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\sect{Chemical equilibrium: H2[3D]-2H[2D]}

An ${H_{2}}$ molecule (mass ${2\mathsf{m}_{H})}$ decomposes into ${H}$ atoms when it
is absorbed upon a certain metallic surface with an energy gain
${\varepsilon}$ per ${H}$ atom due to binding on the surface. This
binding is not to particular sites on the surface: the ${H}$ atoms
are free to move on the surface. Consider the ${H_{2}}$ as an ideal
gas, and express the surface density of the ${H}$ atoms as a
function of the ${H_{2}}$ pressure.