Simulating high-pressure surface reactions with molecular beams

Abstract

Using a reactive molecular beam with high kinetic energy (E_kin), it is possible to speed gas–surface reactions involving high activation barriers (E_act), which would require elevated pressures (P₀) if a random gas with a Maxwell–Boltzmann distribution is used. By simply computing the number of molecules that overcome the activation barrier in a random gas at P₀ and in a molecular beam at E_kin = E_act, we establish an E_kin–P₀ equivalence curve, through which we postulate that molecular beams are ideal tools to investigate gas–surface reactions that involve high activation energies. In particular, we foresee the use of molecular beams to simulate gas surface reactions within the industrial-range (>10 bar) using surface-sensitive ultra-high vacuum (UHV) techniques, such as X-ray photoemission spectroscopy (XPS). To test this idea, we revisit the oxidation of the Cu(111) surface combining O₂ molecular beams and XPS experiments. By tuning the kinetic energy of the O₂ beam in the range of 0.24–1 eV, we achieve the same sequence of surface oxides obtained in ambient pressure photoemission (AP-XPS) experiments, in which the Cu(111) surface was exposed to a random O₂ gas up to 1 mbar. We observe the same surface oxidation kinetics as in the random gas, but with a much lower dose, close to the expected value derived from the equivalence curve.