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(1 1 1) 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(1 1 1) 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.