Geometries, Electronic g-tensor Elements, Hyperfine Coupling Constants, and Vertical Excitation Energies for Small Gallium Arsenide Doublet Radicals, GaxAsy (x + y = 3, 5)

DFT calculation of vibrational frequencies of clusters in GaAs and the Raman spectra

We have calculated the vibrational frequencies of clusters of Ga and As atoms from the first principles using the density-functional theory (DFT) method and the local-density approximation (LDA). We find that the calculated value of 286.2 cm(-1) for a linear cluster of Ga(2)As(2) is very near the experimental value of 292 ± 4 cm(-1). The calculated value of 289.4 cm(-1) for Ga(2)As(6) (dumb bell) cluster is indeed very near the experimental value. There are strong phonon correlations so that the cluster frequency is within the dispersion relation of the crystal LO value. There is a weak line in the experimental Raman spectrum at 268 cm(-1) which is very near the value of 267.3 cm(-1) calculated for the Ga(2)As (triangular) cluster. The weak lines corresponding to the linear bonds provide the strength to the amorphous samples. There are clusters of atoms in the glassy state of GaAs.

Theoretical study of the low lying states of Ga(2)X (X = P, As)

Since the low lying electronic states of Ga(2)X (X = P, As) are nearly degenerate, an accurate determination of the electronic structure of these states was incomplete in the previous works. In this study, the geometry optimization and vibrational frequency calculation have been performed at the various levels of theory, using the effective core pseudopotentials of the Ga and As atoms and the associated cc-pVTZ basis sets. The ground state of Ga(2)P is found to be the (2)A' state correlating with the (2)B(2) state in C(2v) structure, which lies 0.031 eV above the global minimum at the RCCSD(T) level. The equilibrium of the (2)A(1) state is optimized above that of the (2)B(2) state, and the (2)B(1) state, predicted previously as the ground state, has a stable minimum above the optimized geometries of the former states. Concerning the low lying states of Ga(2)As, our results confirm in general the previous studies. Because all the states of both compounds can be considered monoconfigurational around their equilibrium, the RCCSD(T) method is well adapted to acurrately describe both systems. In turn, we have applied our obtained information to analyze the Ga(2)P(-) and Ga(2)As(-) anion photoelectron spectra, to which it remains difficult to appropriately assign the low lying states of Ga(2)P and Ga(2)As. As a consequence, our analysis supports the previous assignment for Ga(2)P, but contrasts partially to that for Ga(2)As.

X- (X = O, S) Ions in Alkali Halide Lattices through Density Functional Calculations. 1. Substitutional Defect Models

Monoatomic X- (X = O, S) chalcogen centers in MZ (M = Na, K, Rb and Z = Cl, Br, I) alkali halide lattices are investigated within the framework of density functional theory with the principal aim to establish defect models. In electron paramagnetic resonance (EPR) experiments, X- defects with tetragonal, orthorhombic, and monoclinic g-tensor symmetry have been observed. In this paper, models in which X- replaces a single halide ion, with a next nearest neighbor and a nearest neighbor halide vacancy, are validated for the X- centers with tetragonal and orthorhombic symmetry, respectively. As such defect models are extended, the ability to reproduce experimental data is a stringent test for various computational approaches. Cluster in vacuo and embedded cluster schemes are used to calculate energy and EPR parameters for the two vacancy configurations. The final assignment of a defect structure is based on the qualitative and quantitative reproduction of experimental g and (super)hyperfine tensors.

X- (X = O, S, Se) Ions in Alkali Halide Lattices through Density Functional Calculations. 2. Interstitial Defect Models

Density functional theory techniques are used to investigate the defect structure of X- (X = O, S, Se) ions in MZ (M = Na, K, Rb and Z = Cl, Br) alkali halides which exhibit monoclinic-I g-tensor symmetry, using cluster in vacuo, embedded cluster, and periodic embedding schemes. Although a perturbed interstitial defect model was suggested from electron paramagnetic resonance experiments (EPR), the nature of the perturbation is still unknown. An appropriate defect model is developed theoretically by comparing structural and energetical properties of various defect configurations. Further validation is achieved by cross referencing experimental and computed EPR data. On the basis of the computational results, the following defect model is proposed: the X- ion is located interstitially with a charge compensating halide vacancy in its first coordination shell.