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15 Nov 2005

Volume 123, Issue 19, Articles (19xxxx)

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back to top Theoretical Methods and Algorithms

Fundamental limits of the dispersion of the two-photon absorption cross section

Javier Pérez Moreno and Mark G. Kuzyk

J. Chem. Phys. 123, 194101 (2005); http://dx.doi.org/10.1063/1.2104407 (13 pages) | Cited 18 times

Online Publication Date: 11 November 2005

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We rigorously apply the sum rules to the sum-over-states expression to calculate the fundamental limits of the dispersion of the two-photon absorption cross section. A comparison of the theory with the data suggests that the truncated sum rules in the three-level model give a reasonable fundamental limit. Furthermore, we posit that the two-photon absorption cross section near the limit must have only three dominant states, so by default, the three-level model is appropriate. This ansatz is supported by a rigorous analytical calculation that the resonant term gets smaller as more states are added. We also find that the contributions of the nonexplicitly resonant terms cannot be neglected when analyzing real molecules with many excited states, even near resonance. However, puzzling as it may be, extrapolating an off-resonant result to resonance using only the resonant term of the three-level model is shown to be consistent with the exact result. In addition, the off-resonant approximation is shown to scale logarithmically when compared with the full three-level model. This scaling can be used to simplify the analysis of measurements. We find that existing molecules are still far from the fundamental limit; so, there is room for improvement. But, reaching the fundamental limit would require precise control of the energy-level spacing, independently of the transition dipole moments—a task that does not appear possible using today’s synthetic approaches. So, we present alternative methods that can still lead to substantial improvements which only require the control of the transition moment to the first excited state. While it is best to normalize measured two-photon absorption cross sections to the fundamental limits when comparing molecules, we show that simply dividing by the square of the number of electrons per molecule yields a good metric for comparison.
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33.80.Wz Other multiphoton processes
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Theoretical study of the electronic spectra of square-planar platinum (II) complexes based on the two-component relativistic time-dependent density-functional theory

Fan Wang and Tom Ziegler

J. Chem. Phys. 123, 194102 (2005); http://dx.doi.org/10.1063/1.2104427 (10 pages) | Cited 16 times

Online Publication Date: 11 November 2005

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In the present work the electronic spectra of [PtCl4]2−, [PtBr4]2−, and [Pt(CN)4]2− are studied with a recently proposed relativistic time-dependent density-functional theory (TDDFT) based on the two-component zeroth-order regular approximation and a noncollinear exchange-correlation (XC) functional. The contribution to the double group excited states in terms of singlet and triplet single group excited states is estimated through the inner product of the transition density matrix obtained from two-component and scalar relativistic TDDFT calculations to better understand the double group excited states. Spin-orbital coupling effects are found to be very important in order to simulate the electronic spectra of these complexes. The results show that the two-component TDDFT formalism can afford excitation energies with high accuracy for the transition-metal systems studied here when use is made of a proper XC potential.
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31.15.E- Density-functional theory
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

Nonlinear response theory with relaxation: The first-order hyperpolarizability

Patrick Norman, David M. Bishop, Hans Jørgen Aa. Jensen, and Jens Oddershede

J. Chem. Phys. 123, 194103 (2005); http://dx.doi.org/10.1063/1.2107627 (18 pages) | Cited 34 times

Online Publication Date: 11 November 2005

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Based on the Ehrenfest theorem, an equation of motion that takes relaxation into account has been presented in wave-function theory, and the resulting response functions are nondivergent in the off-resonant as well as the resonant regions of optical frequencies. The derivation includes single- and multideterminant reference states. When applied to electric dipole properties, the response functions correspond to the phenomenological sum-over-states expressions of Orr and Ward [Mol. Phys. 20, 513 (1971)] for polarizabilities and hyperpolarizabilities of an isolated system. A universal dispersion formula is derived for the complex second-order response function. Response theory calculations are performed on lithium hydride and para-nitroaniline for off-resonant and resonant frequencies in the electro-optical Kerr effect and second-harmonic generation.
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33.57.+c Magneto-optical and electro-optical spectra and effects
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Differential virial theorem in relation to a sum rule for the exchange-correlation force in density-functional theory

A. Holas, N. H. March, and Angel Rubio

J. Chem. Phys. 123, 194104 (2005); http://dx.doi.org/10.1063/1.2114848 (4 pages)

Online Publication Date: 11 November 2005

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Holas and March [ Phys. Rev. A. 51, 2040 (1995) ] gave a formally exact theory for the exchange-correlation (xc) force Fxc(r) = −∇υxc(r) associated with the xc potential υxc(r) of the density-functional theory in terms of low-order density matrices. This is shown in the present study to lead, rather directly, to the determination of a sum rule 〈nFxc〉 = 0 relating the xc force with the ground-state density n(r). Some connection is also made with an earlier result relating to the external potential by Levy and Perdew [ Phys. Rev. A. 32, 2010 (1985) ] and with the quite recent study of Joubert [ J. Chem. Phys. 119, 1916 (2003) ] relating to the separation of the exchange and correlation contributions.
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31.15.E- Density-functional theory

Hartree-Fock orbitals for complex-scaled configuration interaction calculation of highly excited Feshbach resonances

Petra R. Žďánská and Nimrod Moiseyev

J. Chem. Phys. 123, 194105 (2005); http://dx.doi.org/10.1063/1.2110169 (8 pages) | Cited 6 times

Online Publication Date: 11 November 2005

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We examine a complex-scaled configuration interaction [(CS)CI] for highly excited Feshbach resonances, where we study the 2s2 resonance of helium as a test case. Sizable full-CI calculations are reduced by using a correctly defined minimum active space. We compare the convergence of the minimum active space for conventional Hartree-Fock (HF) orbitals obtained as solutions to Hermitian HF equations, to the convergence of minimum active space for complex orbitals obtained as solutions to complex-scaled HF equations. Ground-state optimized orbitals are compared to a simple modification of the HF method using the excited-state mean-field potential.
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31.15.xr Self-consistent-field methods
31.15.vj Electron correlation calculations for atoms and ions: excited states

An efficient approach for self-consistent-field energy and energy second derivatives in the atomic-orbital basis

WanZhen Liang, Yi Zhao, and Martin Head-Gordon

J. Chem. Phys. 123, 194106 (2005); http://dx.doi.org/10.1063/1.2114847 (8 pages) | Cited 7 times

Online Publication Date: 14 November 2005

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Based on self-consistent-field (SCF) perturbation theory, we recast the SCF and the coupled-perturbed SCF (CPSCF) equations for time-independent molecular properties into the atomic-orbital basis. The density matrix and the perturbed density matrix are obtained iteratively by solving linear equations. Only matrix multiplications and additions are required, and this approach can exploit sparse matrix multiplications and thereby offer the possibility of evaluating second-order properties in computational effort that scales linearly with system size. Convergence properties are similar to conventional molecular-orbital-based CPSCF procedures, in terms of the number of derivative Fock matrices that must be constructed. We also carefully address the issue of the numerical accuracy of the calculated second derivatives of the energy, in order to specify the minimum precision necessary in the CPSCF procedure. It is found that much looser tolerances for the perturbed density matrices are adequate when using an expression for the second derivatives that is correct through second order in the CPSCF error.
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31.15.xr Self-consistent-field methods

Nested stochastic simulation algorithm for chemical kinetic systems with disparate rates

Weinan E, Di Liu, and Eric Vanden-Eijnden

J. Chem. Phys. 123, 194107 (2005); http://dx.doi.org/10.1063/1.2109987 (8 pages) | Cited 46 times

Online Publication Date: 14 November 2005

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An efficient simulation algorithm for chemical kinetic systems with disparate rates is proposed. This new algorithm is quite general, and it amounts to a simple and seamless modification of the classical stochastic simulation algorithm (SSA), also known as the Gillespie [J. Comput. Phys. 22, 403 (1976) ; J. Phys. Chem. 81, 2340 (1977) ] algorithm. The basic idea is to use an outer SSA to simulate the slow processes with rates computed from an inner SSA which simulates the fast reactions. Averaging theorems for Markov processes can be used to identify the fast and slow variables in the system as well as the effective dynamics over the slow time scale, even though the algorithm itself does not rely on such information. This nested SSA can be easily generalized to systems with more than two separated time scales. Convergence and efficiency of the algorithm are discussed using the established error estimates and illustrated through examples.
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82.20.Uv Stochastic theories of rate constants

Nonadiabatic quantum-classical reaction rates with quantum equilibrium structure

Hyojoon Kim and Raymond Kapral

J. Chem. Phys. 123, 194108 (2005); http://dx.doi.org/10.1063/1.2110140 (10 pages) | Cited 14 times

Online Publication Date: 14 November 2005

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Time correlation function expressions for quantum reaction-rate coefficients are computed in a quantum-classical limit. This form for the correlation function retains the full quantum equilibrium structure of the system in the spectral density function but approximates the time evolution of the operator by quantum-classical Liouville dynamics. Approximate analytical expressions for the spectral density function, which incorporate quantum effects in the many-body environment and reaction coordinate, are derived. The results of numerical simulations of the reaction rate are presented for a reaction model in which a two-level system is coupled to a bistable oscillator which is, in turn, coupled to a bath of harmonic oscillators. The nonadiabatic quantum-classical dynamics is simulated in terms of an ensemble of surface-hopping trajectories and the effects of the quantum equilibrium structure on the reaction rate are discussed.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Ej Quantum theory of reaction cross section
82.20.Fd Collision theories; trajectory models
82.20.Sb Correlation function theory of rate constants and its applications

Behavior of polarizable models in presence of strong electric fields. I. Origin of nonlinear effects in water point-charge systems

Riccardo Chelli, Alessandro Barducci, Luca Bellucci, Vincenzo Schettino, and Piero Procacci

J. Chem. Phys. 123, 194109 (2005); http://dx.doi.org/10.1063/1.2110107 (8 pages) | Cited 2 times

Online Publication Date: 15 November 2005

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In the current opinion, the inclusion of polarization response in classical computer simulations is considered as one of the most important and urgent improvements to be implemented in modern empirical potential models. In this work we focus on the capability of polarizable models, based on the pairwise Coulomb interactions, to model systems where strong electric fields enter into play. As shown by Masia, Probst, and Rey (MPR) [in J. Chem. Phys. 121, 7362 (2004) ], when a molecule interacts with point charges, polarizable models show underpolarization with respect to ab initio methods. We prove that this underpolarization, clearly related to nonlinear polarization effects, cannot be simply ascribed to the lack of hyperpolarization in the polarizable models, as suggested by MPR. Analysis of the electron-density rearrangement induced on a water molecule by a point charge reveals a twofold level of polarization response. One level involves intramolecular charge transfer on the whole molecular volume, with the related polarization exhibiting a seemingly linear behavior with the external electric field. The other nonlinear polarization level occurs only at strong electric fields and is found to be strictly correlated to the quantum-mechanical nature of the water molecule. The latter type of polarization has a local character, being limited to the space region of the water lone pairs.
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61.20.Ja Computer simulation of liquid structure
61.25.Em Molecular liquids

Multireference calculations of the electronic structure of VF2 and VCl2

M. Vogel and W. Wenzel

J. Chem. Phys. 123, 194110 (2005); http://dx.doi.org/10.1063/1.2126588 (5 pages) | Cited 1 time

Online Publication Date: 16 November 2005

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We investigated the electronic structure of two members of the transition-metal dihalide family, VF2 and VCl2. Using the configuration-interaction method in large basis sets we calculated the lowest 17 states of these molecules in the vicinity of their ground-state geometry. We compute the ground-state bond lengths, vibrational frequencies, and dissociation energies. In contrast to predictions of ligand-field theory, we find math ground states for both molecules.
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31.15.ve Electron correlation calculations for atoms and ions: ground state
33.15.Dj Interatomic distances and angles
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants

The directional contact distance of two ellipsoids: Coarse-grained potentials for anisotropic interactions

Leonid Paramonov and Sophia N. Yaliraki

J. Chem. Phys. 123, 194111 (2005); http://dx.doi.org/10.1063/1.2102897 (11 pages) | Cited 17 times

Online Publication Date: 16 November 2005

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We obtain the distance of closest approach of the surfaces of two arbitrary ellipsoids valid at any orientation and separation measured along their intercenter vector. This directional distance is derived from the elliptic contact function. The geometric meaning behind this approach is clarified. An elliptic pair potential for modeling arbitrary mixtures of elliptic particles, whether hard or soft, is proposed based on this distance. Comparisons with Gay-Berne potentials are discussed. Analytic expressions for the forces and torques acting on the elliptic particles are given.
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87.15.A- Theory, modeling, and computer simulation
87.14.E- Proteins
87.15.B- Structure of biomolecules
87.15.K- Molecular interactions; membrane-protein interactions

Geometry optimization of crystals by the quasi-independent curvilinear coordinate approximation

Károly Németh and Matt Challacombe

J. Chem. Phys. 123, 194112 (2005); http://dx.doi.org/10.1063/1.2121569 (8 pages) | Cited 3 times

Online Publication Date: 17 November 2005

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The quasi-independent curvilinear coordinate approximation (QUICCA) method [ K. Németh and M. Challacombe, J. Chem. Phys. 121, 2877 (2004) ] is extended to the optimization of crystal structures. We demonstrate that QUICCA is valid under periodic boundary conditions, enabling simultaneous relaxation of the lattice and atomic coordinates, as illustrated by tight optimization of polyethylene, hexagonal boron nitride, a (10,0) carbon nanotube, hexagonal ice, quartz, and sulfur at the Γ-point RPBE/STO-3G level of theory.
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61.66.Fn Inorganic compounds
61.66.Hq Organic compounds
61.41.+e Polymers, elastomers, and plastics
61.46.-w Structure of nanoscale materials
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Photon energy dependence of fragmentation of small argon clusters

Mathieu Gisselbrecht, Andreas Lindgren, Maxim Tchaplyguine, Florian Burmeister, Gunnar Öhrwall, Marcus Lundwall, Magnus Lundin, Ricardo R.T. Marinho, Arnaldo Naves de Brito, Svante Svensson, Olle Björneholm, and Stacey L. Sorensen

J. Chem. Phys. 123, 194301 (2005); http://dx.doi.org/10.1063/1.2118527 (7 pages) | Cited 11 times

Online Publication Date: 11 November 2005

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Photofragmentation of small argon clusters with size below ten atoms is reported. In this size range significant modifications from the electronic properties and geometry take place. When tuning the photon energy through the argon 2p edge, the fragmentation pattern is changed. Specifically, cation dimer production is enhanced at the 2p3/2→4s resonance, while above the 2p edge almost complete atomization is observed. In both cases, the widths of the peaks in the mass spectra indicate that a large amount of kinetic energy is imparted to the fragment due to the formation of multiply charged clusters. A model based on “Coulomb explosion”—charge separation, simply resulting in a complete atomization of the cluster with no dependence on the photon energy—is insufficient to explain the observed photofragmentation of small clusters.
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36.40.Jn Reactivity of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Wa Charged clusters
33.15.Ta Mass spectra
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation

An eight-degree-of-freedom quantum dynamics study for the H2+C2H system

Dunyou Wang

J. Chem. Phys. 123, 194302 (2005); http://dx.doi.org/10.1063/1.2122707 (6 pages) | Cited 13 times

Online Publication Date: 11 November 2005

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An eight-degree-of-freedom (8DOF) time-dependent wave-packet approach has been developed to study the H2+C2H→H+C2H2 reaction system. The 8DOF model is obtained by fixing one of the Jacobi torsion angle in the nine-degree-of-freedom AB+CDE reaction system. This study is an extension of the previous seven-degree-of-freedom (7DOF) computation [ J. Chem. Phys. 119, 12057 (2003) ] of this reaction system. This study shows that vibrational excitations of H2 enhance the reaction probability, whereas the stretching vibrational excitations of C2H have only a small effect on the reactivity. Furthermore, the bending excitation of C2H, compared to the ground-state reaction probability, hinders the reactivity. A comparison of the rate constant between the 7DOF calculation and the present 8DOF results has been made. The theoretical and experimental results agree with each other very well when the present 8DOF results are adjusted to account for the lower transition state barrier heights found in recent ab initio calculations.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies

Measurement of the rate coefficient for collisional removal of O2(X math,υ = 1) by O(math)

Konstantinos S. Kalogerakis, Richard A. Copeland, and Tom G. Slanger

J. Chem. Phys. 123, 194303 (2005); http://dx.doi.org/10.1063/1.2110227 (7 pages) | Cited 6 times

Online Publication Date: 11 November 2005

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We report a laboratory measurement of the rate coefficient for the collisional removal of O2(Xmath,υ = 1) by O(math) atoms. In the experiments, 266-nm laser light photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O2(amath) that is rapidly converted to O2(Xmath,υ = 1–3) in a near-resonant electronic energy-transfer process with ground-state O2. In parallel, a large amount of O(math) atoms is generated that promptly relaxes to O(math). Under the conditions of the experiments, only collisions with the photolytically produced O(math) atoms control the lifetime of O2(Xmath,υ = 1), because its removal by molecular oxygen at room temperature is extremely slow. Tunable 193-nm laser light monitors the temporal evolution of the O2(Xmath,υ = 1) population by detection of laser-induced fluorescence near 360 nm. The removal rate coefficient for O2(Xmath,υ = 1) by O(math) atoms is (3.2±1.0)×10−12 cm3 s−1 (2σ) at a temperature of 315±15 K (2σ). This result is essential for the analysis and correct interpretation of the 6.3-μm H2O(ν2) band emission in the Earth’s mesosphere and indicates that the deactivation of O2(X math,υ = 1) by O(math) atoms is significantly faster than the nominal values recently used in atmospheric models.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.50.-m Photochemistry
92.60.H- Atmospheric composition, structure, and properties
82.33.Tb Atmospheric chemistry

Quantum state-to-state rate constants for the rotationally inelastic collision of CH(B math, ν = 0, N→N′) with Ar

Sheng Der Chao, Sheng Hsien Lin, and Millard H. Alexander

J. Chem. Phys. 123, 194304 (2005); http://dx.doi.org/10.1063/1.2118547 (5 pages) | Cited 3 times

Online Publication Date: 14 November 2005

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We have calculated the state-to-state integral cross sections and rate constants for the rotationally inelastic collision of CH(B math, ν = 0, N→N′) with Ar using the quantum coupled-state and close-coupling methods on an ab initio potential-energy surface constructed by Alexander et al. [J. Chem. Phys. 101, 4547 (1994)] . Overall the calculated rate constants are in good agreements with the three available experimental results. The rate constants are comparable to the usual gas kinetic and decrease with increasing N and ΔN. For the multiquantum transition cases, the theory underestimates the experiment. We discuss some possible causes to the discrepancies among the theory and the experiments.
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34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
34.50.-s Scattering of atoms and molecules
31.15.A- Ab initio calculations
33.15.Mt Rotation, vibration, and vibration-rotation constants

Stark slowing asymmetric rotors: Weak-field-seeking states and nonadiabatic transitions

Arne Schwettmann, Jack Franklin, K. Richard Overstreet, and James P. Shaffer

J. Chem. Phys. 123, 194305 (2005); http://dx.doi.org/10.1063/1.2112787 (5 pages) | Cited 9 times

Online Publication Date: 14 November 2005

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Stark deceleration is one of the few methods that can be used to slow polyatomic molecules. We present calculations of Stark shift energies, a quantitative analysis of nonadiabatic transition probabilities, and orientational distribution functions applicable to typical Stark slowing conditions for the two small asymmetric rotors nitromethane and acetaldehyde. We show that asymmetric polyatomic molecules are good candidates for Stark slowing.
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37.10.Mn Slowing and cooling of molecules
37.10.Pq Trapping of molecules
33.57.+c Magneto-optical and electro-optical spectra and effects
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
37.10.Vz Mechanical effects of light on atoms, molecules, and ions

Dissociative recombination of the weakly bound NO-dimer cation: Cross sections and three-body dynamics

Annemieke Petrignani, Patrik U. Andersson, Jan B. C. Pettersson, Richard D. Thomas, Fredrik Hellberg, Anneli Ehlerding, Mats Larsson, and Wim J. van der Zande

J. Chem. Phys. 123, 194306 (2005); http://dx.doi.org/10.1063/1.2116927 (11 pages) | Cited 6 times

Online Publication Date: 14 November 2005

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Dissociative recombination (DR) of the dimer ion (NO)2+ has been studied at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm. The experiments were aimed at determining details on the strongly enhanced thermal rate coefficient for the dimer, interpreting the dissociation dynamics of the dimer ion, and studying the degree of similarity to the behavior in the monomer. The DR rate reveals that the very large efficiency of the dimer rate with respect to the monomer is limited to electron energies below 0.2 eV. The fragmentation products reveal that the breakup into the three-body channel NO+O+N dominates with a probability of 0.69±0.02. The second most important channel yields NO+NO fragments with a probability of 0.23±0.03. Furthermore, the dominant three-body breakup yields electronic and vibrational ground-state products, NO(υ = 0)+N(math)+O(math), in about 45% of the cases. The internal product-state distribution of the NO fragment shows a similarity with the product-state distribution as predicted by the Franck-Condon overlap between a NO moiety of the dimer ion and a free NO. The dissociation dynamics seem to be independent of the NO internal energy. Finally, the dissociation dynamics reveal a correlation between the kinetic energy of the NO fragment and the degree of conservation of linear momentum between the O and N product atoms. The observations support a mechanism in which the recoil takes place along one of the NO bonds in the dimer.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Hf Product distribution

Evanescent high pressure during hypersonic cluster-surface impact characterized by the virial theorem

A. Gross and R. D. Levine

J. Chem. Phys. 123, 194307 (2005); http://dx.doi.org/10.1063/1.2110207 (11 pages) | Cited 3 times

Online Publication Date: 15 November 2005

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Matter under extreme conditions can be generated by a collision of a hypersonic cluster with a surface. The ultra-high-pressure interlude lasts only briefly from the impact until the cluster shatters. We discuss the theoretical characterization of the pressure using the virial theorem and develop a constrained molecular-dynamics procedure to compute it. The simulations show that for rare-gas clusters the pressures reach the megabar range. The contribution to the pressure from momentum transfer is comparable in magnitude and is of the same sign as that (“the internal pressure”) due to repulsive interatomic forces. The scaling of the pressure with the reduced mechanical variables is derived and validated with reference to the simulations.
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36.40.Jn Reactivity of clusters
34.35.+a Interactions of atoms and molecules with surfaces
34.20.Cf Interatomic potentials and forces
34.10.+x General theories and models of atomic and molecular collisions and interactions (including statistical theories, transition state, stochastic and trajectory models, etc.)
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Development of a cooled He*(2 math) beam source for measurements of state-resolved collision energy dependence of Penning ionization cross sections: Evidence for a stereospecific attractive well around methyl group in CH3CN

Takuya Horio, Masakazu Yamazaki, Satoshi Maeda, Takuro Hatamoto, Naoki Kishimoto, and Koichi Ohno

J. Chem. Phys. 123, 194308 (2005); http://dx.doi.org/10.1063/1.2114808 (13 pages) | Cited 6 times

Online Publication Date: 15 November 2005

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A low-temperature discharge nozzle source with a liquid-N2 circulator for He*(2 math) metastable atoms has been developed in order to obtain the state-resolved collision energy dependence of Penning ionization cross sections in a low collision energy range from 20 to 80 meV. By controlling the discharge condition, we have made it possible to measure the collision energy dependence of partial ionization cross sections (CEDPICS) for a well-studied system of CH3CN+He*(2 math) in a wide energy range from 20 to 350 meV. The anisotropic interaction potential energy surface for the present system was obtained starting from an ab initio model potential via an optimization procedure based on classical trajectory calculations for the observed CEDPICS. A dominant attractive well depth was found to be 423 meV (ca. 10 kcal/mol) at a distance of 3.20 Å from the center of mass of CH3CN in the N-atom side along the CCN axis. In addition, a weak attractive well (ca. 0.9 kcal/mol) surrounding the methyl group (−CH3) has been found and ascribed to the interaction between an unoccupied molecular orbital of CH3CN and 2s atomic orbital of He*(2 math).
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34.50.Gb Electronic excitation and ionization of molecules
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.15.A- Ab initio calculations

Quantum approaches for the insertion dynamics of the H++D2 and D++H2 reactive collisions

Tomas González-Lezana, Alfredo Aguado, Miguel Paniagua, and O. Roncero

J. Chem. Phys. 123, 194309 (2005); http://dx.doi.org/10.1063/1.2118567 (13 pages) | Cited 22 times

Online Publication Date: 16 November 2005

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The H++D2 and D++H2 reactive collisions are studied using a recently proposed adiabatic potential energy surface of spectroscopic accuracy. The dynamics is studied using an exact wave packet method on the adiabatic surface at energies below the curve crossing occurring at ≈ 1.5 eV above the threshold. It is found that the reaction is very well described by a statistical quantum method for a zero total angular momentum (J) as compared with the exact ones, while for higher J some discrepancies are found. For J>0 different centrifugal sudden approximations are proposed and compared with the exact and statistical quantum treatments. The usual centrifugal sudden approach fails by considering too high reaction barriers and too low reaction probabilities. A new statistically modified centrifugal sudden approach is considered which corrects these two failures to a rather good extent. It is also found that an adiabatic approximation for the helicities provides results in very good agreement with the statistical method, placing the reaction barrier properly. However, both statistical and adiabatic centrifugal treatments overestimate the reaction probabilities. The reaction cross sections thus obtained with the new approaches are in rather good agreement with the exact results. In spite of these deficiencies, the quantum statistical method is well adapted for describing the insertion dynamics, and it is then used to evaluate the differential cross sections.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Nr Association, addition, insertion, cluster formation
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Kh Potential energy surfaces for chemical reactions

Calculating molecular Rydberg states using the one-particle Green’s function: Application to HCO and C(NH2)3

Sven Feuerbacher and Robin Santra

J. Chem. Phys. 123, 194310 (2005); http://dx.doi.org/10.1063/1.2122687 (8 pages) | Cited 1 time

Online Publication Date: 16 November 2005

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A simple but accurate and computationally efficient method for routine ab initio calculations of molecular Rydberg states is described. The method, which can be applied to Rydberg states associated with a nondegenerate ion core, consists in the self-consistent solution of an effective one-electron problem. First, the restricted Hartree-Fock problem of the ion core is solved. The orbital energies and certain two-electron Coulomb matrix elements with respect to the molecular orbital basis are then used to construct an energy-dependent many-body correction to the Hartree-Fock mean field. This correction is derived from the Dyson equation satisfied by the one-particle Green’s function. The method is applied to calculate Rydberg potential-energy curves of HCO. The presented data confirm and extend recent large-scale multireference configuration-interaction calculations and help develop a detailed theoretical description of the astrophysically important dissociative recombination of a low-energy electron with HCO+. As further illustration of the utility of the method, the first ab initio calculations of the excited states of an electron bound to the guanidinium cation [C(NH2)3]+ are reported.
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31.15.A- Ab initio calculations
31.15.xr Self-consistent-field methods
31.15.vq Electron correlation calculations for polyatomic molecules
31.50.Df Potential energy surfaces for excited electronic states
33.15.Fm Bond strengths, dissociation energies

Solvent effects on the vibronic one-photon absorption profiles of dioxaborine heterocycles

Yan-Hua Wang, Marcus Halik, Chuan-Kui Wang, Seth R. Marder, and Yi Luo

J. Chem. Phys. 123, 194311 (2005); http://dx.doi.org/10.1063/1.2121590 (7 pages) | Cited 6 times

Online Publication Date: 17 November 2005

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The vibronic profiles of one-photon absorption spectra of dioxaborine heterocycles in gas phase and solution have been calculated at the Hartree-Fock and density-functional-theory levels. The polarizable continuum model has been applied to simulate the solvent effect, while the linear coupling model is used to compute the Franck-Condon and Herzberg-Teller contributions. It is found that a good agreement between theory and experiment can be achieved when the solvent effect and electron correlation are taken into account simultaneously. For the first excited charge-transfer state, the maximum of its Herzberg-Teller profile is blueshifted from that of the Franck-Condon profile. The shifted energy is found to be around 0.2 eV, which agrees well with the measured energy difference between two- and one-photon absorptions of the first excited state.
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33.80.-b Photon interactions with molecules
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.70.Dk Environmental and solvent effects
31.15.E- Density-functional theory
31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.xr Self-consistent-field methods
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Determination of the helicity of oriented photofragments

Andrew J. Alexander

J. Chem. Phys. 123, 194312 (2005); http://dx.doi.org/10.1063/1.2122667 (9 pages) | Cited 8 times

Online Publication Date: 17 November 2005

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Equations to enable determination of the helicity (angular momentum orientation) of photofragments resulting from single-photon dissociation of an isotropic sample of molecules are presented. The symmetry of the photofragment distribution is illustrated by three-dimensional vector plots of the expectation values of projections of the fragment total angular momentum. Equations describing circular polarization of light in the spherical tensor basis are presented. Methods for the optical measurement of angular momentum orientation are discussed, including determination of the helicity of circularly polarized light by a quarter-wave plate or single Fresnel rhomb.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
42.25.Ja Polarization
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

A theory of transport properties in molten salts

Takahiro Koishi and Shigeru Tamaki

J. Chem. Phys. 123, 194501 (2005); http://dx.doi.org/10.1063/1.2102901 (11 pages) | Cited 3 times

Online Publication Date: 11 November 2005

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Expressions for diffusion constants in molten salts have been obtained in terms of the inter-ionic pair potentials and the pair distribution functions. Numerical attempts for diffusion constants in molten alkali halides are carried out and results agreed fairly with those obtained by molecular-dynamics simulation and with some experimental data. Based on the coupling of generalized Langevin equation and damped Einstein oscillator equation, ions’ velocity autocorrelation functions have also been described and are numerically applied for molten potassium fluoride. The deviation from the Nernst-Einstein relation was also discussed in detail. In Appendixes A , B , C, the short-time expansion of velocity correlation functions in relation to the partial conductivities and the diffusion constants were obtained up to the term of t4 and these were compared with a model function described by the form of cos(ωt)∙sech(t/τ).
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61.20.Gy Theory and models of liquid structure
66.10.C- Diffusion and thermal diffusion
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