Ab initio MRD-CI study of GaAs -, GaAs 2(±), Ga 2As 2(±) and As 4 clusters

Abstract

Using pseudopotentials and double zeta basis sets with s, p diffuse functions and two sets of d functions, MRD-CI calculations were performed on As 2 (±), As 4 (+), GaAs −, GaAs 2 (±) and Ga 2As 2 (±). This study complements previous theoretical investigations on Ga (±) to Ga 4 (±) and GaAs (+). For As 4 tetrahedral symmetry was assumed, and Re of X 1A 1 determined as 4.73 a 0. Vertical ionization potentials to several states of As 4 + were calculated. For GaAs 2, GaAs 2 + and GaAs 2 −, ground and one low-lying state were geometry-optimized, both in C 2v (Ga-As-As), and linear symmetry (GaAsAs, C ∞h and AsGaAs, D ∞h). The lowest state of GaAs 2 is 2B 2 in C 2v. For Ga 2As 2, the lowest state and low-lying excited states were optimized in various geometries. The most stable state has rhombic structure ( 1A g in D 2h), but T-form and other forms (C 2v, C ∞v, D ∞h) are only 1–2 eV less stable. In D 2h symmetry, several low-lying excited states of Ga 2As 2 were studied. The ground states of Ga 2As 2 + and Ga 2As 2 − were found to be 2B 2u, and 2B 2g, respectively. Trends in ionization potentials (IP), electron affinities (EA), atomization energies and fragmentation energies for the molecules Ga x As y and the pure compounds Ga n and As n up to 4 atoms, were studied. Ga x As y clusters, with x + y even, have higher IP's than odd-numbered clusters. An experimentally observed alternation of EA, whereby an odd number of atoms have higher EA than their even neighbors, is confirmed. The mixed compounds Ga x As y have atomization energies between those of Ga n and As n ( x + y = n), usually closer to those of Ga n . Fragmentation of Ga x As y occurs such that As-As bonds are retained, and if possible, also Ga-As bonds, since the dissociation energy of As 2 is higher than that of GaAs, which in turn is higher than that of Ga 2. Calculated fragmentation energies agree qualitatively with experimental observations about the composition of 3-atomic and 4-atomic clusters Ga x As y .