Measuring physisorption well depth with fast atom scattering

The physisorption of atoms and molecules at surfaces is a crucial step governing future reactions, but this 10-100 meV well-depth is still difficult to measure and calculate.

The use of fast atom scattering on the surface appears counter-intuitive because the atoms have kinetic energy on the order of a keV.

However, under grazing incidence, a very strong decoupling has been demonstrated so that the effective energy governing the interaction with the surface is given by that of the motion perpendicular to the surface which can be brought to the ten's of meV range [1].

Four different manifestations of the well-depth have been observed with fast atom scattering.

-1) Observing resonances associated with the bound states inside the potential well [2].

-2) Comparing diffracted intensities with an exact quantum scattering calculation [3].

-3) Measuring the rainbow scattering angle and interpreting its evolution as a refraction effect due to the attractive forces [3]. No diffraction is required and we have extended the technique to heavier noble gazes up to Xe.

-4) Measuring the sharp increase of the width of the inelastic polar profile due to the attractive forces. We have derived a model based on Morse's potential to link the observation to the well-depth [4] even without crystalline order.