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The valence photoabsorption spectra of the series of the monoanionic closed shell icosahedral clusters MAu12- with M = V, Nb and Ta have been calculated with the time-dependent density functional theory (TDDFT), employing the zero-order regular approximation (ZORA) at both scalar relativistic and spin orbit coupling levels. The calculated photoabsorption spectra show interesting variations according to the nature of the encapsulated metal atom. Spin-orbit coupling plays an important role in these systems. The comparison with the neutral isoelectronic clusters WAu12 and MoAu12 suggests a curious relationship along the diagonal of the periodic table. (C) 2008 Elsevier B.V. All rights reserved.

A new experimental approach for determining the photoionization matrix elements with the use of multi-coincidence velocity-map imaging spectrometry is presented for the case of C1s photoionization of CO2 molecule over the energy region of a shape resonance. Our experimental results provide the complete information on the shape resonance mechanism and clearly demonstrate the inadequacy of presently available theoretical calculations for this system.

With the full use of the experimental data obtained by multi- coincidence velocity- map imaging spectrometry, we have determined the dipole amplitudes and their relative phases that describe the C 1s photoionization dynamics in CS2. It has been found that the sigma shape resonance at about 15 eV above the C 1s ionization threshold appears only in the dipole amplitude d(f sigma). We have also carried out time- dependent density functional theory calculations so as to better understand the photoemission dynamics and shape resonances in the C 1s photoionization of CS2 molecules with heavy atoms ( sulfur) having 3p valence orbitals, which distinguish the present work from well- studied works such as on CO and CO2 molecules.

The valence shell electronic states of pyrimidine and pyrazine have been studied experimentally and theoretically. The absolute photoabsorption cross sections have been measured between 4 and 40 eV, using synchrotron radiation, and are dominated by prominent bands associated with intravalence transitions. In contrast, the structure due to Rydberg excitations is weak, but series have been observed converging onto the (X) over tilde (2)B(2) or (D) over tilde (2)B(1) limits in pyrimidine and the (X) over tilde (2)A(g) or (D) over tilde (2)B(3g) limits in pyrazine. A comparison between the photoabsorption spectrum of pyrazine-h(4) and that for pyrazine-d(4), together with calculated transition energies, has helped clarify the assignments of the 6a(g)-> npb(1u) and npb(2u) Rydberg series. The vibrational progressions associated with these states have been assigned through analogy with those in the corresponding photoelectron band. The time-dependent version of density functional theory has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet-singlet valence transitions, and also to obtain the excitation energies for electric-dipole-forbidden and/or spin-forbidden transitions. These theoretical results have allowed many of the experimentally observed bands to be assigned and provide a generally satisfactory description of the valence shell photoabsorption spectrum. Several of the prominent bands appearing above the ionization threshold can be correlated with predicted intense intravalence transitions.

We report the X-ray crystal structure of Au36-xAgx(SPh-tBu)24 alloy nanomolecules, optical properties, and the substitutional disorder refinement protocol to obtain a reliable structural model. Single crystal X-ray crystallography (SC-XRD) revealed a composition of Au33.17Ag2.83(SPh-tBu)24 with 2 Ag doped on the 28-atom face centered cubic core surface and 0.83 Ag distributed over metal atoms on dimeric staple motifs. Electrospray ionization mass spectrometry revealed a composition of Au32.5Ag3.5(SPh-tBu)24 complementing the SC-XRD data. Optical properties were investigated by steady-state and transient absorption spectroscopies and computational studies, showing faster excited-state decay for Ag-doped clusters due to enhanced electronic coupling. A previously published SC-XRD based Au36-xAgx(SPh-tBu)24 structure used positional disorder refinement and concluded that the structure is "solely motif-doped". But the structure has unusually large and small thermal ellipsoids indicating potential problems with the atom assignment. Here, we have modeled our SC-XRD data using both positional disorder and substitutional disorder. Subtitutional disorder modeling gave better R1 and other refinement indicators, and similarly sized thermal ellipsoids. The resulting substitutional disorder model structure has Ag atoms not as "solely motif-doped" but is found both in the staple motifs and in the core. The substitutional disorder refinement for alloy nanomolecules must be performed at each metal site with independent free variables to determine the partial occupancy of hetero atoms. The positional disorder refinement should be performed for atoms or groups disordered over different positions typically found in disordered tBu group ligands.

A multicenter (LCAO) B-spline basis is described in detail, and its capabilities concerning affording convergent solutions for electronic continuum states and wavepacket propagation are presented. It forms the core of the Tiresia code, which implements static-DFT and TDDFT hamiltonians, as well as single channel Dyson-DFT and Dyson-TDDFT descriptions to include correlation in the bound states. Together they afford accurate and computationally efficient descriptions of photoionization properties of complex systems, both in the single photon and strong field environments. A number of examples are provided.

A series of core@shell and layered ordered phases of AgPt bimetallic nanoparticles has been studied with Ag:Pt = 3:1 and 6:1 atomic compositions and sizes from 116 to 201 atoms. The elementary chemical order has been established by using a recent method (TOP), which assigns energy according to different topological degrees of freedom. The TOP lowest-energy structures, confirmed by density functional calculations, are then studied by time dependent density functional theory in order to calculate optical properties. The present study shows that for AgPt nanoparticles with core@shell structure the optical properties are sensitive to both the Pt concentration and system size. Spectral trends related to chemical order have also been identified.

The near-edge x-ray-absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) spectra of benzo[b]thiophene (BBT) and dibenzothiophene (DBT) in the gas phase have been measured at the carbon Kedge and sulfur L-II,L-III-edge regions. The assignment of the spectral features has been provided by theoretical calculations based on density functional theory (DFT) and its time-dependent generalization (TDDFT) in the linear response regime. Observed trends in computed C 1s and S 2p ionization potentials (IPs) have been rationalized in terms of both the inductive effects due to the presence of S and the increased pi-electrons delocalization arising from the benzoannulation process. The analysis of the NEXAFS carbon K-edge and sulfur L-II,L-III-edge regions regions provided information on both low-lying delocalized virtual pi orbitals, and higher-lying localized sigma*(C-S) states. The evolution of the NEXAFS carbon K-edge spectral features along the series thiophene (T) and derivatives, BBT and DBT, is informative of a stabilizing effect due to increased aromaticity. This effect is however more pronounced in going from T to BBT compared to the introduction of a second annulated phenyl ring in DBT. The nature of the most intense sulfur L-II,L-III-edge NEXAFS spectral features is instead conserved along the series reflecting thus the localized nature of the virtual states involved in the S 2p core-excitation process.

The Near-Edge X-ray-Absorption Fine-Structure (NEXAFS) and X-ray Photoemission Spectroscopy (XPS) of short-chain oligothiophenes (thiophene, 2,2'-bithiophene, and 2,2':5',2 ''-terthiophene) in the gas phase have been measured in the sulfur L-2,L-3-edge region. The assignment of the spectral features is based on the relativistic two-component zeroth-order regular approximation time dependent density functional theory approach. The calculations allow us to estimate both the contribution of the spin-orbit splitting and of the molecular-field splitting to the sulfur binding energies and give results in good agreement with the experimental measurements. The deconvolution of the calculated S2p NEXAFS spectra into the two manifolds of excited states converging to the L-III and L-II edges facilitates the attribution of the spectral structures. The main S2p NEXAFS features are preserved along the series both as concerns the energy positions and the nature of the transitions. This behaviour suggests that the electronic and geometrical environment of the sulfur atom in the three oligomers is relatively unaffected by the increasing chain length. This trend is also observed in the XPS spectra. The relatively simple structure of S2p NEXAFS spectra along the series reflects the localized nature of the virtual states involved in the core excitation process. Published by AIP Publishing.

The geometrical and electronic structures of a series of small CdSe quantum dots protected by various ligands have been studied by density functional theory. UV-vis spectra have been calculated by time-dependent density functional theory (TDDFT). The goal of this investigation is the rationalization of the basic properties of these systems, in particular, the nature of the exciton peaks. This study has been focused on the (CdSe)(x), x = 13, 19, 33, and 66, "magic-size" clusters that are characterized by high stability and large optical gaps. The geometries of the cluster are relaxed both in vacuum and in the presence of the surfactant ligands. To describe the interaction between the bare clusters and the surfactants, model types of ligands are introduced: fatty acids are modeled using formic and acetic acid and amines are modeled using ammonia and methyl amine. Present calculations demonstrate that the ligands play a crucial role in stabilizing the structure in a bulklike geometry and strongly affect the optical gap of the clusters, due to an optimal coordination of the surface atoms. For these "magic-size" clusters, the UV-vis spectrum is calculated at the TDDFT level. The calculated spectra are in good agreement with the experimental ones for clusters with the same dimension capped with the same type of Uganda. This suggests that our structures are realistic models of the actual quantum dots.

The coordination chemistry of a series of 3-alkyl-substituted-1,10-phenanthrolines (3-R-phen) to palladium as well as the catalytic behavior of the corresponding bischelated derivatives, [Pd(3-R-phen)2][PF6]2, in the CO/vinyl arenes copolymerization reaction has been investigated in detail. The alkyl substituents differ in length and steric hindrance. The crystal structure characterization reveals that the two molecules of 3-R-phen are bound to palladium in a syn arrangement with the alkyl groups on the same side of the square planar geometry. In solution a dynamic process involving the equilibrium between syn and anti isomers is evidenced by NMR spectroscopic analysis. This is in agreement with the results of DFT calculations, which indicate similar stabilities for the two isomers. The severe distortions from the ideal square planar coordination geometry observed in the solid state are rationalized, through the DFT analysis, in terms of the HOMO orbitals responsible for the Pd−N bonds. The [Pd(3-R-phen)2][PF6]2 complexes efficiently promote the CO/styrene and CO/p-Me-styrene copolymerizations to the corresponding syndiotactic polyketones. Yields and molecular weights show an increasing trend on increasing the steric demand of the R substituent, and the values recorded are the best ones ever reported for copolymerization reactions of this kind in the absence of the oxidant. From the TON numbers this result seems related to an increase of the olefin insertion rate, which proceeds faster when 3-R-phen are used as ligands. A change of the physical nature of the active species, from homogeneous to heterogeneous, occurs during the polymerization process, and the time at which this variation takes place depends on the nature of the olefin. The positive effect of the alkyl substitution is less evident in the CO/ethylene copolymerization.

The potentiality of the time dependent density functional theory (TDDFT) for the description of core excitation spectra (XAS) in transition metal oxides is analyzed, considering the rutile form of TiO(2) as a test case. Cluster models are adopted to mimic the bulk, embedded within an array of point charges to simulate the Madelung potential. All of the edges, titanium and oxygen K and titanium L edges, are considered, and the TDDFT results are compared with the experimental data in order to assess the performance of the theoretical approach in dealing with this complex class of compounds. Satisfactory results have been obtained for the Ti and O K edges, while in the case of the Ti L edge some discrepancies with the experiment are still present. The configuration mixing explicitly included in the TDDFT model strongly influences the distribution of the 2p metal oscillator strength. The origin of the spectral features is investigated with the help of the partial density of the virtual states (PDOS) calculated for each core hole considered, which can be qualitatively compared with the theoretical spectra calculated in the Kohn-Sham one-electron approach.

The valence electrons of fullerenes may be regarded as spherical distributions with a finite width of a jellium-like potential giving rise to collective motions of this orange peel electron cloud. They cause strong enhancement of the photoionization cross section, a resonant behavior phenomenon know as plasmon excitations. The number and characteristic features of these excitations will be discussed.