@[email protected] with @[email protected] into two or more fragments, both or all of which are bound to the surface of the @[email protected] 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 $$\text{M}^{n+}$$ and $$\text{O}^{2- }$$ are surface sites in which the ions are of lower @[email protected] than the ions in the bulk phase. Associative desorption is the reverse of dissociative adsorption.