Rotation of hydrogen molecules during the dissociative adsorption on the Mg(0001) surface: a first-principles study

Using first-principles calculations, we systematicallystudy the potential energy surfaces and dissociation processes ofthe hydrogen molecule on the Mg(0001) surface. It is found thatduring the dissociative adsorption process with the minimum energybarrier, the hydrogen molecule firstly orients perpendicular, andthen rotates to get parallel to the surface. It is also found thatthe orientation of the hydrogen molecule in the transition state isneither perpendicular nor parallel to the surface. Most importantly,we find that the rotation causes a reduction of the calculateddissociation energy barrier for the hydrogen molecule. Theunderlying electronic mechanism for the rotation of the hydrogenmolecule is also discussed in the paper.