Theory of Adsorption and Desorption of $H_2$ Molecules on the Si(111)- $(7\times 7)$ surface

The physics of adsorption and desorption of ${\mathrm{H}}_2$ molecules on the Si(111)- $(7\times 7)$ surface is investigated through first-principles density functional theory calculations. The calculated adsorption and desorption energy barriers are 0.8 and 2.4 eV, respectively. The process of adatom backbond breaking is identified as the fundamental microscopic mechanism determining the adsorption energy barrier and the kinetic energy of the desorbed ${\mathrm{H}}_2$ molecule. These results shed light on controversial experimental findings for this classic surface–molecule system.