Recombinative desorption of D2 from clean Pd(100) was studied with rotational-state selectivity by using resonantly enhanced two-photon ionization. The amount of vibrationally excited D2 molecules increased exponentially with the surface temperature T s , associated with an activation energy, in addition to that for desorption of D2(i/'=0), of 210 ±60 meV, considerably less than the vibrational energy of free D2 molecules. This activation energy can be accounted for by a temporary trapping of D2 in a molecular chemisorption state with a softened D-D bond, in qualitative agreement with a recently proposed model. PACS numbers: 79.20.Nc, 33.20.Ni, The interaction of molecular hydrogen with metal surfaces is often regarded as a model for more complicated systems. Theoretical studies of the adsorption process on jellium 1 " 4 and cluster 5 substrates proposed a physisorption well followed by a molecular chemisorption state. In this precursor state the molecules are thought to be dynamically trapped and highly mobile along the surface. After surmounting an energy barrier the molecule finally adsorbs in a dissociative state. These barriers show various heights on different substrates which range from being larger than room-temperature thermal energy to not existent.The adsorption of hydrogen in the physisorption well has been observed by Avouris, Schmeisser, and Demuth 6 and Andersson and Harris 7 at low temperatures on Ag and Cu substrates, respectively. On both metals the barriers for crossing to more tightly bound adsorption states are known to be high (e.g., Cu, E^-0.2 eV, Ref. 8) compared to transition-metal surfaces. The physisorption potentials show well depths between 32 and 55 meV for a variety of metal surfaces, as determined with HD molecular-beam scattering. 9 In this potential the hydrogen molecule was observed to behave as a free rotor with energy levels unchanged from the gas-phase values. 6 ' 7 First evidence for molecular adsorption in a chemisorption state has been recently reported by Martensson, Nyberg, and Andersson. 10 They observed electron energy losses on the hydrogen-saturated stepped Ni(510) surface, which were attributed to internal vibrations of a molecularly adsorbed species, with high-resolution electron-energy-loss spectroscopy. Although these energies-398 meV and 286 meV for H 2 and D 2 , respectively-differed markedly from the vibrational energies of free hydrogen molecules (516 meV and 371 meV, respectively), the frequency shift upon isotopic substitution showed the expected behavior. This energy shift may be rationalized by a softening of the hydrogen bond in the chemisorption state. Such a bond softening has very recently been proposed by Miiller 5 in a cluster calculation of potential energies for hydrogen approaching a Pt(lll) surface. The lowest-energy state was found at a H2-Pt distance of 3.0 bohrs with a much weakened H-H bond. An even softer potential was calculated at a distance of 3.5 bohrs. The shape of the H 2 interatomic potential changed drastically upon approach to the ...

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