Search results for "Orbital"
showing 10 items of 679 documents
Theoretical characterization of the lowest-energy absorption band of pyrrole
2002
The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory (CASPT2) and its multistate extension (MS–CASPT2) in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5–6.5 eV and confirm that the bulk of the intensity of the band arises from a ππ∗ intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new…
New insights in chemical reactivity from quantum chemical topology.
2021
International audience; Based on the quantum chemical topology of the modified electron localization function ELFx, an efficient and robust mechanistic methodology designed to identify the favorable reaction pathway between two reactants is proposed. We first recall and reshape how the supermolecular interaction energy can be evaluated from only three distinct terms, namely the intermolecular coulomb energy, the intermolecular exchange‐correlation energy and the intramolecular energies of reactants. Thereafter, we show that the reactivity between the reactants is driven by the first‐order variation in the coulomb intermolecular energy defined in terms of the response to changes in the numbe…
Bandgap lowering in mixed alloys of Cs2Ag(SbxBi1−x)Br6 double perovskite thin films
2020
Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air and long charge-carrier lifetimes. However, most double perovskites, including Cs2AgBiBr6, have wide bandgaps, which limit photo conversion efficiencies. The bandgap can be reduced through hallowing with Sb3+, but Sb-rich alloys are difficult to synthesise due to the high formation energy of Cs2AgSbBr6, which itself has a wide bandgap. We develop a solution-based route to synthesis phase-pure Cs2Ag(SbxBi1-x)Br6 thin films, with the mixing parameter x continuous varying over the entire composition range. We reveal that the mixed alloys (x between 0.5 and 0.9) dem…
Ferromagnetic kinetic exchange interaction in magnetic insulators
2020
The superexchange theory predicts dominant antiferromagnetic kinetic interaction when the orbitals accommodating magnetic electrons are covalently bonded through diamagnetic bridging atoms/groups. Here we show that explicit consideration of magnetic and (leading) bridging orbitals, together with the electron transfer between the former, reveals a strong ferromagnetic kinetic exchange contribution. First principle calculations show that it is comparable in strength with antiferromagnetic superexchange in a number of magnetic materials with diamagnetic metal bridges. In particular, it is responsible for a very large ferromagnetic coupling ($-10$ meV) between the iron ions in a Fe$^{3+}$-Co$^{…
Circular dichroism in X-ray photoemission from Pd(111) and CO/Pd(111)
1995
It is shown experimentally that in the soft X-ray region a large circular dichroism in the photoelectron angular distribution (CDAD) exists for both valence orbitals and core levels of CO molecules adsorbed on Pd(111). From theoretical consideration it follows that in the case of a spherically symmetric ground state wave function, like the 1σ and 2σ orbitals of CO, CDAD appears due to the lack of spherical symmetry in the final state. For carbonK-shell experimental results are compared to model calculations. Investigations at the Pd core levels proved that CDAD does also arise in X-ray photo-emission from non-magnetic crystals.
Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808
2016
We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of $\sim339.97$ Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of $44.3$ minutes and a projected semi-major axis of $\sim17.6$ lt-ms. Based on the mass function we estimate a minimum companion mass of 0.024 M$_{\odot}$, which assumes a neutron star mass of 1.4 M$_{\odot}$ and a maximum inclination angle of $75^{\…
Experimental and Theoretical Investigations on the Synthesis, Structure, Reactivity, and Bonding of the Stannylene-Iron Complex Bis{{bis(2-tert-butyl…
2000
The pi-(arene)bis(stannylene) complex bis(bis(2-tert-butyl-4,5,6-trimethylphenyl)SnFe(eta6-toluene) (Sn-Fe-Sn, 15) is accessible in high yields by a metal-atom-mediated synthesis between iron atoms, toluene, and the stannylene [bis(2-tert-butyl-4,5,6-trimethylphenyl)Sn](3). Complex 15 has a half-sandwich structure with short Fe -Sn bonds (2.432(1) A) and a trigonal-planar coordination at both the Fe and Sn atoms. The distance between the two Sn centers is 3.56 A. Complex 15 is stable under ambient conditions and displays a pi-arene lability, so far rarely observed for (arene)iron complexes; this leads to an irreversible substitution of the arene and formation of fivefold-coordinated zeroval…
Méthodes géometriques en mécanique spatiale et aspects numériques
2005
We present in this thesis two research projectson the optimal control of the space vehicles.In the first, we have dealt with the orbit transferproblem. We study the minimum time control of a satellite that we want to reach a geostationary orbit. Our contribution is of two kinds. Geometric, first, since we study the controllability of the system together with the geometry of the transfer (structure of the command) by means of geometric control without state constraint tools (minimum principle). Then we present shootingalgorithm and homotopy method. These approaches allow the numerical resolution of problems with strong or low thrust satellites.The second project concerns to the calculation o…
3D Geosynchronous Transfer of a Satellite: Continuation on the Thrust
2003
The minimum-time transfer of a satellite from a low and eccentric initial orbit toward a high geostationary orbit is considered. This study is preliminary to the analysis of similar transfer cases with more complicated performance indexes (maximization of payload, for instance). The orbital inclination of the spacecraft is taken into account (3D model), and the thrust available is assumed to be very small (e.g. 0.3 Newton for an initial mass of 1500 kg). For this reason, many revolutions are required to achieve the transfer and the problem becomes very oscillatory. In order to solve it numerically, an optimal control model is investigated and a homotopic procedure is introduced, namely cont…
Averaging and optimal control of elliptic Keplerian orbits with low propulsion
2006
This article deals with the optimal transfer of a satellite between Keplerian orbits using low propulsion. It is based on preliminary results of Geffroy [Generalisation des techniques de moyennation en controle optimal, application aux problemes de rendez-vous orbitaux a poussee faible, Ph.D. Thesis, Institut National Polytechnique de Toulouse, France, Octobre 1997] where the optimal trajectories are approximated using averaging techniques. The objective is to introduce the appropriate geometric framework and to complete the analysis of the averaged optimal trajectories for energy minimization, showing in particular the connection with Riemannian problems having integrable geodesics.