Fundamentals of molecular and dissociative adsorption

There is a very simple model for estimating the trapping probability in atomic adsorption due to a phonon-excitation mechanism. In the hard-cube model (HCM) [6, 7], the impact of the atom on the surface is treated as a binary elastic collision between a gas phase atom (mass m) and a substrate atom (mass Mc) which is moving freely with a velocity distribution Pc(uc). This model is schematically illustrated in Fig. 1. If the depth of the adsorption well is denoted by Ead, the adsorbate will impinge 

Assuming a weighted Maxwellian velocity distribution for uc, the trapping probability in the hard-cube model can be analytically expressed as [7] 

When an atom hits a surface, the initial kinetic energy of the atom can not only be transfered to the substrate. If the surface is corrugated, i.e. if the atom-surface interaction varies as a function of lateral coordinates of the atom, then the impinging atom can also change its lateral component of the initial velocity upon the collision. In the case of molecules, there are also the internal degrees of molecular vibration and rotation that can be excited (or de-excited) during the collision with the surface. 

In the case of dissociative adsorption on surfaces there is an additional channel into which energy can be transfered, namely the conversion of the kinetic and internal energy of the molecule into translational energy of the fragments on the surface with respect to each other. In fact, in the dissociation of light molecules such as 1I2 on metal surfaces the dissociative adsorption probability is 

This is different at semiconductor surfaces where the covalent bonds between the substrate atoms are often strongly perturbed by the presence of adsorbates. This can result in a significant surface restructuring. Hence the dynamics of the substrate atoms has to be explicitly taken into account which of course increases the complexity of the modelling of the adsorption/desorption dynamics, as will be shown below for the H2/Si system. 

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