THEORETICAL-STUDY OF AS2 DESORPTION FROM THE GA DANGLING-BOND SITE

The cluster model As2-(GaH2)4 is used for ab initio complete active sp ace self-consistent-field (CASSCF) molecular-orbital calculations, to investigate the electronic mechanism of As2 desorption from the fourfo ld-hollow Ga dangling-bond (DB) site. The electronic state of the adso rbed As2 is revealed to be the sigma(g)2pi(u)2pi(g)2 doubly excited co nfiguration, where the free As2 Molecule is characterized by sigma(g)2 pi(u)4 with the triple bond. Namely, the four pi orbitals of the As2 u nit are used to form the four Ga-As covalent bonds with the Ga-DB site and a weak sigma bond remains between the two As atoms. The calculate d 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-configura tional picture is completely invalid in understanding the As2 desorpti on. The transition state is found to be of the ''late-barrier'' type, and from this the ''cold'' vibrational distribution of the desorbed AS 2 ig predicted.