There is a true precursor for hydrogen adsorption after all: the system H2/Pd(1 1 1) + subsurface V

The adsorption dynamics of H2 on Pd(111) alloyed with V has been studied using molecular beam experiments (MB) and ab initio density functional theory (DFT) calculations. The experiments show that dissociation on pure Pd(111) is dominated by dynamical steering and therefore shows strong rotational hindering. The alloy surface exhibits a sticking probability independent of rotational excitation and an isotropic distribution of inelastically scattered molecules, attributed to a dynamical precursor. DFT calculations of the potential energy surface and ab initio simulations of scattering events demonstrate that the different behavior of Pd and Pd/V surfaces arises from the existence of a flat physisorption state.