorption experiments as well
as Born–Oppenheimer molecular dynamics calculations, it follows
that the effects of surface atom motion and electron–hole pair
excitation on the reactivity fall within chemical accuracy, that is,
modeling these effect shifts extracted reaction probability curves
by less than 1 kcal/mol translational energy. We found no evidence
in our fully state-resolved calculations for the “slow”
reaction channel that was recently reported for associative desorption
of H2 from Cu(111) and Cu(211), but our results for the
fast channel are in good agreement with the experiments on H2 + Cu(211).