= Dissociative adsorption
In <dissociative adsorption>, an arriving <molecule> splits into bound fragments. For the idealized process $A_2+2*\rightleftharpoons2A*$, let $*$ denote a vacant <adsorption site> and let $\theta$ count sites occupied by $A$. A <mean-field approximation> with <independent> vacancies takes the forward rate proportional to $P(1-\theta)^2$ and the reverse rate proportional to $\theta^2$. Equating them yields
$$
\frac{\theta}{1-\theta}=\sqrt{KP},\qquad
\theta=\frac{\sqrt{KP}}{1+\sqrt{KP}},
$$
with a positive coefficient $K$ of inverse-<pressure> units that includes the chosen pair-counting convention. This square-root dependence illustrates why the intact-<molecule> <Langmuir isotherm> cannot simply be assumed for splitting <molecules>. Requirements for neighboring vacancies or interactions can further change the <mean-field approximation>.
Back to article page
