  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 ($P_0$) 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 $P_0$ and
in a molecular beam at $E_{kin}$=$E_{act}$, we establish an $E_{kin}$-$P_0$
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 O$_2$ molecular beams and XPS experiments. By
tuning the kinetic energy of the O$_2$ beam in the range 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
O$_2$ 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.
