Simulating high-pressure surface reactions with molecular beams

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.