Molecular beam surface scattering (MBSS)

Introduction

A better understanding of the detailed mechanisms of surface processes that take place for instance in heterogeneous catalysis or in the fabrication of multilayer semiconductor structures would be highly interesting both from industrial and academic points of view. Unfortunately, the direct, time-resolved imaging of the dynamics of individual molecules reacting with each other is still not possible. However, over the past two decades surface science methods have made significant contributions to our understanding of the microscopic details of surface chemical reactions and other surface processes. One of the most prominent of these methods is the Molecular Beam Surface Scattering (MBSS).

Principle of operation

Investigating the details of surface reactions at atmospheric pressures is difficult due to the gas-phase interactions that make it almost impossible to probe of the molecule-surface interactions. In contrast, a molecular beam is literally a beam of molecules in the collisionless environment of vacuum that prevents the perturbation of the energy and direction of the molecules before they interact with the surface. This allows one to probe the molecule-surface interaction in great detail.

In the MBSS technique a molecular beam is directed at a surface and the changes in the chemical identity, energy and direction of the molecules caused by the interaction with the surface are detected. Since the only interaction the molecules experience is with the surface, surface chemical reactions can be better investigated as the absence of collisions guarantees that all the detected reaction products were formed on the surface and not in the gas-phase. The figure on the right shows schematically how a molecular beam can be employed to study the oxidation of CO. The surface is first covered with oxygen atoms and a CO molecular beam with a well known kinetic energy distribution is allowed to react with it. The backscattered CO and the reaction product, carbon dioxide, are then detected by a quadrupole mass spectrometer.

MBSS experiments can be made both with effusive molecular beams as well as with supersonic molecular beams. The advantage in using the latter is that the energy distribution of the supersonic molecular beam is nearly monoenergetic and that the kinetic, vibrational and rotational energy of the neutral molecules in the beam can be varied independently. This property makes it an invaluable tool in the investigation of the dynamics of surface processes.

Applicability

The reactive molecular beam surface scattering can be used to investigate both the kinetics and dynamics of surface processes on all solid surfaces.