ABSTRACT
Molecular reaction dynamics is concerned with understanding elementary chemical reactions in terms of the individual atomic and molecular forces and the motions that occur during the process of chemical change. In gas phase and condensed phase reactions (discussed in sections A3.7 and A3.8) the reactants, products and all intermediates are in the same phase. This ‘reduces’ the complexity of such systems such that we need ‘only’ develop experimental and theoretical tools to treat one medium. In a surface reaction, the reactants derive from the gas phase, to which the products may or may not return, but the surface is a condensed phase exchanging energy with reactants and products and any intermediates in a nontrivial fashion. The electronic states of the surface may also play a role by changing the bonding within and between the various species, affecting the reaction as a heterogeneous catalyst (see section A3.10). Of course, the surface itself may be one of the reactants, as in the etching of silicon surfaces by halide molecules. Indeed, it might be argued that if the reactants achieve thermal equilibrium with the surface, they have become part of a new surface, with properties differing from those of the clean surface.
