INFLUENCE OF ELECTROSTATIC FIELDS ON BINDING-ENERGY (NO ON RH)

Pulsed field desorption mass spectrometry (PFDMS) has been applied to measure binding energies of adsorbates on metal surfaces in the presence of a high electric field. The electric field modifies the charge state of adsorbed particles, i.e. it acts in a manner similar to work function changes or alkali coadsorption. Furthermore, it modifies the binding energy and possibly even the orbital configuration of the adsorbate. Recent progress in the theoretical treatment of the chemisorption of small molecules on metal clusters has made it possible to calculate electrostatic field influences on adsorbates. Comparison between experiment and quantum chemical calculations becomes possible. For this purpose, the adsorption/thermal desorption equilibrium of NO on Rh(111) is observed as a function of temperature (300-630 K) and field strength (0-3 V/nm). The PFDMS apparatus is merely used as a tool to measure the equilibrium coverage. The data treatment is performed according to Frenkel's equation so that the binding energy is obtained from an Arrhenius plot. In this way, the influence of the electric field on the kinetic energy of the impinging particles, as well as the diffusion from the shank towards the apex can be eliminated.