1 There is a true precursor for hydrogen adsorption after all: The system H2/Pd(111) + 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 disso-ciation on pure Pd(111) is dominated by dynamical steering and there-fore shows strong rotational hindering. The alloy surface exhibits a sticking probability independent of rotational excitation and an iso-tropic 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 dif-ferent behavior of Pd and Pd/V surfaces arises from the existence of a flat physisorption state. The concept of a precursor to chemisorption has a long history [1,2,3]. Already the simple Lennard-Jones model [4] for dissociative chemisorption included the possibil-ity of a molecularly adsorbed state. Precursors were first postulated as extrinsic pre