Molecular Beam Scattering from Ultrathin Metallic Films

One of the goals of studying molecule-surface interactions is to understand the details of the chemical reactivity at surfaces. The exchange of energy between a gas and a solid is a complex sequence of events even when focused on a model system like a diatomic molecule scattering from a monocrystalline metal surface. Due to the number of energy channels of the molecule (translational, rotational and vibrational energy) and the surface (phonons, electrons) involved, a detailed knowledge about the role of each of these degrees of freedom in the collision is necessary for the modeling of such processes. Over the last decades, the importance of electronically nonadiabatic interactions in molecule-surface interactions, where the Born-Oppenheimer Approximation is not valid anymore, has become clear. New theoretical models going beyond this approximation are needed to adequately describe such processes. Equally important is the acquisition of experimental data of model systems as a standard for comparison that can challenge the validity of such models. Although many ideas and concepts could be learned during the investigation of flat single crystals, “real” catalysts used in industrial processes are substances complex in structure and chemical composition. The introduction of model systems with a higher degree of complexity like bimetallic layers in the field of surface science bridges the gap between the ideal behavior of models and the diversity of “real world” catalysts. The present work investigates the role of the surface in molecule-surface scattering processes. In contrast to prior studies employing a single material surface, layers of Ag/Au(111) with defined thickness are chosen as a target. Beam scattering methods, combined with laser-based detection techniques, are used to investigate the molecules NO and CO with many similar properties but different strength of nonadiabatic coupling. We examined translational energy exchange of vibrationally excited NO(v = 2) and CO(v = 2) from ultrathin metallic films of Ag/Au(111) as a function of film thickness at an incidence translational energy of 0.6 eV. For NO, arrival time distributions were measured for vibrationally elastic (v = 2 → 2 and v = 0 → 0) and inelastic scattering (v = 2 → 1) for Ag film thicknesses up to 33 ML. For CO, vibrationally elastic (v = 2 → 2) and inelastic (v = 2 → 1) scattering channels were examined in a similar manner. For both molecules and for all investigated vibrational states, we observe a gradual decrease of the mean final translational energy for 0 - 3 ML Ag/Au films before reaching constant behavior for higher thicknesses (3-30 ML). The results are consistent with experimental data for pure Au and Ag crystal surfaces, and can be explained as head-on elastic collisions between NO molecules and surface elements with effective masses of 200amu (1.0 mAu) and 130 amu (1.2 mAg). Due to the substantial change of translational inelasticity up to a film thickness of 3 ML, we propose that subsurface layers take part in the scattering process. The thickness dependence of the final translations energy suggests that this kind of inelasticity is purely dominated by mechanical properties of a surface. Furthermore, we measured the thickness-dependent relaxation probabilities of NO(v = 2) and CO(v = 2) scattered off Ag/Au(111). Both molecules show a significantly different trend. For NO(v = 2), which is characterized as a molecule with strong nonadiabatic coupling strength, relaxation is gradually increasing between 0-3 ML Ag/Au until reaching a constant value, dominated by multiquantum relaxation into the vibrational ground state. A linear correlation between the survival probability of NO(v = 2) and the work function strongly suggests that this property determines the amount of vibrational energy loss during the collision event. The data is discussed in the context of previous relaxation experiments exhibiting a trend when plotting the relaxation probability against the difference of the vertical electron binding energy (VEBE) of a vibrationally excited molecule and the work function of a bulk surface. For CO(v = 2) which is understood as a molecule with only weak nonadiabatic coupling, scattering from Au(111) and films above 3 ML Ag/Au(111) exhibits similar low relaxation probabilities. However, for a thickness of 1 ML Ag/Au, a maximum of vibrational relaxation into the ground state is observed. The equal amount of energy loss when scattered from Au(111) and Ag(111)-like surfaces shows that in the case of CO, the surface work function does not substantially influence vibrational relaxation, probably due to the low electron affinity of CO (-1.5 eV) in comparison to NO (0.026 eV). The peaking of CO(v = 2) relaxation at 1 ML Ag/Au cannot be assigned to a single surface property with a maximum/minimum emerging for this film thickness. Moreover, the dependence of vibrational relaxation does not follow the trend of the previously suggested model on the basis of VEBE and surface work function. It is therefore interpreted as a special effect characteristic for a bimetallic layer system as observed previously for many metal-metal bilayer systems. Possible effects that explain the high relaxation probability are discussed