Rotational orientation control of a ground state ortho-H₂ dissociation on a metal surface

When hydrogen molecules collide with a surface, they can either scatter away from the surface or undergo dissociative chemisorption. The relative probabilities of these different outcomes could depend on the rotational orientation of the impinging molecules, however, due to the lack of steric control techniques for ground state hydrogen, they could not be measured directly. Here, we demonstrate that magnetic field manipulation can be used to control the rotational orientation of H₂ molecules colliding with a nickel surface and change the balance between reactive and scattering collision events. Our measurements show that molecules which approach the surface while rotating within a plane parallel to the surface are less likely to undergo specular scattering than those rotating within a perpendicular plane. An opposite trend was measured for the likelihood of dissociative chemisorption. A possible link between these two findings, and its potential impact on the interpretation of dissociation mechanisms is discussed.