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We investigated the inner shell photofragmentation of thiophene by time of flight (TOF) mass spectroscopy using multi-coincidence electron-ion techniques. Our main purpose was to understand aspects of molecular relaxation process after inner shell excitation and to search for bond break selectivity. Analyses of mass and branching ratios are presented and the results suggest different mechanisms and channel of fragmentation when compared with S 2p and C 1s excitations.

A multi-reference configuration interaction (CI) method based on multiple Hartree–Fock (HF) solutions is applied to study the permanent dipole moment of the LiH, BH, FH, and H2O systems with the minimal atomic basis. The LiH was also calculated with the double-zeta atomic basis. This method uses several HF solutions as references to expand the state functions. For each one of the systems studied, several HF solutions were obtained in the appropriate point and spin symmetry classes, i.e., 1Σ+ and 1A1. With each of these solutions is generated a distinct basis of the same full CI space. The set of these bases is a system of generators of the full CI space and the multi-reference HF (MRHF) bases are built mixing configuration state functions (CSFs) originated from distinct HF solutions. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

The authors present a study of the instability and convergence of Hartree-Fock (HF) ab initio solutions for the diatomic systems H{sub 2}, LiH, CH, C{sub 2}, and N{sub 2}. In the study, they consider real molecular orbitals (MOs) and analyze the classes of single-determinant functions associated to Hartree-Fock-Roothaan (HFR) and Hartree-Fock-Pople-Nesbet (HFPN) equations. To determine the multiple HF solutions, they used either an SCF iterative procedure with aufbau and non-aufbau ordering rules or the algebraic method (AM). Stability conditions were determined using TICS and ASDW stability matrices, derived from the maximum and minimum method of functions (MMF). They examined the relationship between pure SCF convergence criterion with the aufbau ordering rule, and the classification of the HF solution as an extremum point in its respective class of functions. The results show that (1) in a pure converged SCF calculation, with the aufbau ordering rule, the solutions are not necessarily classified as a minimum of the HF functional with respect to the TICS or ASDW classes of solutions, and (2) for all studied systems, they obtained local minimum points associated only with the aufbau rule and the solutions of lower energies.

Realizamos um estudo sistematico de impurezas de metal de transicao das series 3d, 4d e 5d em semicondutores iii-v atraves do modelo de aglomerados moleculares utilizando duas tecnicas de campo medio diferentes: espalhamento multiplo-xALFA e a aproximacao semiempirica indo. Efeito de muitos-eletrons foram introduzidos para a descricao do espectro de excitacao atraves de correlacoes de multipletos e interacao de configuracoes. Apresentamos um estudo comparativo entre os espectros obtidos a partir do calculo de interacao de configuracoes e o espectro experimental, onde observamos uma melhor concordancia entre o espectro experimental e os resultados obtidos a partir de bases mais completas. Observamos tambem, uma melhor concordancia entre o espectro experimental e os nossos resultados quando os orbitais com maior carater d apresentam-se fortemente localizados. Nossos resultados para a correcao de multipletos comparados com nossos resultados do calculo de interacao de configuracao nos mostra uma boa concordancia, quando o calculo de interacao de configuracoes e realizado apenas com os orbitais e e T IND.2 com forte carater d. No estudo envolvendo relaxacoes e distorcoes no sistema GAAS:V POT.2+, observamos um favorecimento ao estado fundamental de alto-spin, ANTPOT.4 T IND.1. A systematical study about 3d, 4d and 5d transition metal impurities in III-V semiconductors were realized through the molecular approach using two differents mean-field techniques: Multiple Scattering-X and the semiempirical approximation INDO. Many electron effects was introduced through the multiplets corrections and the configuration interaction calculation to the excitation spectrum description. A comparative study between configuration interaction and experimental spectra is presented, where we observed a good agreement between the experimental spectrum and the results achieved from more complete basis. We also discuss the agreement of our results and the experimental study, when the orbitals with higher d-character are strongly localized. Our multiplets correction results compared with our configuration interaction results show us a good concordance when the configuration interaction calculation is realized with only e and t2 strong d-character orbitals. In the study involving relaxations and distortions in GaAs:v2+ system, we observe a favouring to the high-spin 4t1 ground-state.

The multiple-scattering Xalpha molecular cluster model is used to carry out self-consistent calculations of the electronic states of an isolated P vacancy in GaP. A fully occupied singly degenerate a1 level and a threefold-degenerate t2 level occupied by one electron are found within the band gap. This configuration indicates that the centre is unstable with respect to Jahn-Teller distortions. The vacancy states a1 and t2 are quite extended showing an appreciable charge distribution in the next-nearest neighbours of the vacancy (P atoms). The role played by these results in the explanation of the properties of the GaP:O centre is discussed.

We study the electronic structure of the Mo 4d and W 5d impurities in GaAs, using the cluster approach within the multiple-scattering X\ensuremath{\alpha} formalism; results are also given for the Cr 3d isovalent impurity. Multiplet structures for the systems are evaluated through the approach of Fazzio, Caldas, and Zunger. Our results indicate that the Mo and W impurities will present a low-spin ground state in the 2+ oxidation state (acceptor), and that the observed luminescence lines are not related to the $^{5}\mathrm{}^{5}$${\mathrm{T}}_{2}$ internal transition of the isolated substitutional Mo and W impurities.

The multiple-scattering Xα method within the framework of the Watson-sphere-terminated cluster model was used to carry out electronic structure calculations of a phosphorus vacancy and an oxygen substitutional impurity replacing phosphorus in GaP. The calculations were performed by considering different electronic configurations of the oxygen 2p atomic states, and the results were compared with those derived from the two theoretical models recently proposed to describe the experimental data on the GaP:Op system. An alternative description is suggested which may explain the well known large relaxation that occurs after the capture of a second electron by the center.

We present a comparison between two approaches\char22{}the linear combination of atomic orbitals\char21{}configuration interaction approach and the multiplet method developed by Fazzio, Caldas, and Zunger\char22{}to study excitation spectra of transition-metal impurities in a semiconductor. If we place the physically reasonable restriction on the configuration-interaction basis that only excitations between the highest (d-related) orbitals ${t}_{2}$ and e are allowed, we conclude that the results of both approaches are in excellent agreement.