The orientation and
motion of reactants play important roles in
reactions. The small rotational excitations involved render the reactants
susceptible to dynamical steering, making direct comparison between
experiments and theory rather challenging. Using space-quantized molecular
beams, we directly probed the (polar and azimuthal) orientation dependence
of O
2
chemisorption on Cu(110) and Cu
3
Au(110).
We observed polar and azimuthal anisotropies on both surfaces. Chemisorption
proceeded rather favorably with the O–O bond axis oriented
parallel (vs perpendicular) to the surface and rather favorably with
the O–O bond axis oriented along [001] (vs along [1̅10]).
The presence of Au hindered the surface from further oxidation, introducing
a higher activation barrier to chemisorption and rendering an almost
negligible azimuthal anisotropy. The presence of Au also prevented
the cartwheel-like rotations of O
2
.