adsorption with dissociation into two or more fragments, both or all of which are bound to the surface of the adsorbent The process may be homolytic, as in the chemisorption of hydrogen: \[\text{H}_{2}+2\text{*}\rightarrow 2\text{H}^{\text{*}}\] where * represents a surface site, or heterolytic, as in: \[\text{H}_{2}+\text{M}^{n+}+\text{O}^{2-}\rightarrow \text{H}^{-}\text{M}^{n+}+\text{HO}^{-}\] where M n + and O 2- are surface sites in which the ions are of lower coordination than the ions in the bulk phase. Associative desorption is the reverse of dissociative adsorption.