| 1. |
- Bast, R., et al.
(författare)
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Role of noncollinear magnetization for the first-order electric-dipole hyperpolarizability at the four-component Kohn-Sham density functional theory level
- 2009
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 130:2
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Tidskriftsartikel (refereegranskat)abstract
- The quadratic response function has been derived and implemented at the adiabatic four-component Kohn-Sham density functional theory level with inclusion of noncollinear spin magnetization and gradient corrections in the exchange-correlation functional-a work that is an extension of our previous report where magnetization dependencies in the exchange-correlation functional were ignored [J. Henriksson, T. Saue, and P. Norman, J. Chem. Phys. 128, 024105 (2008)]. The electric-field induced second-harmonic generation experiments on CF3 Cl and CF3 Br are addressed by a determination of Β- (-2ω;ω,ω) for a wavelength of 694.3 nm, and the same property is also determined for CF3 I. The relativistic effects on the static hyperpolarizability for the series of molecules amount to 1%, 5%, and 9%, respectively. At the experimental wavelength, the contributions to Β due to the magnetization dependence in the exchange-correlation functional are negligible for CF3 Cl and CF3 Br and small for CF 3 I. The noticeable effect of magnetization in the latter case is attributed to a near two-photon resonance with the excited state 1 E3 (nonrelativistic notation). It is emphasized, however, that the effect of magnetization on Β for CF3 I is negligible both in comparison to the total relativistic correction as well as to the effects of electron correlation. It is concluded that, in calculations of hyperpolarizabilities under nonresonant conditions, the magnetization dependence in the exchange-correlation functional may be ignored. © 2009 American Institute of Physics.
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| 2. |
- Linares, Mathieu, et al.
(författare)
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Effects of π -stacking interactions on the near carbon K -edge x-ray absorption fine structure : A theoretical study of the ethylene pentamer and the phthalocyanine dimer
- 2009
-
Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 130:10
-
Tidskriftsartikel (refereegranskat)abstract
- X-ray absorption spectra have been determined for ethylene and free base phthalocyanine at the carbon K -edge with use of the complex polarization propagator method combined with Kohn-Sham density functional theory and the Coulomb attenuated method B3LYP exchange-correlation functional. Apart from isolated molecules, the study includes π -stacked systems of the phthalocyanine dimer and the ethylene dimer, trimer, tetramer, and pentamer. For ethylene, π -stacking involves a reduction in transition energy of the valence π* -band by some 70 meV and large spectral changes (regarding also shape and intensity) of the Rydberg bands. For phthalocyanine, there are large spectral changes in the entire valence π * -part of the spectrum. © 2009 American Institute of Physics.
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| 3. |
- Johansson, N., et al.
(författare)
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Electronic structure of tris(8-hydroxyquinoline) aluminum thin films in the pristine and reduced states
- 1999
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 111:5, s. 2157-2163
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of tris(8-hydroxyquinoline) aluminum (Alq(3)) has been studied in the pristine molecular solid state as well as upon interaction (doping) with potassium and lithium. We discuss the results of a joint theoretical and experimental investigation, based on a combination of x-ray and ultraviolet photoelectron spectroscopies with quantum-chemical calculations at the density functional theory level. Upon doping, each electron transferred from an alkali metal atom is stored on one of the three ligands of the Alq(3) molecule, resulting in a new spectral feature (peak) in the valence band that evolves uniformly when going from a doping level of one to three metal atoms per Alq(3) molecule. (C) 1999 American Institute of Physics. [S0021-9606(99)50628-4].
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| 4. |
- Crispin, Xavier, et al.
(författare)
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Chemisorption of acrylonitrile on the Cu(100) surface: A local density functional study
- 1999
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 111:7, s. 3237-3251
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Tidskriftsartikel (refereegranskat)abstract
- The possibility of chemically grafting polyacrylonitrile onto transition metal electrodes via electropolymerization leads to promising applications in the fields of corrosion protection or metal surface functionalization. The initial step of the electrografting mechanism is the adsorption of the acrylonitrile monomer on the metal surface from solution. Here, we investigate theoretically this adsorption process on the copper (100) surface; Density Functional Theory is used in the Local Spin Density approximation to describe the electronic and structural properties of acrylonitrile adsorbed on copper clusters. The chemisorption of acrylonitrile on the copper surface is confirmed experimentally via X-Ray Photoelectron Spectroscopy. The thermodynamic characteristics of the adsorption process are also studied via statistical mechanics. Finally, determining the influence of the copper cluster size on the adsorption of acrylonitrile allows to extrapolate the properties of the acrylonitrile/Cu(100) surface from those of acrylonitrile/copper clusters. (C) 1999 American Institute of Physics. [S0021-9606(99)70231-X].
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| 5. |
- Lou, L, et al.
(författare)
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Electronic structure and kinetics of K on graphite
- 2000
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 112:10, s. 4788-4796
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Tidskriftsartikel (refereegranskat)abstract
- The K/graphite adsorption system is studied in a cluster model using ab initio density-functional methods. From the investigation of the potential energy surface a lower bound for the potassium atom binding energy 1.5 eV is obtained, and a surface diffusion barrier of 0.2 eV. To simulate experimentally reported thermal desorption spectra, a two-phase kinetic model is investigated and a desorption energy of 1 eV is found. The thermally activated surface diffusion of K atoms leads to intercalation at defects or steps, which is followed by desorption when further heating the sample. A normal mode analysis yields a K-graphite in-phase and out-of-phase vibrational mode with an energy split of 8 meV, which indicates a relatively strong dynamical coupling between the adsorbed K atom and the graphite substrate. The calculated electron density distribution is verified by an accurate reproduction of the measured dipole moment. From a projected density of state analysis we find a K 4s and an antibonding K 4p resonance located slightly above and 2.6 eV above the Fermi level, respectively. The location of the K 4s resonance, with a lower occupied tail, is consistent with an incomplete charge transfer, and the location of the K 4p resonance is consistent with a proposed hot-electron model to explain recent photodesorption data. The new assignment of the K-induced states near the Fermi level resolves previous apparent discrepancies of the charge state of the dispersed K atom. (C) 2000 American Institute of Physics. [S0021-9606(00)70310-2].
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| 6. |
- Abel, Martin, et al.
(författare)
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Collision-induced absorption at wavelengths near 5 micrometers by dense hydrogen gas
- 2009
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Ingår i: J. Chem. Phys.. - : AIP Publishing. ; 131
-
Tidskriftsartikel (refereegranskat)abstract
- Based on a recent ab initio interaction-induced dipole surface of collisionally interacting molecular hydrogen pairs H2–H2, we compute the binary absorption coefficients at wavelengths near 5 micrometers at temperatures of 77.5 and 297 K for comparison with existing laboratory measurements. We observe satisfactory agreement of the measurements with our calculations, thereby concluding an earlier study [Gustafsson et al., J. Chem. Phys. 119, 12264 (2003)], which was based on an ab initio interaction-induced dipole surface that was inadequate for the 5 micrometer band.
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| 7. |
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| 8. |
- Afzelius, Mikael, et al.
(författare)
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Semiclassical calculations of collision line broadening in Raman spectra of N-2 and CO mixtures
- 2004
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 120:18, s. 8616-8623
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed theoretical study of pressure-broadened Raman line shapes in binary mixtures of nitrogen and carbon monoxide. The semiclassical Robert-Bonamy theory was used to calculate self-broadened Q-branch linewidths of N-2 and CO, and Lennard-Jones (LJ) potential energy surface parameters were fixed by comparing our results with extensive experimental linewidth data. For the case of N-2, the ab initio PES8 potential energy surface was investigated, however, the anisotropic repulsive part had to be reduced to ensure a good agreement with experimental linewidths. The agreement between calculations and experiments was remarkably good, both for self-broadened N-2 and CO Q-branch linewidths. Yet, our calculations were not able to predict the experimentally observed difference between Q- and S-branch linewidths of self-broadened N-2. The central results of this work are the Q-branch linewidths of N-2-CO and CO-N-2, which have been calculated through an extrapolation of the parameters of the potential energy surfaces used for self-broadened linewidths by common combination rules. (C) 2004 American Institute of Physics.
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| 9. |
- Agosta, Lorenzo, et al.
(författare)
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Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics
- 2017
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 147:2
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio molecular dynamics simulations are reported forwater-embedded TiO2 surfaces to determine the diffusive and reactive behavior at full hydration. A three-domain model is developed for six surfaces [rutile (110), (100), and (001), and anatase (101), (100), and (001)] which describes waters as hard (irreversibly bound to the surface), soft (with reduced mobility but orientation freedom near the surface), or bulk. The model explains previous experimental data and provides a detailed picture of water diffusion near TiO2 surfaces. Water reactivity is analyzed with a graph-theoretic approach that reveals a number of reaction pathways on TiO2 which occur at full hydration, in addition to direct water splitting. Hydronium (H3O+) is identified to be a key intermediate state, which facilitates water dissociation by proton hopping between intact and dissociated waters near the surfaces. These discoveries significantly improve the understanding of nanoscale water dynamics and reactivity at TiO2 interfaces under ambient conditions.
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| 10. |
- Agosta, Lorenzo, et al.
(författare)
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Self-assembly of a triply periodic continuous mesophase with Fddd symmetry in simple one-component liquids
- 2020
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:19
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Tidskriftsartikel (refereegranskat)abstract
- Triply periodic continuous morphologies (networks) arising as a result of the microphase separation in block copolymer melts have so far never been observed self-assembled in systems of particles with spherically symmetric interaction. We report a molecular dynamics simulation where two simple one-component liquids form upon cooling an equilibrium network with the Fddd space group symmetry. This complexity reduction in the liquid network formation in terms of the particle geometry and the number of components evidences the generic nature of this class of phase transition, suggesting opportunities for producing these structures in a variety of new systems.
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| 11. |
- Agosta, Lorenzo, et al.
(författare)
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Supercooled liquid-like dynamics in water near a fully hydrated titania surface : Decoupling of rotational and translational diffusion
- 2021
-
Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 154:9
-
Tidskriftsartikel (refereegranskat)abstract
- We report an ab initio molecular dynamics (MD) simulation investigating the effect of a fully hydrated surface of TiO2 on the water dynamics. It is found that the universal relation between the rotational and translational diffusion characteristics of bulk water is broken in the water layers near the surface with the rotational diffusion demonstrating progressive retardation relative to the translational diffusion when approaching the surface. This kind of rotation-translation decoupling has so far only been observed in the supercooled liquids approaching glass transition, and its observation in water at a normal liquid temperature is of conceptual interest. This finding is also of interest for the application-significant studies of the water interaction with fully hydrated nanoparticles. We note that this is the first observation of rotation-translation decoupling in an ab initio MD simulation of water.
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| 12. |
- Ahlstrand, Emma, et al.
(författare)
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Computer simulations of alkali-acetate solutions : Accuracy of the forcefields in difference concentrations
- 2017
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 147, s. 1-10
-
Tidskriftsartikel (refereegranskat)abstract
- When proteins are solvated in electrolyte solutions that contain alkali ions, the ions interact mostlywith carboxylates on the protein surface. Correctly accounting for alkali-carboxylate interactionsis thus important for realistic simulations of proteins. Acetates are the simplest carboxylates thatare amphipathic, and experimental data for alkali acetate solutions are available and can be comparedwith observables obtained from simulations. We carried out molecular dynamics simulations of alkali acetate solutions using polarizable and non-polarizable forcefields and examined the ionacetateinteractions. In particular, activity coefficients and association constants were studied in a range of concentrations (0.03, 0.1, and 1M). In addition, quantum-mechanics (QM) based energy decomposition analysis was performed in order to estimate the contribution of polarization, electrostatics, dispersion, and QM (non-classical) effects on the cation-acetate and cation-water interactions. Simulations of Li-acetate solutions in general overestimated the binding of Li+ and acetates. In lower concentrations, the activity coefficients of alkali-acetate solutions were too high, which is suggested to be due to the simulation protocol and not the forcefields. Energy decomposition analysis suggested that improvement of the forcefield parameters to enable accurate simulations of Li-acetate solution scan be achieved but may require the use of a polarizable forcefield. Importantly, simulations with some ion parameters could not reproduce the correct ion-oxygen distances, which calls for caution in thechoice of ion parameters when protein simulations are performed in electrolyte solutions.
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| 13. |
- Ahmadi, Sareh, et al.
(författare)
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Molecular layers of ZnPc and FePc on Au(111) surface : Charge transfer and chemical interaction
- 2012
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 137:8, s. 084705-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied zinc phthalocyanine (ZnPc) and iron phthalocyanine (FePc) thick films and monolayers on Au(111) using photoelectron spectroscopy and x-ray absorption spectroscopy. Both molecules are adsorbed flat on the surface at monolayer. ZnPc keeps this orientation in all investigated coverages, whereas FePc molecules stand up in the thick film. The stronger inter-molecular interaction of FePc molecules leads to change of orientation, as well as higher conductivity in FePc layer in comparison with ZnPc, which is reflected in thickness-dependent differences in core-level shifts. Work function changes indicate that both molecules donate charge to Au; through the pi-system. However, the Fe3d derived lowest unoccupied molecular orbital receives charge from the substrate when forming an interface state at the Fermi level. Thus, the central atom plays an important role in mediating the charge, but the charge transfer as a whole is a balance between the two different charge transfer channels; pi-system and the central atom.
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| 14. |
- Ahmadi, Sareh, et al.
(författare)
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Site-dependent charge transfer at the Pt(111)-ZnPc interface and the effect of iodine
- 2014
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 140:17, s. 174702-
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of ZnPc, from sub-monolayers to thick films, on bare and iodated Pt(111) is studied by means of X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and scanning tunneling microscopy. Our results suggest that at low coverage ZnPc lies almost parallel to the Pt(111) substrate, in a non-planar configuration induced by Zn-Pt attraction, leading to an inhomogeneous charge distribution within the molecule and an inhomogeneous charge transfer to the molecule. ZnPc does not form a complete monolayer on the Pt surface, due to a surface-mediated intermolecular repulsion. At higher coverage ZnPc adopts a tilted geometry, due to a reduced molecule-substrate interaction. Our photoemission results illustrate that ZnPc is practically decoupled from Pt, already from the second layer. Pre-deposition of iodine on Pt hinders the Zn-Pt attraction, leading to a non-distorted first layer ZnPc in contact with Pt(111)-I(root 3x root 3) or Pt(111)-I(root 7x root 7), and a more homogeneous charge distribution and charge transfer at the interface. On increased ZnPc thickness iodine is dissolved in the organic film where it acts as an electron acceptor dopant.
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| 15. |
- Ahmadzadeh, Karan, et al.
(författare)
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Efficient implementation of isotropic cubic response functions for two-photon absorption cross sections within the self-consistent field approximation
- 2021
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 154:2
-
Tidskriftsartikel (refereegranskat)abstract
- Within the self-consistent field approximation, computationally tractable expressions for the isotropic second-order hyperpolarizability have been derived and implemented for the calculation of two-photon absorption cross sections. The novel tensor average formulation presented in this work allows for the evaluation of isotropic damped cubic response functions using only similar to 3.3% (one-photon off-resonance regions) and similar to 10% (one-photon resonance regions) of the number of auxiliary Fock matrices required when explicitly calculating all the needed individual tensor components. Numerical examples of the two-photon absorption cross section in the one-photon off-resonance and resonance regions are provided for alanine-tryptophan and 2,5-dibromo-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene. Furthermore, a benchmark set of 22 additional small- and medium-sized organic molecules is considered. In all these calculations, a quantitative assessment is made of the reduced and approximate forms of the cubic response function in the one-photon off-resonance regions and results demonstrate a relative error of less than similar to 5% when using the reduced expression as compared to the full form of the isotropic cubic response function.
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| 16. |
- Ai, Yue-Jie, et al.
(författare)
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Ultrafast deactivation processes in the 2-aminopyridine dimer and the adenine-thymine base pair : Similarities and differences
- 2010
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 133:6, s. 064302-
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Tidskriftsartikel (refereegranskat)abstract
- 2-aminopyridine dimer has frequently been used as a model system for studying photochemistry of DNA base pairs. We examine here the relevance of 2-aminopyridine dimer for a Watson-Crick adenine-thymine base pair by studying UV-light induced photodynamics along two main hydrogen bridges after the excitation to the localized (1)pi pi(*) excited-state. The respective two-dimensional potential-energy surfaces have been determined by time-dependent density functional theory with Coulomb-attenuated hybrid exchange-correlation functional (CAM-B3LYP). Different mechanistic aspects of the deactivation pathway have been analyzed and compared in detail for both systems, while the related reaction rates have also be obtained from Monte Carlo kinetic simulations. The limitations of the 2-aminopyridine dimer as a model system for the adenine-thymine base pair are discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3464485]
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| 17. |
- Aidas, Kestutis, et al.
(författare)
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On the performance of quantum chemical methods to predict solvatochromic effects: The case of acrolein in aqueous solution.
- 2008
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 128:19, s. 1-194503
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Tidskriftsartikel (refereegranskat)abstract
- The performance of the Hartree-Fock method and the three density functionals B3LYP, PBE0, and CAM-B3LYP is compared to results based on the coupled cluster singles and doubles model in predictions of the solvatochromic effects on the vertical n-->pi(*) and pi-->pi(*) electronic excitation energies of acrolein. All electronic structure methods employed the same solvent model, which is based on the combined quantum mechanics/molecular mechanics approach together with a dynamical averaging scheme. In addition to the predicted solvatochromic effects, we have also performed spectroscopic UV measurements of acrolein in vapor phase and aqueous solution. The gas-to-aqueous solution shift of the n-->pi(*) excitation energy is well reproduced by using all density functional methods considered. However, the B3LYP and PBE0 functionals completely fail to describe the pi-->pi(*) electronic transition in solution, whereas the recent CAM-B3LYP functional performs well also in this case. The pi-->pi(*) excitation energy of acrolein in water solution is found to be very dependent on intermolecular induction and nonelectrostatic interactions. The computed excitation energies of acrolein in vacuum and solution compare well to experimental data.
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| 18. |
- Ajitha, D, et al.
(författare)
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Spin-orbit ab initio study of alkyl halide dissociation via electronic curve crossing
- 2004
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 121:12, s. 5761-5766
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Tidskriftsartikel (refereegranskat)abstract
- An ab initio study of the role of electronic curve crossing in the photodissociation dynamics of the alkyl halides is presented. Recent experimental studies show that curve crossing plays a deterministic role in deciding the channel of dissociation. Coupled repulsive potential energy curves of the low-lying n-sigma* states are studied including spin-orbit and relativistic effects. Basis set including effect of core correlation is used. Ab initio vertical excitation spectra of CH3I and CF3I are in agreement with the experimental observation. The curve crossing region is around 2.371 Angstrom for CH3I and CF3I. The potential curves of the repulsive excited states have larger slope for CF3I, suggesting a higher velocity and decreased intersystem crossing probability on fluorination. We also report the potential curves and the region of curve crossing for CH3Br and CH3Cl.
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| 19. |
- Al-Abdalla, A, et al.
(författare)
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Ab initio model potential embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6
- 1998
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Ingår i: Journal of Chemical Physics. - : Springer Science and Business Media LLC. - 0021-9606 .- 1089-7690. ; 108, s. 2005-2014
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present the results of an ab initio model potential (AIMP) embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6; complete active space SCF (CASSCF) and averaged coupled-pair functional (ACPF) calculations are performed on CrCl63- and CrBr63- clusters embedded in ab initio model potential representations of the surrounding lattices Cs2NaYCl6 and Cs2NaYBr6. The experimental structural data are revisited and some new results are found which differ significantly from those available in the literature. The calculated local structure parameters and electronic transition energies which can be compared to experiments are found to be very good; new structural and spectroscopic results are produced which have been neither measured nor calculated, which are complementary to the available ones, and whose quality is expected to be high as well. In particular, the question of the competition of the excited-state absorptions with the potential vibronic laser emission has been adressed: A considerable overlap between the broad E-2(g) –> (2)A(1g) excited-state absorption and (4)A(2g) <– T-4(2g) emission bands is predicted in both materials, which must result in a reduction in the emission efficiency. Finally, it is shown that the quantum mechanical embedding effects due to the fact that the external Cs+, Na+, Y3+, Cl-, and Br- ions are not point charges, are non-negligible; lacking of these effects must be one of the reasons which make previous Density Functional Theory calculations show significantly larger discrepancies with the available experiments.
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| 20. |
- Al-Khalili, A, et al.
(författare)
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Dissociative recombination cross section and branching ratios of protonated dimethyl disulfide and N-methylacetamide
- 2004
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 121:12, s. 5700-5708
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Tidskriftsartikel (refereegranskat)abstract
- Dimethyl disulfide (DMDS) and N-methylacetamide are two first choice model systems that represent the disulfide bridge bonding and the peptide bonding in proteins. These molecules are therefore suitable for investigation of the mechanisms involved when proteins fragment under electron capture dissociation (ECD). The dissociative recombination cross sections for both protonated DMDS and protonated N-methylacetamide were determined at electron energies ranging from 0.001 to 0.3 eV. Also, the branching ratios at 0 eV center-of-mass collision energy were determined. The present results give support for the indirect mechanism of ECD, where free hydrogen atoms produced in the initial fragmentation step induce further decomposition. We suggest that both indirect and direct dissociations play a role in ECD.
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| 21. |
- Alagia, Michele, et al.
(författare)
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Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene : Experiment and theory
- 2005
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Ingår i: Journal of chemical physics Online. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 122:12, s. 124305-
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Tidskriftsartikel (refereegranskat)abstract
- The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288 eV photon energy, due to absorption to * virtual orbitals, and broader structures at higher energy, involving * virtual orbitals. The sharp absorption structures to the * empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of * symmetry, from the six chemically shifted C 1s core orbitals. ©2005 American Institute of Physics
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| 22. |
- Aleksis, Rihards, et al.
(författare)
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Low-power synchronous helical pulse sequences for large anisotropic interactions in MAS NMR : Double-quantum excitation of N-14
- 2020
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 153:24
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Tidskriftsartikel (refereegranskat)abstract
- We develop a theoretical framework for a class of pulse sequences in the nuclear magnetic resonance (NMR) of rotating solids, which are applicable to nuclear spins with anisotropic interactions substantially larger than the spinning frequency, under conditions where the radiofrequency amplitude is smaller than or comparable to the spinning frequency. The treatment is based on average Hamiltonian theory and allows us to derive pulse sequences with well-defined relationships between the pulse parameters and spinning frequency for exciting specific coherences without the need for any detailed calculations. This framework is applied to the excitation of double-quantum spectra of N-14 and is used both to evaluate the existing low-power pulse schemes and to predict the new ones, which we present here. It is shown that these sequences can be designed to be gamma -encoded and therefore allow the acquisition of sideband-free spectra. It is also shown how these new double-quantum excitation sequences are incorporated into heteronuclear correlation NMR, such as H-1-N-14 dipolar double-quantum heteronuclear multiple-quantum correlation spectroscopy. The new experiments are evaluated both with numerical simulations and experiments on glycine and N-acetylvaline, which represent cases with moderate and large quadrupolar interactions, respectively. The analyzed pulse sequences perform well for the case of a moderate quadrupolar interaction, however poorly with a large quadrupolar interaction, for which future work on pulse sequence development is necessary.
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| 23. |
- Aleksis, Rihards, et al.
(författare)
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Separation of quadrupolar and paramagnetic shift interactions in high-resolution nuclear magnetic resonance of spinning powders
- 2021
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 155:9
-
Tidskriftsartikel (refereegranskat)abstract
- Separation and correlation of the shift anisotropy and the first-order quadrupolar interaction of spin I = 1 nuclei under magic-angle spinning (MAS) are achieved by the phase-adjusted spinning sideband (PASS) nuclear magnetic resonance (NMR) experiment. Compared to methods for static samples, this approach has the benefit of higher sensitivity and resolution. Moreover, the PASS experiment has the advantage over previous MAS sequences in the ability to completely separate the shift anisotropy and first-order quadrupolar interactions. However, the main drawback of the pulse sequence is the lower excitation bandwidth. The sequence is comprehensively evaluated using theoretical calculations and numerical simulations and applied experimentally to the H-2 NMR of a range of paramagnetic systems: deuterated nickel(II) acetate tetrahydrate, deuterated copper(II) chloride dihydrate, and two forms of deuterated oxyhydride ion conductor BaTiO3-xHy. Our results show that despite the issue with broadband excitation, the extracted shift and quadrupolar interaction tensors and the Euler angles relating the two tensors match well with the NMR parameters obtained with static NMR methods. Therefore, the new application of the PASS experiment is an excellent addition to the arsenal of NMR experiments for H-2 and potentially N-14 in paramagnetic solids.
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| 24. |
- Alfredsson, Ylvi, et al.
(författare)
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Electronic structure of a vapor-deposited metal-free phthalocyanine thin film
- 2005
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 122:21
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of a vapor-sublimated thin film of metal-free phthalocyanine (H2Pc) is studied experimentally and theoretically. An atom-specific picture of the occupied and unoccupied electronic states is obtained using x-ray-absorption spectroscopy (XAS), core- and valence-level x-ray photoelectron spectroscopy (XPS), and density-functional theory (DFT) calculations. The DFT calculations allow for an identification of the contributions from individual nitrogen atoms to the experimental N1s XAS and valence XPS spectra. This comprehensive study of metal-free phthalocyanine is relevant for the application of such molecules in molecular electronics and provides a solid foundation for identifying modifications in the electronic structure induced by various substituent groups.
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| 25. |
- Allum, Felix, et al.
(författare)
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Coulomb explosion imaging of CH3I and CH2CII photodissociation dynamics
- 2018
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 149:20
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Tidskriftsartikel (refereegranskat)abstract
- The photodissociation dynamics of CH3I and CH2CII at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815nmprobe pulse. Fragment ion momenta over a widem/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH2ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion.
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