Surface reaction kinetics and dynamics
An experimental setup for MBSS consists of a supersonic molecular beam source, which generates a highly collimated beam of neutral molecules the translational energy distribution of which is highly monochromatic, and a scattering chamber with a mass spectrometer and a sample off which the beam is scattered. The most probable translational energy of molecules can be adjusted typically in a range of 30 meV - 1000 meV by diluting of the beam with lighter or heavier molecules and by cooling or annealing the nozzle. The translational energy distribution can be measured with time-of-flight techniques.
The figure on the right demonstrates how MBSS, in conjunction with other surface analytical techniques such as scanning tunneling microscopy (STM), can reveal molecular level details of molecule-surface interaction mechanisms. The figure shows an STM image of a clean Cu(100) surface and the initial sticking probability of O2 on Cu(100) as a function of translational energy. The dramatic increase in sticking probability as the energy is increased indicates that the adsorption process is activated, i.e. there is an activation barrier for O2 chemisorption on Cu(100).
The next figure shows results from experiments in which the surface was covered with oxygen prior to the molecular beam experiments. As shown by the STM image, the pre-exposure of O2 led to a surface reconstruction. The subsequent MBSS results indicate that there has been a significant change in the interaction between O2 molecules and Cu(100). Instead of an increase in the sticking probability with increasing translational energy, we see just the opposite kind of behaviour. Furthermore, for the low energy O2 molecules (<100 meV) the oxygen covered surface actually appears to be more reactive towards O2 dissociation than the clean Cu(100). This surprising result indicates that a new, non-activated adsorption channel opened up when the surface was covered with oxygen and, since the low energy O2 molecules dominate in the ambient gas, this phase is critical to the eventual formation of Cu oxides.