JCP Nameplate
  • Volume/Page
  • Keyword
  • DOI
  • Citation
  • Advanced
   
 
 
 

You Tube Flickr Twitter UniPHY Group iResearch App Facebook

BROWSE VOLUMES

Year Range: 
Search Issue | RSS Feeds RSS
Previous Issue

14 Feb 2012

Volume 136, Issue 6 (partial)

Issue Cover Spotlight Figure

J. Chem. Phys. 136, 064501 (2012); doi:10.1063/1.3676408 (17 pages)

Florian Göltl and Jürgen Hafner
Enlarge Image | Read More
Page 1 of 2 Pages Next Page | Jump to Page
back to top
RSS Feeds
FREE

Communication: Angle-resolved thermal dissociative sticking of CH4 on Pt(111): Further indication that rotation is a spectator to the gas-surface reaction dynamics

J. K. Navin, S. B. Donald, D. G. Tinney, G. W. Cushing, and I. Harrison

J. Chem. Phys. 136, 061101 (2012); doi:10.1063/1.3685833 (4 pages)

Online Publication Date: 9 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Effusive molecular beam measurements of angle-resolved thermal dissociative sticking coefficients for CH4 impinging on a Pt(111) surface, at a temperature of 700 K, are reported and compared to theoretical predictions. The reactivity falls off steeply as the molecular angle of incidence increases away from the surface normal. Successful modeling of the thermal dissociative sticking behavior, consistent with existent CH4 supersonic molecular beam experiments involving rotationally cold molecules, required that rotation be treated as a spectator degree of freedom.
Show PACS
68.43.Mn Adsorption kinetics
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
37.20.+j Atomic and molecular beam sources and techniques
back to top
RSS Feeds
back to top Theoretical Methods and Algorithms

Accurate thermochemistry from a parameterized coupled-cluster singles and doubles model and a local pair natural orbital based implementation for applications to larger systems

Lee M. J. Huntington, Andreas Hansen, Frank Neese, and Marcel Nooijen

J. Chem. Phys. 136, 064101 (2012); doi:10.1063/1.3682325 (17 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We have recently introduced a parameterized coupled-cluster singles and doubles model (pCCSD(α, β)) that consists of a bivariate parameterization of the CCSD equations and is inspired by the coupled electron pair approximations. In our previous work, it was demonstrated that the pCCSD(−1, 1) method is an improvement over CCSD for the calculation of geometries, harmonic frequencies, and potential energy surfaces for single bond-breaking. In this paper, we find suitable pCCSD parameters for applications in reaction thermochemistry and thermochemical kinetics. The motivation is to develop an accurate and economical methodology that, when coupled with a robust local correlation framework based on localized pair natural orbitals, is suitable for large-scale thermochemical applications for sizeable molecular systems. It is demonstrated that the original pCCSD(−1, 1) method and several other pCCSD methods are a significant improvement upon the standard CCSD approach and that these methods often approach the accuracy of CCSD(T) for the calculation of reaction energies and barrier heights. We also show that a local version of the pCCSD methodology, implemented within the local pair natural orbital (LPNO) based CCSD code in ORCA, is sufficiently accurate for wide-scale chemical applications. The LPNO based methodology allows us for routine applications to intermediate sized (20–100 atoms) molecular systems and is a significantly more accurate alternative to MP2 and density functional theory for the prediction of reaction energies and barrier heights.
Show PACS
31.15.bw Coupled-cluster theory
31.50.-x Potential energy surfaces
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Fm Bond strengths, dissociation energies
82.20.Kh Potential energy surfaces for chemical reactions
82.60.-s Chemical thermodynamics

Revised self-consistent continuum solvation in electronic-structure calculations

Oliviero Andreussi, Ismaila Dabo, and Nicola Marzari

J. Chem. Phys. 136, 064102 (2012); doi:10.1063/1.3676407 (20 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The solvation model proposed by Fattebert and Gygi [J. Comput. Chem. 23, 662 (2002)10.1002/jcc.10069] and Scherlis et al. [J. Chem. Phys. 124, 074103 (2006)10.1063/1.2168456] is reformulated, overcoming some of the numerical limitations encountered and extending its range of applicability. We first recast the problem in terms of induced polarization charges that act as a direct mapping of the self-consistent continuum dielectric; this allows to define a functional form for the dielectric that is well behaved both in the high-density region of the nuclear charges and in the low-density region where the electronic wavefunctions decay into the solvent. Second, we outline an iterative procedure to solve the Poisson equation for the quantum fragment embedded in the solvent that does not require multigrid algorithms, is trivially parallel, and can be applied to any Bravais crystallographic system. Last, we capture some of the non-electrostatic or cavitation terms via a combined use of the quantum volume and quantum surface [M. Cococcioni, F. Mauri, G. Ceder, and N. Marzari, Phys. Rev. Lett. 94, 145501 (2005)10.1103/PhysRevLett.94.145501] of the solute. The resulting self-consistent continuum solvation model provides a very effective and compact fit of computational and experimental data, whereby the static dielectric constant of the solvent and one parameter allow to fit the electrostatic energy provided by the polarizable continuum model with a mean absolute error of 0.3 kcal/mol on a set of 240 neutral solutes. Two parameters allow to fit experimental solvation energies on the same set with a mean absolute error of 1.3 kcal/mol. A detailed analysis of these results, broken down along different classes of chemical compounds, shows that several classes of organic compounds display very high accuracy, with solvation energies in error of 0.3-0.4 kcal/mol, whereby larger discrepancies are mostly limited to self-dissociating species and strong hydrogen-bond-forming compounds.
Show PACS
31.15.xv Molecular dynamics and other numerical methods
31.70.Dk Environmental and solvent effects
33.15.Fm Bond strengths, dissociation energies

A neural network potential-energy surface for the water dimer based on environment-dependent atomic energies and charges

Tobias Morawietz, Vikas Sharma, and Jörg Behler

J. Chem. Phys. 136, 064103 (2012); doi:10.1063/1.3682557 (11 pages)

Online Publication Date: 9 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Understanding the unique properties of water still represents a significant challenge for theory and experiment. Computer simulations by molecular dynamics require a reliable description of the atomic interactions, and in recent decades countless water potentials have been reported in the literature. Still, most of these potentials contain significant approximations, for instance a frozen internal structure of the individual water monomers. Artificial neural networks (NNs) offer a promising way for the construction of very accurate potential-energy surfaces taking all degrees of freedom explicitly into account. These potentials are based on electronic structure calculations for representative configurations, which are then interpolated to a continuous energy surface that can be evaluated many orders of magnitude faster. We present a full-dimensional NN potential for the water dimer as a first step towards the construction of a NN potential for liquid water. This many-body potential is based on environment-dependent atomic energy contributions, and long-range electrostatic interactions are incorporated employing environment-dependent atomic charges. We show that the potential and derived properties like vibrational frequencies are in excellent agreement with the underlying reference density-functional theory calculations.
Show PACS
31.50.-x Potential energy surfaces
33.20.Tp Vibrational analysis
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.15.E- Density-functional theory
31.15.xv Molecular dynamics and other numerical methods

Coherent control and time-dependent density functional theory: Towards creation of wave packets by ultrashort laser pulses

Shampa Raghunathan and Mathias Nest

J. Chem. Phys. 136, 064104 (2012); doi:10.1063/1.3682980 (6 pages)

Online Publication Date: 10 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Explicitly time-dependent density functional theory (TDDFT) is a formally exact theory, which can treat very large systems. However, in practice it is used almost exclusively in the adiabatic approximation and with standard ground state functionals. Therefore, if combined with coherent control theory, it is not clear which control tasks can be achieved reliably, and how this depends on the functionals. In this paper, we continue earlier work in order to establish rules that answer these questions. Specifically, we look at the creation of wave packets by ultrashort laser pulses that contain several excited states. We find that (i) adiabatic TDDFT only works if the system is not driven too far from the ground state, (ii) the permanent dipole moments involved should not differ too much, and (iii) these results are independent of the functional used. Additionally, we find an artifact that produces fluence-dependent excitation energies.
Show PACS
31.15.E- Density-functional theory
37.10.-x Atom, molecule, and ion cooling methods
32.80.Rm Multiphoton ionization and excitation to highly excited states

Transient-time correlation function applied to mixed shear and elongational flows

Remco Hartkamp, Stefano Bernardi, and B. D. Todd

J. Chem. Phys. 136, 064105 (2012); doi:10.1063/1.3684753 (7 pages)

Online Publication Date: 10 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The transient-time correlation function (TTCF) method is used to calculate the nonlinear response of a homogeneous atomic fluid close to equilibrium. The TTCF response of the pressure tensor subjected to a time-independent planar mixed flow of shear and elongation is compared to directly averaged non-equilibrium molecular dynamics (NEMD) simulations. We discuss the consequence of noise in simulations with a small rate of deformation. The generalized viscosity for planar mixed flow is also calculated with TTCF. We find that for small rates of deformation, TTCF is far more efficient than direct averages of NEMD simulations. Therefore, TTCF can be applied to fluids with deformation rates which are much smaller than those commonly used in NEMD simulations. Ultimately, TTCF applied to molecular systems is amenable to direct comparison between NEMD simulations and experiments and so in principle can be used to study the rheology of polymer melts in industrial processes.
Show PACS
47.11.Mn Molecular dynamics methods
66.20.Ej Studies of viscosity and rheological properties of specific liquids
62.10.+s Mechanical properties of liquids

Critical lines for an unequal size of molecules in a binary gas-liquid mixture around the van Laar point using the combination of the Tompa model and the van der Waals equation

Mustafa Gençaslan and Mustafa Keskin

J. Chem. Phys. 136, 064106 (2012); doi:10.1063/1.3683443 (7 pages)

Online Publication Date: 10 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We combine the modified Tompa model with the van der Waals equation to study critical lines for an unequal size of molecules in a binary gas-liquid mixture around the van Laar point. The van Laar point is coined by Meijer and it is the only point at which the mathematical double point curve is stable. It is the intersection of the tricritical point and the double critical end point. We calculate the critical lines as a function of x1 and x2, the density of type I molecules and the density of type II molecules for various values of the system parameters; hence the global phase diagrams are presented and discussed in the density-density plane. We also investigate the connectivity of critical lines at the van Laar point and its vicinity and discuss these connections according to the Scott and van Konynenburg classifications. It is also found that the critical lines and phase behavior are extremely sensitive to small modifications in the system parameters.
Show PACS
64.75.Cd Phase equilibria of fluid mixtures, including gases, hydrates, etc.
64.60.Kw Multicritical points

Calculations of nonlinear response properties using the intermediate state representation and the algebraic-diagrammatic construction polarization propagator approach: Two-photon absorption spectra

S. Knippenberg, D. R. Rehn, M. Wormit, J. H. Starcke, I. L. Rusakova, A. B. Trofimov, and A. Dreuw

J. Chem. Phys. 136, 064107 (2012); doi:10.1063/1.3682324 (15 pages)

Online Publication Date: 10 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
An earlier proposed approach to molecular response functions based on the intermediate state representation (ISR) of polarization propagator and algebraic-diagrammatic construction (ADC) approximations is for the first time employed for calculations of nonlinear response properties. The two-photon absorption (TPA) spectra are considered. The hierarchy of the first- and second-order ADC/ISR computational schemes, ADC(1), ADC(2), ADC(2)-x, and ADC(3/2), is tested in applications to H2O, HF, and C2H4 (ethylene). The calculated TPA spectra are compared with the results of coupled cluster (CC) models and time-dependent density-functional theory (TDDFT) calculations, using the results of the CC3 model as benchmarks. As a more realistic example, the TPA spectrum of C8H10 (octatetraene) is calculated using the ADC(2)-x and ADC(2) methods. The results are compared with the results of TDDFT method and earlier calculations, as well as to the available experimental data. A prominent feature of octatetraene and other polyene molecules is the existence of low-lying excited states with increased double excitation character. We demonstrate that the two-photon absorption involving such states can be adequately studied using the ADC(2)-x scheme, explicitly accounting for interaction of doubly excited configurations. Observed peaks in the experimental TPA spectrum of octatetraene are assigned based on our calculations.
Show PACS
33.80.Wz Other multiphoton processes
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Gas-phase structures of neutral silicon clusters

Marko Haertelt, Jonathan T. Lyon, Pieterjan Claes, Jorg de Haeck, Peter Lievens, and André Fielicke

J. Chem. Phys. 136, 064301 (2012); doi:10.1063/1.3682323 (6 pages)

Online Publication Date: 9 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Vibrational spectra of neutral silicon clusters Sin, in the size range of n = 6–10 and for n = 15, have been measured in the gas phase by two fundamentally different IR spectroscopic methods. Silicon clusters composed of 8, 9, and 15 atoms have been studied by IR multiple photon dissociation spectroscopy of a cluster-xenon complex, while clusters containing 6, 7, 9, and 10 atoms have been studied by a tunable IR-UV two-color ionization scheme. Comparison of both methods is possible for the Si9 cluster. By using density functional theory, an identification of the experimentally observed neutral cluster structures is possible, and the effect of charge on the structure of neutrals and cations, which have been previously studied via IR multiple photon dissociation, can be investigated. Whereas the structures of small clusters are based on bipyramidal motifs, a trigonal prism as central unit is found in larger clusters. Bond weakening due to the loss of an electron leads to a major structural change between neutral and cationic Si8.
Show PACS
36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Ea Infrared spectra
33.20.Lg Ultraviolet spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation

Absorption by DNA single strands of adenine isolated in vacuo: The role of multiple chromophores

Lisbeth Munksgaard Nielsen, Sara Øvad Pedersen, Maj-Britt Suhr Kirketerp, and Steen Brøndsted Nielsen

J. Chem. Phys. 136, 064302 (2012); doi:10.1063/1.3679444 (5 pages)

Online Publication Date: 10 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The degree of electronic coupling between DNA bases is a topic being up for much debate. Here we report on the intrinsic electronic properties of isolated DNA strands in vacuo free of solvent, which is a good starting point for high-level excited states calculations. Action spectra of DNA single strands of adenine reveal sign of exciton coupling between stacked bases from blueshifted absorption bands (∼3 nm) relative to that of the dAMP mononucleotide (one adenine base). The bands are blueshifted by about 10 nm compared to those of solvated strands, which is a shift similar to that for the adenine molecule and the dAMP mononucleotide. Desolvation has little effect on the bandwidth, which implies that inhomogenous broadening of the absorption bands in aqueous solution is of minor importance compared to, e.g., conformational disorder. Finally, at high photon energies, internal conversion competes with electron detachment since dissociation of the bare photoexcited ions on the microsecond time scale is measured.
Show PACS
87.15.Pc Electronic and electrical properties
36.20.Kd Electronic structure and spectra
87.14.gk DNA
87.15.M- Spectra of biomolecules

New determination of the adiabatic ionization potential of the BaOH radical from laser photoionization-molecular beam experiments and ab initio calculations

Maximiliano Rossa, Iván Cabanillas-Vidosa, Gustavo A. Pino, and Juan C. Ferrero

J. Chem. Phys. 136, 064303 (2012); doi:10.1063/1.3682283 (8 pages)

Online Publication Date: 10 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The adiabatic ionization potential of the BaOH radical, as generated in a laser vaporization-supersonic expansion source has been determined by laser photoionization experiments to be (4.55 ± 0.03) eV. This value supports the three lowest out of seven previous experimental estimates, the former ranging from 4.35 to 4.62 eV. The present result is compared to ab initio calculations, as performed using both quantum chemistry at different levels of theory and density functional theory, and trying several effective core potentials and their accompanying basis sets for Ba. The most satisfactory agreement is obtained for either the adiabatic or vertical ionization potentials that derive from post-Hartree-Fock [MP2 and CCSD(T)] treatments of electron correlation, along with consideration of relativistic effects and extensive basis sets for Ba, in both BaOH and BaOH+. Such conclusions extend to the results of related calculations on the Ba−OH dissociation energies of BaOH and BaOH+, which were performed to help in calibrating the present computational study. Bonding in BaOH/BaOH+, as well as possible sources of discrepancy with previous experimental determinations of the BaOH adiabatic ionization potential are discussed.
Show PACS
33.80.Eh Autoionization, photoionization, and photodetachment
31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory
31.15.xr Self-consistent-field methods
33.15.Fm Bond strengths, dissociation energies
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
FREE

Structure and properties of metal-exchanged zeolites studied using gradient-corrected and hybrid functionals. I. Structure and energetics

Florian Göltl and Jürgen Hafner

J. Chem. Phys. 136, 064501 (2012); doi:10.1063/1.3676408 (17 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The structural and energetic properties of purely siliceous, proton-, and Cu- and Co-exchanged chabazite have been studied using periodic density-functional (DFT) calculations with both conventional gradient-corrected exchange-correlation functionals and hybrid functionals mixing exact (i.e., Hartree-Fock) and DFT exchange. Spin-polarized and fixed-moment calculations have been performed to determine the equilibrium and excited spin-configurations of the metal-exchanged chabazites. For the purely siliceous chabazite, hybrid functionals predict a slightly more accurate cell volume and lattice geometry. For isolated Al/Si substitution sites, gradient-corrected functionals predict that the lattice distortion induced by the substitution preserves the local tetrahedral symmetry, whereas hybrid functionals lead to a distorted Al coordination with two short and two long Al-O bonds. Hybrid functionals yield a stronger cation-framework binding that conventional functionals in metal-exchanged zeolites, they favor shorter cation-oxygen bonds and eventually also a higher coordination of the cation. Both types of functionals predict the same spin in the ground-state. The structural optimization of the excited spin-states shows that the formation of a high-spin configuration leads to a strong lattice relaxation and a weaker cation-framework bonding. For both Cu- and Co-exchanged chabazite, the prediction of a preferred location of the cation in a six-membered ring of the zeolite agrees with experiment, but the energy differences between possible cation locations and the lattice distortion induced by the Al/Si substitution and the bonding of the cation depends quite significantly on the choice of the functional. All functionals predict similar energy differences for excited spin states. Spin-excitations are shown to be accompanied by significant changes in the cation coordination, which are more pronounced with hybrid functionals. The consequences of electronic spectra and chemical reactivity are analyzed in the following papers.
Show PACS
82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
82.33.Jx Reactions in zeolites
82.39.Wj Ion exchange, dialysis, osmosis, electro-osmosis, membrane processes
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

Structure and properties of metal-exchanged zeolites studied using gradient-corrected and hybrid functionals. II. Electronic structure and photoluminescence spectra

Florian Göltl and Jürgen Hafner

J. Chem. Phys. 136, 064502 (2012); doi:10.1063/1.3676409 (17 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The influence of the choice of the exchange-correlation functional (semilocal gradient corrected or hybrid functionals) on the electronic properties of metal-exchanged zeolites has been investigated for Cu- and Co-exchanged chabazite. The admixture of exact exchange in hybrid functionals increases the fundamental gap of purely siliceous chabazite, leading to better agreement with experiment and many-body perturbation theory for close-packed SiO2 polymorphs where detailed experimental information is available. For the metal-exchanged chabazite the increased exchange splitting strongly influences the position of the cation states relative to the framework bands—in general, gradient-corrected functionals locate the occupied cation states close to the valence-band maximum of the framework, while hybrid functionals shift the occupied cation states to larger binding energies and the empty states to higher energies within the fundamental gap. The photoluminescence spectra have been analyzed using fixed-moment total-energy calculations for excited spin states in structurally relaxed and frozen geometries. The geometrical relaxation of the excited states leads to large differences in excitation and emission energies which are more pronounced in calculations using hybrid functionals. Due to the stronger relaxation effects calculated with hybrid functionals, the large differences in the electronic spectra calculated with both types of functionals are not fully reflected in the photoluminescence spectra.
Show PACS
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)
31.15.vq Electron correlation calculations for polyatomic molecules
31.15.xp Perturbation theory

Structure and properties of metal-exchanged zeolites studied using gradient-corrected and hybrid functionals. III. Energetics and vibrational spectroscopy of adsorbates

Florian Göltl and Jürgen Hafner

J. Chem. Phys. 136, 064503 (2012); doi:10.1063/1.3676410 (31 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The influence of the exchange-correlation functional (semilocal gradient corrected or hybrid functional) on density-functional studies of the adsorption of CO and NO in Cu- and Co-exchanged chabazite has been investigated, extending the studies of the structural and electronic properties of these materials [F. Göltl and J. Hafner, J. Chem. Phys. 136, 064501 (2012)10.1063/1.3676408; F. Göltl and J. Hafner, J. Chem. Phys. 136, 064502 (2012)10.1063/1.3676409] and including for comparison carbonyls and nitrosyls of Cu and Co. Hybrid functionals predict much lower adsorption energies than conventional semilocal functionals, in better agreement with experiment as far as data are available for comparison. The calculated adsorption energies show a strong linear correlation with the stability of the cation sites. For Cu(I)-chabazite the calculated adsorption energies span almost the interval between the adsorption energies calculated for pure neutral and positively charged Cu-carbonyls and nitrosyls. For divalent Cu(II) and Co(II) the adsorption energies at cations in chabazite are much lower than the metal-molecule binding energies in the free carbonyls or nitrosyls, especially for the most stable cation location in a six-membered ring of the chabazite structure. For the stretching modes of adsorbed CO only hybrid functionals reproduce the blueshift of the frequency reported for all Cu(I)- and Co(II)-zeolites. For Cu(II)-chabazite both types of functionals predict a blueshift, the larger value calculated with hybrid functionals being in better agreement with observation. For NO adsorbed on Cu(I)-chabazite all functionals produce a redshift, the smaller value derived with hybrid functionals being in better agreement with experiment. For NO adsorbed in Cu(II)- and Co(II)-chabazite gradient-corrected functionals produce the best agreement with experiment for cations located in a six-membered ring. Semilocal functionals tend to underestimate the frequencies, while hybrid functionals tend to overestimate. The decisive factors determining the influence of the functionals are the larger HOMO-LUMO gap and the larger bandgap of the zeolite host, as well as the larger exchange-splitting of the cation eigenstates predicted with hybrid functionals. For Co(II)-chabazite the tendency to overestimate the exchange-splitting and to stabilize a high-spin state lead to better results with semilocal functionals. Finally, a comprehensive discussion of the influence of the exchange-correlation functional on the physico-chemical properties of these complex systems, based all three papers of this series is presented.
Show PACS
68.43.Mn Adsorption kinetics
31.15.eg Exchange-correlation functionals (in current density functional theory)
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.20.Tp Vibrational analysis
33.70.Jg Line and band widths, shapes, and shifts

Analysis of time resolved femtosecond and femtosecond/picosecond coherent anti-Stokes Raman spectroscopy: Application to toluene and Rhodamine 6G

Kai Niu and Soo-Y. Lee

J. Chem. Phys. 136, 064504 (2012); doi:10.1063/1.3682470 (11 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The third-order polarization for coherent anti-Stokes Raman scattering (CARS) from a pure state is described by 48 terms in perturbation theory, but only 4 terms satisfy the rotating wave approximation. They are represented by Feynman dual time-line diagrams and four-wave mixing energy level diagrams. In time-resolved (tr) fs and fs/ps CARS from the ground vibrational state, one resonant diagram, which is the typical CARS term, with three field interactions—pump, Stokes, followed by probe—on the ket is dominant. Using the separable, displaced harmonic oscillators approximation, an analytic result is obtained for the four-time correlation function in the CARS third-order polarization. Dlott's phenomenological expression for off-resonance CARS from the ground vibrational state is derived using a three-state model. We calculated the tr fs and fs/ps CARS for toluene and Rhodamine 6G (R6G), initially in the ground vibrational state, to compare with experimental results. The observed vibrational features and major peaks for both tr fs and fs/ps CARS, from off-resonance (for toluene) to resonance (for R6G) pump wavelengths, can be well reproduced by the calculations. The connections between fs/ps CARS, fs stimulated Raman spectroscopy, and impulsive stimulated scattering for toluene and R6G are discussed.
Show PACS
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis
33.80.Be Level crossing and optical pumping
31.15.xp Perturbation theory

Selective probe of the morphology and local vibrations at carbon nanoasperities

Toshihiko Fujimori (藤森 利彦), Koki Urita (瓜田 幸幾), David Tománek, Tomonori Ohba (大場 友則), Isamu Moriguchi (森口 勇), Morinobu Endo (遠藤 守信), and Katsumi Kaneko (金子 克美)

J. Chem. Phys. 136, 064505 (2012); doi:10.1063/1.3682771 (5 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We introduce a way to selectively probe local vibration modes at nanostructured asperities such as tips of carbon nanohorns. Our observations benefit from signal amplification in surface-enhanced Raman scattering (SERS) at sites near a silver surface. We observe nanohorn tip vibration modes in the range 200–500 cm−1, which are obscured in regular Raman spectra. Ab initio density functional calculations assign modes in this frequency range to local vibrations at the nanohorn cap resembling the radial breathing mode of fullerenes. Careful interpretation of our SERS spectra indicates presence of caps with 5 or 6 pentagons, which are chemically the most active sites. Changes in the peak intensities and frequencies with time indicate that exposure to laser irradiation may cause structural rearrangements at the cap.
Show PACS
63.22.Kn Clusters and nanocrystals
68.55.J- Morphology of films
61.46.Hk Nanocrystals
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Rx Nanocrystalline materials
78.30.Hv Other nonmetallic inorganics

Elucidation of spin echo small angle neutron scattering correlation functions through model studies

Chwen-Yang Shew and Wei-Ren Chen

J. Chem. Phys. 136, 064506 (2012); doi:10.1063/1.3678228 (11 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Several single-modal Debye correlation functions to approximate part of the overall Debey correlation function of liquids are closely examined for elucidating their behavior in the corresponding spin echo small angle neutron scattering (SESANS) correlation functions. We find that the maximum length scale of a Debye correlation function is identical to that of its SESANS correlation function. For discrete Debye correlation functions, the peak of SESANS correlation function emerges at their first discrete point, whereas for continuous Debye correlation functions with greater width, the peak position shifts to a greater value. In both cases, the intensity and shape of the peak of the SESANS correlation function are determined by the width of the Debye correlation functions. Furthermore, we mimic the intramolecular and intermolecular Debye correlation functions of liquids composed of interacting particles based on a simple model to elucidate their competition in the SESANS correlation function. Our calculations show that the first local minimum of a SESANS correlation function can be negative and positive. By adjusting the spatial distribution of the intermolecular Debye function in the model, the calculated SESANS spectra exhibit the profile consistent with that of hard-sphere and sticky-hard-sphere liquids predicted by more sophisticated liquid state theory and computer simulation.
Show PACS
61.20.Ja Computer simulation of liquid structure
76.60.Lz Spin echoes

First principles analysis of the initial oxidation of Si(001) and Si(111) surfaces terminated with H and CH3

Huashan Li, Zhibin Lin, Zhigang Wu, and Mark T. Lusk

J. Chem. Phys. 136, 064507 (2012); doi:10.1063/1.3682782 (8 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Transition state analyses have been carried out within a density functional theory setting to explain and quantify the distinctly different ways in which hydrogen and methyl terminations serve to protect silicon surfaces from the earliest onset of oxidation. We find that oxidation occurs via direct dissociative adsorption, without any energy barrier, on Si(111) and reconstructed Si(001) that have been hydrogen terminated; oxidation initiates with a barrier of only 0.05 eV on unreconstructed Si(001). The commonly measured protection afforded by hydrogen is shown to derive from a coverage-dependent dissociation rate combined with barriers to the hopping of adsorbed oxygen atoms. Methyl termination, in contrast, offers an additional level of protection because oxygen must first undergo interactions with these ligands in a three-step process with significant energy barriers: adsorption of O2 into a C–H bond to form a C–O–O–H intermediate; decomposition of C–O–O–H into C–O–H and C=O intermediates; and, finally, hopping of oxygen atoms from ligands to the substrate.
Show PACS
81.05.Cy Elemental semiconductors
81.65.Mq Oxidation
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
68.43.Mn Adsorption kinetics

Dynamics and efficiency of a self-propelled, diffusiophoretic swimmer

Benedikt Sabass and Udo Seifert

J. Chem. Phys. 136, 064508 (2012); doi:10.1063/1.3681143 (15 pages)

Online Publication Date: 9 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Active diffusiophoresis—swimming through interaction with a self-generated, neutral, solute gradient—is a paradigm for autonomous motion at the micrometer scale. We study this propulsion mechanism within a linear response theory. First, we consider several aspects relating to the dynamics of the swimming particle. We extend established analytical formulae to describe small swimmers, which interact with their environment on a finite lengthscale. Solute convection is also taken into account. Modeling of the chemical reaction reveals a coupling between the angular distribution of reactivity on the swimmer and the concentration field. This effect, which we term “reaction induced concentration distortion,” strongly influences the particle speed. Building on these insights, we employ irreversible, linear thermodynamics to formulate an energy balance. This approach highlights the importance of solute convection for a consistent treatment of the energetics. The efficiency of swimming is calculated numerically and approximated analytically. Finally, we define an efficiency of transport for swimmers which are moving in random directions. It is shown that this efficiency scales as the inverse of the macroscopic distance over which transport is to occur.
Show PACS
87.15.Vv Diffusion
05.70.Ln Nonequilibrium and irreversible thermodynamics

Time-resolved simultaneous polarized and depolarized light scattering system with high sensitivity to optical anisotropy: Application to phase separation of an optically isotropic liquid mixture

Takehito Koyama and Hajime Tanaka

J. Chem. Phys. 136, 064509 (2012); doi:10.1063/1.3682469 (8 pages)

Online Publication Date: 9 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Depolarized light scattering is widely used to probe the spatial correlation of optical anisotropy in crystals, liquid crystals, and viscoelastic materials under stress, and a powerful means to study a non-equilibrium pattern evolution process of such a system. To follow the temporal change in the diagonal and off-diagonal contributions of the dielectric tensor, it is highly desirable to measure two-dimensional (2D) polarized (HH: horizontally transmitted, horizontally received) and depolarized (VH: vertically transmitted, horizontally received) scattering patterns simultaneously in a time-resolved manner. We develop a light scattering system with a video-rate time resolution as well as very high sensitivity to optical anisotropy. To detect extremely weak VH scattering from a sample without suffering from residual birefringence of the optical system itself and leakage of strong HH scattering signals, we use an objective lens specially designed for polarizing microscopy and Glan-laser prisms, respectively. This system enables us to experimentally elucidate the origin of VH scattering: we use the ratio of the VH and HH scattering intensity as a fingerprint for whether a 2D VH scattering pattern is caused by (i) optical anisotropy (intrinsic birefringence) or merely by (ii) spatial inhomogeneity of optically isotropic materials. We verify the validity of this method for a process of phase separation in a binary mixture of isotropic liquids. The simultaneous HH and VH measurement allows us to directly estimate the ratio of VH and HH scattering intensity accurately. The careful comparison of this ratio with a simple theory unambiguously demonstrates that the 2D VH scattering pattern is caused by the scattering angle dependence of the diffraction efficiency of light with the two polarization directions. That is, the origin of VH scattering is due to geometrical effects of the inhomogeneous distribution of the refractive index and not due to optical birefringence, as it should be for the optically isotropic sample. This method using the ratio of VH and HH scattering intensity may be widely used for distinguishing the two types of origins for a VH scattering pattern in an unambiguous manner.
Show PACS
78.47.je Time resolved light scattering spectroscopy
64.75.Va Phase separation and segregation in polymer blends/polymeric solutions
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.20.Fm Birefringence

Mechanism of OH radical hydration: A comparative computational study of liquid and supercritical solvent

Dorota Swiatla-Wojcik and Joanna Szala-Bilnik

J. Chem. Phys. 136, 064510 (2012); doi:10.1063/1.3683436 (9 pages)

Online Publication Date: 10 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Flexible models of the radical and water molecules including short-range interaction of hydrogen atoms have been employed in molecular dynamic simulation to understand mechanism of •OH hydration in aqueous systems of technological importance. A key role of H-bond connectivity patterns of water molecules has been identified. The behavior of •OHaq strongly depends on water density and correlates with topological changes in the hydrogen-bonded structure of water driven by thermodynamic conditions. Liquid and supercritical water above the critical density exhibit the radical localization in cavities existing in the solvent structure. A change of mechanism has been found at supercritical conditions below the critical density. Instead of cavity localization, we have identified accumulation of water molecules around •OH associated with the formation of a strong H-donor bond and diminution of non-homogeneity in the solvent structure. For all the systems investigated, the computed hydration number and the internal energy of hydration ΔhU showed approximately linear decrease with decreasing density of the solvent but a degree of radical-water hydrogen bonding exhibited non-monotonic dependence on density. The increase in the number of radical-water H-acceptor bonds is associated with diminution of extended nets of four-bonded water molecules in compressed solution at ∼473 K. Up to 473 K, the isobaric heat of hydration in compressed liquid water remains constant and equal to −40 ± 1 kJ mol−1.
Show PACS
82.30.Rs Hydrogen bonding, hydrophilic effects
82.30.Nr Association, addition, insertion, cluster formation
back to top Surfaces, Interfaces, and Materials

The role of surface defects in multi-exciton generation of lead selenide and silicon semiconductor quantum dots

Heather M. Jaeger, Sean Fischer, and Oleg V. Prezhdo

J. Chem. Phys. 136, 064701 (2012); doi:10.1063/1.3682559 (10 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Multi-exciton generation (MEG), the creation of more than one electron-hole pair per photon absorbed, occurs for excitation energies greater than twice the bandgap (Eg). Imperfections on the surface of quantum dots, in the form of atomic vacancies or incomplete surface passivation, lead to less than ideal efficiencies for MEG in semiconductor quantum dots. The energetic onset for MEG is computed with and without surface defects for nanocrystals, Pb4Se4, Si7, and Si7H2. Modeling the correlated motion of two electrons across the bandgap requires a theoretical approach that incorporates many-body effects, such as post-Hartree-Fock quantum chemical methods. We use symmetry-adapted cluster with configuration interaction to study the excited states of nanocrystals and to determine the energetic threshold of MEG. Under laboratory conditions, lead selenide nanocrystals produce multi-excitons at excitation energies of 3 Eg, which is attributed to the large dielectric constant, small Coulomb interaction, and surface defects. In the absence of surface defects the MEG threshold is computed to be 2.6 Eg. For lead selenide nanocrystals with non-bonding selenium valence electrons, Pb3Se4, the MEG threshold increases to 2.9 Eg. Experimental evidence of MEG in passivated silicon quantum dots places the onset of MEG at 2.4 Eg. Our calculations show that the lowest multi-exciton state has an excitation energy of 2.5 Eg, and surface passivation enhances the optical activity of MEG. However, incomplete surface passivation resulting in a neutral radical on the surface drives the MEG threshold to 4.4 Eg. Investigating the mechanism of MEG at the atomistic level provides explanations for experimental discrepancies and suggests ideal materials for photovoltaic conversion.
Show PACS
71.55.Cn Elemental semiconductors
71.55.Ht Other nonmetals
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.21.La Quantum dots
71.35.Gg Exciton-mediated interactions
71.35.Lk Collective effects (Bose effects, phase space filling, and excitonic phase transitions)

Mesoscopic analysis of Gibbs’ criterion for sessile nanodroplets on trapezoidal substrates

F. Dutka, M. Napiórkowski, and S. Dietrich

J. Chem. Phys. 136, 064702 (2012); doi:10.1063/1.3682775 (20 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
By taking into account precursor films accompanying nanodroplets on trapezoidal substrates we show that on a mesoscopic level of description one does not observe the phenomenon of liquid-gas-substrate contact line pinning at substrate edges. This phenomenon is present in a macroscopic description and leads to non-unique contact angles which can take values within a range determined by the so-called Gibbs’ criterion. Upon increasing the volume of the nanodroplet the apparent contact angle evaluated within the mesoscopic approach changes continuously between two limiting values fulfilling Gibbs’ criterion, while the contact line moves smoothly across the edge of the trapezoidal substrate. The spatial extent of the range of positions of the contact line, corresponding to the variations of the contact angle between the values given by Gibbs’ criterion, is of the order of ten fluid particle diameters.
Show PACS
68.43.Mn Adsorption kinetics

Activation of water on the TiO2 (110) surface: The case of Ti adatoms

Meng Miao, Yingchun Liu, Qi Wang, Tao Wu, Liping Huang, Keith E. Gubbins, and Marco Buongiorno Nardelli

J. Chem. Phys. 136, 064703 (2012); doi:10.1063/1.3682781 (5 pages)

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Using first-principles calculations we have studied the reactions of water over Ti adatoms on the (110) surface of rutile TiO2. Our results provide fundamental insights into the microscopic mechanisms that drive this reaction at the atomic level and assess the possibility of using this system to activate the water dissociation reaction. In particular, we show that a single water molecule dissociates exothermically with a small energy barrier of 0.17 eV. After dissociation, both H+ and OH− ions bind strongly to the Ti adatom, which serves as an effective reactive center on the TiO2 surface. Finally, clustering of Ti adatoms does not improve the redox activity of the system and results in a slightly higher energy barrier for water dissociation.
Show PACS
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
36.40.Jn Reactivity of clusters
71.15.-m Methods of electronic structure calculations
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions

Electronic states and the influence of oxygen addition on the optical absorption behaviour of manganese phthalocyanine

R. Friedrich, T. Hahn, J. Kortus, M. Fronk, F. Haidu, G. Salvan, D. R. T. Zahn, M. Schlesinger, M. Mehring, F. Roth, B. Mahns, and M. Knupfer

J. Chem. Phys. 136, 064704 (2012); doi:10.1063/1.3683253 (8 pages)

Online Publication Date: 10 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The effects of electronic states and air exposure on the spectroscopic properties of manganese phthalocyanine (MnPc) have been examined. The observed features of the Q-band in the absorption spectra can be explained by intrinsic electronic properties of MnPc, i.e., the formation of singly charged molecules by charge transfer excitations. However, the reaction of MnPc with atmospheric molecular oxygen leads to deviations in peak intensities but does not change the fundamental characteristics of the spectra. Nevertheless, the reaction with oxygen changes the spin state from S = 3/2 to S = 1/2. X-ray diffraction measurements also indicate a slow diffusion process of the oxygen into the MnPc crystal. We discuss both influences to explain the behaviour of MnPc in various spectroscopic methods (EELS, ellipsometry, PES). Furthermore, we support the experimental investigations by detailed ab-initio calculations of spectroscopic properties using methods of the density functional theory framework.
Show PACS
71.20.Rv Polymers and organic compounds
78.40.Me Organic compounds and polymers
78.30.Jw Organic compounds, polymers
78.40.Fy Semiconductors
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
Page 1 of 2 Pages Next Page | Jump to Page
Close

Google Calendar

ADVERTISEMENT

Go to AIP Home
Copyright © American Institute of Physics
close