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The optical spectrum of diatomic IrSi has been investigated for the first time, with transitions observed in the range from 17 178 to 23 858 cm-1 (582-419 nm). A rich spectrum has been recorded, consisting of 14 electronic band systems and a number of unclassified bands. Thirty-one bands have been investigated with rotational resolution, allowing the ground state to be identified as X2Δ5/2 arising from the 1σ21π42σ21δ33σ2 configuration. The ground X2Δ5/2 state is characterized by ΔG1/2 = 533 cm-1 and r0 = 2.0899(1) Å for the more abundant isotopic form, 193Ir28Si (57.8%). The measured excited electronic states have equilibrium bond lengths ranging from 2.17 to 2.25 Å and vibrational frequencies ranging from 365 to 452 cm-1. Ab initio calculations were also carried out on the molecule using the complete active space self-consistent field and multistate complete active space second-order perturbation theory methods, with relativistic and spin-orbit effects included through the restricted active space state-interaction with spin-orbit coupling method. The calculated ground state agrees with experiment, and a large number of excited states lying within 20 000 cm-1 of the ground state are reported. [ABSTRACT FROM AUTHOR]

The ground and excited states of the UO2 molecule have been studied using a Dirac-Coulomb intermediate Hamiltonian Fock-space coupled cluster approach (DC-IHFSCC). This method is unique in describing dynamic and nondynamic correlation energies at relatively low computational cost. Spin-orbit coupling effects have been fully included by utilizing the four-component Dirac-Coulomb Hamiltonian from the outset. Complementary calculations on the ionized systems UO2+ and UO22+ as well as on the ions U4+ and U5+ were performed to assess the accuracy of this method. The latter calculations improve upon previously published theoretical work. Our calculations confirm the assignment of the ground state of the UO2 molecule as a 3[uppercase_phi_synonym]2u state that arises from the 5f17s1 configuration. The first state from the 5f2 configuration is found above 10 000 cm-1, whereas the first state from the 5f16d1 configuration is found at 5 047 cm-1. [ABSTRACT FROM AUTHOR]

In this paper we explore the use of the novel relativistic intermediate Hamiltonian Fock-space coupled-cluster method in the calculation of the electronic spectrum for small actinyl ions (NpO2+, NpO22+, and PuO22+). It is established that the method, in combination with uncontracted double-zeta quality basis sets, yields excitation energies in good agreement with experimental values, and better than those obtained previously with other theoretical methods. We propose the reassignment of some of the peaks that were observed experimentally, and confirm other assignments. [ABSTRACT FROM AUTHOR]

In this paper we explore the use of the novel relativistic intermediate Hamiltonian Fock-space coupled-cluster method in the calculation of the electronic spectrum for small actinyl ions (NpO2+, NpO22+, and PuO22+). It is established that the method, in combination with uncontracted double-zeta quality basis sets, yields excitation energies in good agreement with experimental values, and better than those obtained previously with other theoretical methods. We propose the reassignment of some of the peaks that were observed experimentally, and confirm other assignments. [ABSTRACT FROM AUTHOR]

We present calculations at the relativistic coupled cluster theory that predict the 1Σ0+ ground state of CUO to lie 58.2 kJ/mol below the first excited state, 3[uppercase_phi_synonym]2. This can be contrasted with the outcome of earlier density functional theory and complete active space second order perturbation theory (CASPT2) calculations that both predicted a 3[uppercase_phi_synonym]2 ground state upon inclusion of spin-orbit coupling in the calculations. Our result gives further justification to the interpretation of the measured frequency shifts of this species in various noble gas matrices as being caused by significant interaction between the uranium and the heavier noble gas atoms. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The La+O and La+[O.sub.2] chemiionization reactions are investigated using quantum chemical methods. During the study potential energy curves are computed for a number of states of LaO, La[O.sup.*] and La[O.sup.+].

A computational study results of several species with the general formula [M.sub.2][at the rate of][C.sub.60], where the dimetal [M.sub.2] = [Cr.sub.2], [Mo.sub.2], [W.sub.2] and [U.sub.2], which resides inside and outside the fullerene cage, is presented. The transition metal diatomic molecules [Cr.sub.2], [Mo.sub.2], [W.sub.2] are observed to form exohedral complexes with [C.sub.60] while [U.sub.2] forms a highly symmetric endohedral compound placed in the center of the [C.sub.60] cavity.

The codeposition of laser-ablated tungsten atoms with neat hydrogen at 4 K which form a single major product identified as the largest possible physically stable tungsten hydride species is reported. The complex product is tungsten tetrahydride tetradihydrogen (W[H.sub.4][([H.sub.2]).sub.4]) complex with average binding energy of 15 kcal/mol per dihydrogen molecule.

The studies of the wavelength-resolved multiple photon photodissociation of discrete, gas-phase uranyl complexes containing a single anionic ligand with or without ligated solvent molecule is presented. The comparison between the uranyl frequencies and [[U[O.sub.2]AS].sup.+] species with different solvent neutrals showed the values to decrease with increase in neutral nucleophilicity.