Theoretical study ofAs2desorption from the Ga dangling-bond site

The cluster model ${\mathrm{As}}_2$-(${\mathrm{GaH}}_2$ ${)}_4$ is used for ab initio complete active space self-consistent-field (CASSCF) molecular-orbital calculations, to investigate the electronic mechanism of ${\mathrm{As}}_2$ desorption from the fourfold-hollow Ga dangling-bond (DB) site. The electronic state of the adsorbed ${\mathrm{As}}_2$ is revealed to be the ${\mathrm{\sigma }}_{\mathit{g}}^2$ ${\mathrm{\pi }}_{\mathit{u}}^2$ ${\mathrm{\pi }}_{\mathit{g}}^2$ doubly excited configuration, where the free ${\mathrm{As}}_2$ molecule is characterized by ${\mathrm{\sigma }}_{\mathit{g}}^2$ ${\mathrm{\pi }}_{\mathit{u}}^4$ with the triple bond. Namely, the four π orbitals of the ${\mathrm{As}}_2$ unit are used to form the four Ga-As covalent bonds with the Ga-DB site and a weak σ bond remains between the two As atoms. The calculated CASSCF wave functions indicate that the desorption process could be just the chemical reaction of multiple bond conversions having strong electron correlations due to the near degeneracy. The single-configurational picture is completely invalid in understanding the ${\mathrm{As}}_2$ desorption. The transition state is found to be of the ‘‘late-barrier’’ type, and from this the ‘‘cold’’ vibrational distribution of the desorbed ${\mathrm{As}}_2$ is predicted.