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The electronic spectra up to 50,000 cm(-1) of uranyl(V) both as a bare ion, UO2(+), and coordinated with three carbonate ligands, [UO2(CO3)3]5-, are presented. Solvent effects were treated by a nonequilibrium continuum solvent model. The transition energies were obtained at the spin-orbit level using relativistic wave function based multiconfigurational methods such as the complete active space self-consistent field method (CASSCF)and the complete active space with second-order perturbation theory (CASPT2) followed by a calculation of the spin-orbit effects at the variation-perturbation level. Earlier relativistic intermediate Hamiltonian Fock space coupled-cluster calculations on the spectrum of the bare uranyl(V) ion were extended to investigate the influence of electron correlation effects on spacings between the electronic states. This study is an attempt to contribute to an enhanced understanding of the electronic structure of actinyl ions. Both spectra show transitions within nonbonding orbitals and between nonbonding and antibonding orbitals as well as charge transfers from the uranyl oxygens to uranium. The ground state in UO2(+) is found to be 2Φ(5/2u), corresponding to the σ(u)(2)φ(u)(1) configuration, while in [UO2(CO3)3]5-, it is 2Δ(3/2u), arising from the σ(u)(2)δ(u)(1) configuration. It is remarkable that the excited state corresponding to an excitation from the nonbonding δ(u) to the uranyl antibonding 3π(u)* molecular orbital is significantly lower in energy in the carbonate complex, 6623 cm(-1), than that in the bare ion, 17,908 cm(-1). The first ligand (carbonate) to metal charge-transfer excitation is estimated to occur above 50,000 cm(-1). The reported results compare favorably with experiment when available.

The 5 f 1 6 d ( t 2 g ) 1 emission spectrum of Cs 2 GeF 6 :U 4+ single crystals has been calculated using the relativistic ab initio model potential (AIMP) embedded cluster method. It consists of a series of broad, intense bands in the UV followed by broad and increasingly weaker bands in the visible and near infrared spectral range, which appear to be associated with emissions from the lowest 5 f 1 6 d ( t 2 g ) 1 excited state, 1 E u , to a number of levels of the 5 f 2 manifold. The electric dipole allowed emission of highest energy, 1 E u → 1 T 1 g , is found to have very low oscillator strength, - 0.115 × 10 - 2 ; this value is only 4% of the most intense electronic transition: 1 E u → 1 T 2 g whose electronic origin is found to be at 34 200 cm - 1 . A very large Stokes shift of 6200 cm - 1 is obtained, which should result in low luminescence quenching due to reabsorption and should favourably contribute to the emission quantum efficiency. The lifetime of the spontaneous emission from 1 E u has been calculated to be in the nanoseconds range: 33 ns.

The aim of this paper is to present the methodology of ab initio simulation in the field of photoluminescent materials using the local embedded cluster technique. In our approach the effect of the crystalline environment is introduced via ab initio embedding model potentials and the resulting hamiltonian of the perturbed cluster is treated using state-of-the-art quantum chemistry methods including a refined spin–orbit coupling treatment. The method is applied to the case study of a Bi 3+ ion embedded in a site of point symmetry S 6 of Y 2 O 3 sesquioxide. Comparison of the results obtained with two kinds of embedded clusters (a monoatomic Bi 3+ cluster and a more realistic molecular (BiO 6 ) 9− cluster) makes it possible to separate the respective parts of crystal field and molecular bonding. Both effects contribute to a marked stabilization of Bi 3+ 6s6p z versus Bi 3+ 6s6p x,y , which causes a spectacular quenching of spin–orbit effects. The strong decrease of the 6s 2 -6s6p z energy separation (10 → 4 eV) observed on embedding Bi 3+ is entirely due to the small covalent character of the mainly ionic Bi–O bond, not at all to crystal field. The lowest-lying excited states calculated in the range 4.3–5.1 eV are found to be the four components of the Bi 3+ (6s6p z ) 3 A u and 1 A u complex. The next states, above 6.9 eV, are charge transfer states corresponding to the excitation of an electron of the valence 2p band of oxygen of the ground state into the 6p z bismuth orbital. In the qualitative orbital model this allows to deduce an estimate of the position of the Bi 3+ ground state with respect to the top of the oxygen valence band: 2.6 eV.

High level (CASSCF-MRCI) ab initio calculations are used to investigate the structural, electronic and vibrational properties of the electronic ground state of thiazyl chloride (NSCl) and of the low-lying electronic states of NSCl + . A new high resolution HeI photoelectron spectrum of NSCl has been recorded in the 10–16 eV energy region. From the results of the calculations, the first band is assigned to the (1) 2 A ′ state of NSCl + . The second one corresponds to the (2) 2 A ′ and (1) 2 A ″ states which are quasi-degenerate. Despite the high resolution, the two first bands show no vibrational fine structure. For the first one, Franck–Condon analysis shows that it is due to the overlapping of two vibrational progressions involving the S–Cl stretching and the NSCl bending modes. In the case of the second band, it is explained by the highly repulsive character of the potential energy surfaces of two states of NSCl + in the Franck–Condon region of the neutral molecule. For the third band, which shows vibrational peaks, the calculation of Franck–Condon factors permits the determination of the adiabatic ionisation energy of the (3) 2 A ′ electronic state of NSCl + at 13.798 eV. Finally, the fourth band, which is due to three different ionic states with vibrational progressions, is too complicated to be assigned quantitatively.

International audience; The carbon and oxygen K-shell spectra of gaseous 2-propenal (acrolein) have been measured using the inner-shell electron energy loss spectroscopy method. Large scale ab initio configuration interaction calculations have been carried out to enable firm assignments of the observed bands. The overall shapes of the spectra are similar to previous low resolution monolayer and multilayer phases NEXAFS spectra recorded by photoabsorption of synchrotron radiation, but the spectral bands are much better resolved than the earlier ones. The spectra are dominated by excitation of π* type states and by interaction between the C=C and C=O π* orbitals.

The carbon and nitrogen K-shell excitation spectra of gaseous pyridine and pyridazine were recorded using the electron-energy loss spectroscopy under electric-dipole conditions (2 keV, small angle) with a resolution of 0.2 eV. Ab initio Configuration interaction calculations in the frame of the equivalent core model were performed in order to help in the assignment of the spectral features. The spectra are dominated by the transitions to the 1π* and σ* type orbitals. The C1s spectra of both molecules are close to that of benzene: The intensity of Rydberg transitions are enhanced by an important valence σC–H* character; the 1s→3π* transition is mixed with double excitations and give rise to several states, some of them lying above the ionization thresholds. Finally, the N1s spectra of both molecules are similar to the s-triazine one.

Three flexible models (PW, CMP and TCPE) are presented to study interactions occurring in water clusters. The total interaction energy is decomposed into five terms; repulsive, charge–charge, intramolecular relaxation, polarization, and hydrogen bonding. The parameters of the first three terms are the same for all of the three models. The polarization term is taken as a classical molecular many-body polarization potential for the CMP and TCPE models, while nonpairwise effects are omitted in the PW model. As nonpairwise effects occurring in water clusters greater than the dimer have a topological origin, such effects are introduced in the hydrogen bonding term of the TCPE model. Parameters of the three models are derived from ab initio calculations at the MP2/6-311+G(2df,2p) level on three water dimer structures and the cyclic water trimer. Application of the three models to water clusters (H2O)n from n=3 to 20 exhibits that the nonpairwise effects in such clusters represent about 20% of their total bindin...

The three-dimensional potential energy functions for the two components of the X2П ground state of HCSi isomer have been determined by multi-reference contracted CI calculations. These potentials have been used in beyond Born-Oppenheimer variational calculations of the ro-vibronic energy levels including the couplings of rotation-vibration, the electron spin and the electronic angular momenta: band origins are given up to 4000 cm−1. Selected MRCI calculations on the HSiC isomer have shown that the more stable structure of this isomer is a bent 2A′ , stabilized however by a small barrier of only 2800 cm1. The geometry of the saddle point between the two isomers has been located.

The predissociation or dissociation of selected vibronic levels of the X 2 Π, A 2 Π, and B 2 Σ + states of the COS+ ions has been studied experimentally. The product state distribution has been probed by photoabsorption of the synchrotron radiation in the 13.5–16.5 eV range and threshold photoelectron-photoion coincidence spectroscopy. Ab initio calculations have been performed to model the potential energy surfaces of the X 2 Π, A 2 Π, and B 2 Σ + electronic states and also of the lowest energy 4Σ− state. This latter is coupled by spin-orbit to X 2 Π and is confirmed to be responsible of the predissociation in the A − X Franck-Condon gap. In the A − X Franck-Condon energy gap the predissociation mechanism by 4Σ− is not statistical and favours the production of vibrationally excited CO (v″) with maximum probability for the highest energy accessible levels. It involves also a highly effective transfer into rotational motion. For the A 2 Π state, there is internal conversion to X 2 Π followed by predissociation to the lowest limit and, for the levels above the second limit, the predissociation by 4Σ− is in competition with direct dissociation of X 2 Π to the second limit. The B 2 Σ + state also undergoes internal conversion to X 2 Π followed by dissociation to the second limit and predissociation by 4Σ− to the first limit.

International audience; The chloromethane cation and dication have been studied using the complete active space self-consistent field method followed by a multireference perturbative configuration interaction. The 2E ground state of CH3Cl presents a weak Jahn-Teller distortion, the 2A' and 2A" components being separated by only 3.4 kcal/mol with very similar geometries. Both states are equilibrium structures. Contrary to previous calculations, the lowest singlet and triplet states of CH3Cl++ are calculated to be stable. While the 3A2 state keeps the neutral C3v symmetry, the 1E singlet state shows a large Jahn-Teller effect, with a splitting of 32.7 kcal/mol in favor of the 1A' state, which is the ground state of the molecule. The planar 1A' state of CH2Cl++ is found to be the most stable isomer, lying 41.4 kcal/mol below the corresponding state of CH3Cl++. The 3A" state, which lies 22.59 kcal/mol above the 1A' state, has a nonplanar Cs-symmetry geometry. Finally, the CH3Cl++ (1A') → CH2ClH++ (1A') isomerization takes place via a C1-symmetry transition state, with an energy barrier of 32.85 kcal/mol.

International audience; The vibrational structure of the K-shell O1s → π∗ of acenaphthenequinone C(12)H(6)O(2) and its halogenated compound C(12)H(2)Br(2)Cl(2)O(2) has been simulated using an entirely ab initio approach. For both molecules, analysis of the calculated Franck-Condon factors confirm without ambiguity that, contrary to initial claims, the C-H stretching modes are not modified in the core states and are not excited. For C(12)H(6)O(2), the vibrational fine structure appears to be mainly due to three modes, involving C=O∗ asymmetric stretch and in-plane ring deformation modes, due to the symmetry breaking of the core state. For C(12)H(2)Br(2)Cl(2)O(2), the vibrational excitation arises essentially from the C=O∗ asymmetric stretch, with numerous secondary peaks arising from hot and combination bands. For both molecules, these bands are probably responsible for the asymmetry deduced in the experimental fits using a unique Morse potential and initially assigned to anharmonic effects.

International audience; issociations of the ethyne dication following its production by photoionization in the photon energy range of 35-65 eV have been investigated by the photoelectron-ion-ion coincidence technique using both synchrotron radiation and laboratory light sources. New quantum mechanical calculations identify and locate the electronic states of the molecular dication in this energy range and show that the dissociation products are formed in their ground states by heterogeneous processes. Five reaction channels leading to three molecular fragments have been - identified and are interpreted as sequential processes, several faster than fragment rotation and one possibly involving dissociation of CH+ to H+ with a lifetime of the order of 25 fs.

Ab initio calculations have been carried out in order to assign the bands observed in the carbon and oxygen K-shell spectra of gaseous tetrahydrofuran (THF), measured using inner-shell electron energy loss spectroscopy (ISEELS). The good agreement between the theoretical and the measured spectra allows us to re-assign precisely most of the peaks. The ionisation energies are calculated for the first time and found to be consistent with a quantum defect analysis of the core excitation energies. The calculated spectra for the C 2 v , C 2 and C s geometries of the molecule show very small differences. Thus, K-shell spectroscopy cannot be used to solve the controversy on the geometry of THF.

International audience; The first ab initio calculations (vertical energies and oscillator strengths) are reported for the neutral electronic transitions of isoprene (2-methyl-1,3-butadiene), CH(2)CHC(CH(3))CH(2). The VUV photoabsorption spectroscopy of the molecule is presented in the energy range 4.6 to 10.8 eV (270-125 nm) with the highest resolution yet reported above 6.05 eV, revealing new spectral features. Valence and Rydberg transitions have been assigned in accordance with the theoretical results and the associated vibronic series have been analysed. The absolute photoabsorption cross sections at energies below 6.89 eV have been used to calculate the photolysis lifetime of isoprene in the upper stratosphere (20-50 km). Electron energy loss spectroscopy (EELS) measurements have enabled further photoabsorption cross sections to be derived in the range 9-28 eV. The first ab initio calculations have been carried out to determine excitation energies to the lowest energy ionic states of isoprene. The calculations are compared with the He(i) photoelectron spectrum (8 to 17 eV) and new vibrational structure is observed in the first photoelectron band.

15 pages; International audience; The electronic spectra of the 5f2 isoelectronic actinide compounds U4+, NpO2+, and PuO22+ have been investigated theoretically both in gas phase and in solution. In the latter case the solvent was modelled by a saturated first hydration sphere, five water molecules for NpO2+, and PuO22+ and eight for U4+, and a continuum model describing the remaining solvent. The transition energies and oscillator strengths were obtained at the spin-orbit level using the relativistic wave function based multi-configuration methods CASPT2 (complete active space with second-order perturbation theory) and MRCI+DC (Davidson corrected multi-reference configuration interaction), followed by a spin-orbit CI based on a dressed effective spin-orbit Hamiltonian. This study is an attempt to contribute to an enhanced understanding of the electronic structure of tetravalent actinide ions and actinyl (V) and (VI) ions. The spin-orbit MRCI and spin-orbit CASPT2 transitions energies have been compared for the bare ions, leading us to the conclusion that the spin-orbit CASPT2 approach is reasonably accurate and can be used with confidence for the calculation of the hydrated species. The first hydration sphere and the bulk solvent lift degeneracies, but the effect on the transition energies is fairly small for the two actinyl ions, while it is larger, up to several thousands of wave numbers for U4+. The calculations allowed us to make assignments of the experimentally observed absorption spectra for all species. The computed transition energies and intensities compared favourably with experiment.

8 pages; International audience; In order to assess the accuracy of a recently adjusted relativistic energy-consistent small-core pseudopotential for uranium, the U5+ (5f1 subconfiguration) spin-orbit splitting as well as the fine structure of the U4+ (5f2 subconfiguration) spectrum have been calculated. The pseudopotential has been adjusted to four-component all-electron data, i.e., at the multiconfiguration Dirac-Hartree-Fock level using the Dirac-Coulomb Hamiltonian with a Fermi nucleus charge distribution and perturbatively including the Breit interaction. Its performance in a dressed effective Hamiltonian spin-orbit configuration interaction framework is compared to that of an older scalar-relativistic Wood−Boring adjusted pseudopotential, supplemented by a valence spin-orbit term, as well as to all-electron calculations using the Douglas-Kroll-Hess Hamiltonian. Electron correlation is accounted for by the multireference configuration interaction method with and without the Davidson correction and with different frozen-orbital spaces. Our best calculations show satisfactory agreement with experimental data; i.e., the mean absolute (relative) deviations amount to 183 (2.4%) and 948 cm-1 (5.1%) for the U5+ and the U4+ fine-structure energy levels, respectively. Even better agreement, comparable to the one for rigorous highly correlated four-component all-electron data, is obtained in intermediate Hamiltonian Fock-space coupled-cluster calculations applying the new pseudopotential.

International audience; Ab initio Configuration Interaction calculations have been carried out in order to assign the bands observed in the carbon and oxygen K-shell spectra of gaseous acetic acid, measured using the inner-shell electron energy loss spectroscopy (ISEELS) method with better energy resolution than in previous studies. The good agreement between the theoretical and the measured spectra allows us to assign precisely most of the peaks, especially for the Rydberg states. Some of them have been shown to have strong valence character. The lowest energy band at the carbon edge is assigned to the transitions 1sC1 --> 3p[pi]/[sigma]*(C1-H) and 1sC1 --> [pi]*(C2-O2). Simple Franck-Condon calculations, based on the linear coupling approximation, were performed in order to reproduce the vibrational structure observed for the first time in the oxygen and carbon core excited species as well as in the previously measured X-ray photoelectron spectroscopy spectra of the core ionised molecule. Finally, the calculated structural parameters of the core states of acetic acid match well those of the corresponding valence states of their Z + 1 molecules, as predicted by the equivalent core approximation. However, significant differences between the geometry of the 1sC1 --> [pi]*(C2-O2) state and the CH3NOOH ground state are obtained.

9 pages; International audience; We discuss the accuracy of density functional theory (DFT) in the gas phase for the water-exchange reactions in the uranyl(VI) aqua ion taking place both in the electronic ground state and in the first excited state (the luminescent 3Δg state). The geometries of the reactant and intermediates have been optimized using DFT and the B3LYP functional, with a restricted closed-shell formalism for the electronic ground state and either an unrestricted open-shell formalism or the time-dependent DFT method for the 3Δg state. The relative energies have been computed with wave-function-based methods such as Møller–Plesset second-order perturbation theory, or a minimal multireference perturbative calculation (minimal CASPT2); coupled-cluster method (CCSD(T)); DFT with B3LYP, BLYP, and BHLYP correlation and exchange functionals; and the hybrid DFT−multireference configuration interaction method. The results obtained with second-order perturbative methods are in excellent agreement with those obtained with the CCSD(T) method. However, DFT methods overestimate the energies of low coordination numbers, yielding to too high and too low reaction energies for the associative and dissociative reactions, respectively. Part of the errors appears to be associated with the amount of Hartree–Fock exchange used in the functional; for the dissociative intermediate in the ground state, the pure DFT functionals underestimate the reaction energy by 20 kJ/mol relative to wave-function-based methods, and when the amount of HF exchange is increased to 20% (B3LYP) and to 50% (BHLYP), the error is decreased to 13 and 4 kJ/mol, respectively.

nate, with higher resolution than in the previous studies. They are interpreted with the aid of ab initio configuration interaction calculations. The spectrum at the C 1s edge is dominated by an intense * band. The calculated chemical shift due to the different chemical environment at the carbon 1s edge calculated is in agreement with the experimental observations within a few tenths of an eV. The transition energies of the most intense bands in the C 1s excitation spectrum are discussed at different levels of calculations. In the nitrogen 1s excitation spectrum the most intense bands are due to Rydberg-valence transitions involving the * -type molecular orbitals, in agreement with the experiment. This assignment is different from that of extended Huckel molecular orbital calculations. The geometries of the core excited states have been calculated and compared to their equivalent core molecules and benzene.

Ab initio study of excitation energies and oscillator strengths for absorption towards the (3)P(1) and (1)P(1) states of the Bi(3+) ion has been performed for the Bi(3+) ion in gas phase and as a dopant of the cubic elpasolite Cs(2)NaYCl(6) and the yttria Y(2)O(3) crystal using the ab initio embedded-cluster method. The ground and excited states were computed with a relativistic spin-orbit configuration interaction approach suited for heavy elements. Electron correlation was treated in the scalar relativistic scheme with perturbative, variational, and coupled-cluster methods. Intermediate coupling is included via an effective-Hamiltonian based spin-orbit configuration interaction approach. Small-core (60 electrons) and large-core (78 electrons) relativistic effective core potentials (ECPs) have been used to describe the bismuth ion. The best match with experiment was obtained with the small-core ECP. The accuracy of excitation energies strongly depends on the electron correlation method used. The agreement between experimental data and the results obtained using second-order multiconfigurational perturbation theory is greatly improved with the shifted zeroth-order Hamiltonian proposed by Ghido et al. [Chem. Phys. Lett. 396, 142 (2004)]. Although quite time consuming, coupled-cluster and variational methods yield good agreement with experimental data. The first absorption band recorded for the doped elpasolite crystal is positioned with an excellent accuracy while the computed energy of the second absorbing manifold is in poorer agreement with experimental data. This suggests that interactions with neglected close-lying excited states with a ligand-to-metal charge transfer character may be significant. Calculations of the spectrum of Bi(3+) doping yttria in both the S(6) and C(2) site symmetries indicate that the absorbing manifold arises from electronic excitations localized on the Bi(3+) doping ion with main triplet 6s6p character. Our results predict the first absorbing peak to lie about 0.5 eV lower for the S(6) sites than for the C(2) site, thus attributing the violet and the green emission wavelengths to the S(6) and C(2) sites, respectively. A subsequent study of Stokes shift and emission wavelength should hopefully lead to a final assignment of the measured excitation spectra.

Electronic state spectroscopy of CH3COOH has been investigated using Vacuum UltraViolet (VUV) photo-absorption spectroscopy in the energy range 3.5-10.6 eV. The availability of a high-resolution photon beam (similar to 0.075 nm) has allowed detailed analysis of the vibrational progressions and allowed us to propose, for the first time, new assignments for several Rydberg series. Excited states located in the 10-14 eV region have been studied for the first time. The VUV photo-absorption spectrum is compared with the electron energy loss spectrum (EELS) recorded in dipolar transition mode (100 eV incident energy, similar to 0 degrees scattering angle) in the 5.0-14 eV energy loss range. A He(l) photoelectron spectrum has also been recorded from 10.5 to 13 eV and compared to earlier low resolution works. A new value of (10.657 +/- 0.002) eV for the ground ionic state adiabatic ionisation energy is proposed. All spectra presented in this paper represent the highest resolution data yet reported for acetic acid. New ab initio calculations help in the assignment of the spectral bands. (c) 2005 Elsevier B.V. All rights reserved.

International audience; Ab initio configuration interaction calculations have been carried out in order to assign the bands observed in the carbon K-shell spectrum of gaseous cyclopropane, measured using the inner-shell electron energy loss spectroscopy (ISEELS) method. Simple Franck-Condon calculations, based on the linear coupling approximation, were performed in order to reproduce the observed vibrational structure. The good agreement between the theoretical and the measured spectra allows to assign precisely most of the peaks. The spectrum is dominated by an intense band, corresponding to the 1s→σ∗(CC)(1a2') transition, with large vibrational excitation. This is explained by the trimethylenic equilibrium geometry of this core state, very different from the D 3h ground state structure. On the other hand, the Rydberg core-excited states and the core ion are only slightly distorted. Finally, the calculated structural and energetic parameters of the NC 2H 6 valence states match well those of the corresponding core states of C 3H 6, illustrating the validity of the equivalent core approximation.

International audience; The carbon and nitrogen K-shell spectra of gaseous s-triazine have been studied using inner-shell electron energy loss spectroscopy (ISEELS) method. Ab initio Configuration Interaction calculations have been carried out in order to assign the observed bands. As in many similar molecules, both spectra are dominated by an intense π * peak, followed by lower intensity features. At the C1s edge, the calculations show that some previous assignments made using an underestimated core ionisation energy of about 2.5 eV have to be revisited. At the nitrogen edge, the calculations predict a high intensity π* doubly excited state lying below the ionisation threshold, which could be responsible for one the most intense observable bands at 405.32 eV.

International audience; The K-shell spectra of gaseous furan have been measured using the inner-shell electron energy loss spectroscopy (ISEELS) method at the carbon and oxygen thresholds. Large-scale ab initio configuration interaction calculations have been carried out in order to help in the assignments of the observed bands. The spectra are close to previous low resolution ones obtained using ISEELS in the gas phase and photoabsorption with the synchrotron radiation in gaseous and condensed phases. The presence of a new feature located at 287.3 eV in the C1s spectrum and recently detected by photoabsorption with synchrotron radiation is confirmed. At both edges, the calculations confirm the assignments proposed in earlier experimental works, with the exception of several C1s pre-edge features for which a new interpretation is given.

International audience; The carbon K-shell spectrum of gaseous benzene has been recorded by inner-shell electron energy loss spectroscopy (ISEELS) under electric-dipole conditions (2 keV, small angle) with a resolution of 0.17 eV. Ab initio configuration interaction calculations performed in the frame of the equivalent core model have helped in the assignment of the experimental spectrum. They emphasize the importance of the multi-configurational approach and the role of the Rydberg orbitals in the assignment of some features. The band located at 289.09 eV is assigned to Rydberg excitations and a 3 pi* transition mixed with doubly excited configurations, and not to an anti-bonding (C-H) transition as suggested previously. Because of a large 3s/3d character of the (C-H)* orbital, the corresponding peak appears above the ionization threshold. The 293.8 eV band contains in-plane resonances and very likely also several doubly excited states with a 3 pi* contribution.

International audience; The spectroscopy of the unoccupied molecular orbitals of gaseous pyrrole and pyrazole has been studied by electron impact under electric dipole scattering conditions (2 KeV, small angle) and the inner-shell electron energy loss method. The core-excitation spectra have been recorded at the C1s and N1s edges with 0.2 eV resolution, resolution allowing to observe separately the transitions from the carbon atoms with different chemical environment. Ab initio calculations, using the equivalent core model, were performed to help in the interpretation of the spectral features. The spectra are dominated by transitions to the π* and σ* molecular orbitals. The calculations confirm that the 3b1(π*) orbital is the lowest-energy unoccupied one in pyrrole. In several cases, the intensity of the Rydberg excitations is enhanced by an important valence σ* character of the Rydberg orbitals.

The C2H photodissociation processes have been studied using ab initio multireference perturbative configuration interaction methods. The calculated structural parameters of the two linear lowest lying states X 2Σ+ and A 2Π were found to be in good agreement with previous theoretical and experimental works. Construction of the correlation diagram indicates that C2H cannot photodissociate into C+CH. Therefore, two‐dimensional potential energy surfaces of the six lowest lying A’ and A‘ doublet states have been calculated as functions of the bond angle and the C–H bond distance, keeping the C–C distance at the equilibrium C2H value. It appears that the 1 2A’, 2 2A’, and 1 2A‘ states correlating to the X 2Σ+ and A 2Π states in linear geometry remain bound at all angles, whereas the 3 2A’, 4 2A’, and 4 2A‘ are dissociative. However, the structure of the energy surfaces is complicated by the presence of numerous avoided crossings between these states. Transition dipole moments connecting the excited states with ...

New ab initio embedded-cluster calculations devoted to simulating the electronic spectroscopy of Bi(3+) impurities in Y(2)O(3) sesquioxide for substitutions in either S(6) or C(2) cationic sites have been carried out taking special care of the quality of the environment. A considerable quantitative improvement with respect to previous studies [F. Real et al. J. Chem. Phys. 125, 174709 (2006); F. Real et al. J. Chem. Phys. 127, 104705 (2007)] is brought by using environments of the impurities obtained via supercell techniques that allow the whole (pseudo) crystal to relax (WCR geometries) instead of environments obtained from local relaxation of the first coordination shell only (FSR geometries) within the embedded cluster approach, as was done previously. In particular the uniform 0.4 eV discrepancy of absorption energies found previously with FSR environments disappears completely when the new WCR environments of the impurities are employed. Moreover emission energies and hence Stokes shifts are in much better agreement with experiment. These decisive improvements are mainly due to a lowering of the local point-group symmetry (S(6)--C(3) and C(2)--C(1)) when relaxing the geometry of the emitting (lowest) triplet state. This symmetry lowering was not observed in FSR embedded cluster relaxations because the crystal field of the embedding frozen at the genuine pure crystal positions seems to be a more important driving force than the interactions within the cluster, thus constraining the overall symmetry of the system. Variations of the doping rate are found to have negligible influence on the spectra. In conclusion, the use of WCR environments may be crucial to render the structural distortions occurring in a doped crystal and it may help to significantly improve the embedded-cluster methodology to reach the quantitative accuracy necessary to interpret and predict luminescence properties of doped materials of this type.

10 pages; International audience; Cécile Danilo,1,2 Valérie Vallet,1 Jean-Pierre Flament,1 and Ulf Wahlgren2,3 1Université de Sciences et Technologies de Lille 1, Laboratoire PhLAM, CNRS UMR 8523, CERLA, CNRS FR 2416, Bât. P5, F-59655 Villeneuve d'Ascq Cedex, France 2Department of Physics, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden 3NORDITA, AlbaNova University Centre, 106 91 Stockholm, Sweden The energy levels of the 5f configuration of U5+ and 5f2 configuration of U4+ have been calculated in a dressed effective Hamiltonian relativistic spin-orbit configuration interaction framework. Electron correlation is treated in the scalar relativistic scheme with either the multistate multireference second-order multiconfigurational perturbation theory (MS-CASPT2) or with the multireference single and double configuration interaction (MRCI) and its size-extensive Davidson corrected variant. The CASPT2 method yields relative energies which are lower than those obtained with the MRCI method, the differences being the largest for the highest state 1S0 of the 5f2 manifold. Both valence correlation effects and spin-orbit polarization of the outer-core orbitals are shown to be important. The satisfactory agreement of the results with experiments and four-component correlated calculations illustrates the relevance of dressed spin-orbit configuration interaction methods for spectroscopy studies of heavy elements.

The ab initio embedded cluster method coupled with correlated spin-orbit calculations has been used to interpret the excitation spectrum of a Bi(3+)-doped yttria crystal. Our results indicate that the Bi(3+) impurity can absorb light over a wider energy range in the C(2) site than in the S(6) site. Even if the computed absorption energies seem to be about 0.4 eV too high with respect to the experimental peaks for both sites, it is noteworthy that the embedded cluster model renders 93% of the large crystal redshift, about 6 eV. The determination of the geometry relaxation of the first shell of oxygen neighbors upon electronic excitation shows that the Stokes shift is smaller in the S(6) site than in the C(2) site. Combining all these results confirms the assignment of the violet emission to the S(6) site and that of the green emission to the C(2) site, as proposed by Boulon [J. Phys. (Paris) 32, 333 (1971)]. In addition, the nature of the metastable states which lie below the emitting ones and are responsible for the temperature dependence of the fluorescence lifetimes is discussed.