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14 Oct 2007

Volume 127, Issue 14, Articles (14xxxx)

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Determination of structural transitions of atomic clusters from local and global bond orientational order parameters

Lixin Zhan, Jeff Z. Y. Chen, and Wing-Ki Liu

J. Chem. Phys. 127, 141101 (2007); http://dx.doi.org/10.1063/1.2794032 (4 pages) | Cited 6 times

Online Publication Date: 9 October 2007

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Designing an effective order parameter for the identification of geometries in atomic clusters is an important step toward understanding the structural transitions occurring in these systems. We propose a method that simultaneously utilizes the local and global bond orientational order parameters for structural transitions. When applied to Lennard–Jones clusters at finite temperature over the size range 30 ≤ N ≤ 146, this method identified all the major geometries: icosahedra with Mackay overlayers, icosahedra with anti-Mackay overlayers, decahedra, octahedra, and tetrahedra. From the distributions of these geometries as a function of temperatures on clusters containing 38, 75, and 98 atoms, we are able to interpret all transition types without ambiguity.
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36.40.Ei Phase transitions in clusters
36.40.Mr Spectroscopy and geometrical structure of clusters

The origin of deficiency of the supermolecule second-order Møller-Plesset approach for evaluating interaction energies

Slawomir M. Cybulski and Marion L. Lytle

J. Chem. Phys. 127, 141102 (2007); http://dx.doi.org/10.1063/1.2795693 (4 pages) | Cited 23 times

Online Publication Date: 10 October 2007

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Calculations for the complex of thymine and adenine are used to show that the supermolecule second-order Møller-Plesset perturbation theory (MP2) approach for evaluating interaction energies fails in certain cases because of the behavior of one of its components: the uncoupled Hartree-Fock dispersion energy. A simple approach for correcting the MP2 supermolecule interaction energies is proposed. It focuses on correcting a relatively small difference between the MP2 and coupled cluster interaction energies, which is a very appealing feature of the new approach considering a benchmark role played by coupled cluster results.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory
31.15.xr Self-consistent-field methods

Intracule functional models. II. Analytically integrable kernels

Deborah L. Crittenden, Elise E. Dumont, and Peter M. W. Gill

J. Chem. Phys. 127, 141103 (2007); http://dx.doi.org/10.1063/1.2795694 (5 pages) | Cited 16 times

Online Publication Date: 10 October 2007

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We present, within the framework of intracule functional theory (IFT), a class of kernels whose correlation integrals can be found in closed form. This approach affords three major advantages over other kernels that we have considered previously; ease of implementation, computational efficiency, and numerical stability. We show that even the simplest member of the class yields reasonable estimates of the correlation energies of 18 atomic and 56 molecular systems and we conclude that this kernel class will prove useful in the development of future IFT models.
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31.15.E- Density-functional theory
31.15.V- Electron correlation calculations for atoms, ions and molecules

Cobaltocene as a spin filter

Rui Liu, San-Huang Ke, Weitao Yang, and Harold U. Baranger

J. Chem. Phys. 127, 141104 (2007); http://dx.doi.org/10.1063/1.2796151 (4 pages) | Cited 13 times

Online Publication Date: 10 October 2007

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In the context of investigating organic molecules for molecular electronics, doping molecular wires with transition metal atoms provides additional means of controlling their transport behavior. The incorporation of transition metal atoms may generate spin dependence because the conduction channels of only one spin component align with the chemical potential of the leads, resulting in a spin polarized electric current. The possibility to create such a spin polarized current is investigated here with the organometallic moiety cobaltocene. According to our calculations, cobaltocene contacted with gold electrodes acts as a robust spin filter: Applying a voltage less than 0.2 V causes the current of one spin component crossing the molecular bridge to be two orders of magnitude larger than the other. We address the key issue of sensitivity to molecule-lead geometry by showing that a weak barrier generated by CH2 groups between the cobaltocene and the leads is crucial in reducing the sensitivity to the contact geometry while only reducing the current modestly. These results suggest cobaltocene as a robust basic building block for molecular spintronics.
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72.25.-b Spin polarized transport
82.60.-s Chemical thermodynamics
85.65.+h Molecular electronic devices
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
75.47.Pq Other materials
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)

Photoinduced two-dimensional gratings based on dye-doped cholesteric liquid crystal films

Hui-Chen Yeh, Guang-Hao Chen, Chia-Rong Lee, and Ting-Shan Mo

J. Chem. Phys. 127, 141105 (2007); http://dx.doi.org/10.1063/1.2798103 (4 pages) | Cited 8 times

Online Publication Date: 12 October 2007

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This work elucidates photoinduced two-dimensional (2D) gratings in dye-doped cholesteric liquid crystal films. The helical pitch is increased by green-beam-induced trans-cis isomerization and a concomitant thermal effect. Two-dimensional gratings appear when the green beam is turned off. Grating formation results from elastic instability caused by restored strain arising from helical pitch reduction. Grating lifetime increases as green beam intensity increases and declines under irradiation with a strong red beam. Variation in grating spacing with green intensity with various pitches is also examined.
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42.70.Df Liquid crystals
61.30.-v Liquid crystals
42.79.Dj Gratings
82.30.Qt Isomerization and rearrangement
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back to top Theoretical Methods and Algorithms

Bond breaking with auxiliary-field quantum Monte Carlo

W. A. Al-Saidi, Shiwei Zhang, and Henry Krakauer

J. Chem. Phys. 127, 144101 (2007); http://dx.doi.org/10.1063/1.2770707 (8 pages) | Cited 6 times

Online Publication Date: 9 October 2007

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Bond stretching mimics different levels of electron correlation and provides a challenging test bed for approximate many-body computational methods. Using the recently developed phaseless auxiliary-field quantum Monte Carlo (AF QMC) method, we examine bond stretching in the well-studied molecules BH and N2 and in the H50 chain. To control the sign/phase problem, the phaseless AF QMC method constrains the paths in the auxiliary-field path integrals with an approximate phase condition that depends on a trial wave function. With single Slater determinants from unrestricted Hartree-Fock as trial wave function, the phaseless AF QMC method generally gives better overall accuracy and a more uniform behavior than the coupled cluster CCSD(T) method in mapping the potential-energy curve. In both BH and N2, we also study the use of multiple-determinant trial wave functions from multiconfiguration self-consistent-field calculations. The increase in computational cost versus the gain in statistical and systematic accuracy are examined. With such trial wave functions, excellent results are obtained across the entire region between equilibrium and the dissociation limit.
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31.15.xr Self-consistent-field methods
31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.bw Coupled-cluster theory
31.50.-x Potential energy surfaces

Theory of ultrafast nonresonant multiphoton transitions in polyatomic molecules: Basics and application to optimal control theory

Volkhard May, David Ambrosek, Markus Oppel, and Leticia González

J. Chem. Phys. 127, 144102 (2007); http://dx.doi.org/10.1063/1.2766717 (11 pages) | Cited 3 times

Online Publication Date: 9 October 2007

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A systematic approach is presented to describe nonresonant multiphoton transitions, i.e., transitions between two electronic states without the presence of additional intermediate states resonant with the single-photon energy. The method is well suited to describe femtosecond spectroscopic experiments and, in particular, attempts to achieve laser pulse control of molecular dynamics. The obtained effective time-dependent Schrödinger equation includes effective couplings to the radiation field which combine powers of the field strength and effective transition dipole operators between the initial and final states. To arrive at time-local equations our derivation combines the well-known rotating wave approximation with the approximation of slowly varying amplitudes. Under these terms, the optimal control formalism can be readily extended to also account for nonresonant multiphoton events. Exemplary, nonresonant two- and three-photon processes, similar to those occurring in the recent femtosecond pulse-shaping experiments on CpMn(CO)3, are treated using related ab initio potential energy surfaces.
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33.80.Wz Other multiphoton processes
03.65.Ge Solutions of wave equations: bound states
31.15.A- Ab initio calculations
31.50.-x Potential energy surfaces

Spin resolved electron number distribution functions: How spins couple in real space

A. Martín Pendás, E. Francisco, and M. A. Blanco

J. Chem. Phys. 127, 144103 (2007); http://dx.doi.org/10.1063/1.2784392 (10 pages) | Cited 8 times

Online Publication Date: 10 October 2007

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The probabilities of finding arbitrary partitions of the Nαms = 1/2 and Nβms = −1/2 electrons of a molecule into m arbitrary regions that exhaust the physical space are developed and computed, both for atomic and electron localization function basins, in a number of test systems. These spin resolved electron number distribution functions provide access to the coarse-grained distribution of spins in space even for singlet states, a nontrivial result. It is found that atoms within molecules partially retain their in vacuo preferences for certain spin configurations. This may lead to long range spin coupling among basins. An aufbaulike rule favoring spin coupling, particularly for Hartree-Fock wave functions, has also been found.
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31.15.xr Self-consistent-field methods

Basis set limits of the second order Møller-Plesset correlation energies of water, methane, acetylene, ethylene, and benzene

Daisuke Yamaki, Henrik Koch, and Seiichiro Ten-no

J. Chem. Phys. 127, 144104 (2007); http://dx.doi.org/10.1063/1.2794036 (5 pages) | Cited 19 times

Online Publication Date: 10 October 2007

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We report second order Møller-Plesset (MP2) and MP2-F12 total energies on He, Ne, Ar, H2O, CH4, C2H2, C2H4, and C6H6, using the correlation consistent basis sets, aug-cc-pVXZ (X = D−7). Basis set extrapolation techniques are applied to the MP2 and MP2-F12/B methods. The performance of the methods is tested in the calculations of the atoms, He, Ne, and Ar. It is indicated that the two-point extrapolation of MP2-F12/B with the basis sets (X = 5,6) is the most reliable. Similar accuracy is obtained using two-point extrapolated conventional MP2 with the basis sets (X = 6,7). For the molecules investigated the valence MP2 correlation energy is estimated within 1 mEh.
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31.15.V- Electron correlation calculations for atoms, ions and molecules

Dynamic polarizabilities of polyaromatic hydrocarbons using coupled-cluster linear response theory

Jeff R. Hammond, Karol Kowalski, and Wibe A. deJong

J. Chem. Phys. 127, 144105 (2007); http://dx.doi.org/10.1063/1.2772853 (9 pages) | Cited 16 times

Online Publication Date: 11 October 2007

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Coupled-cluster theory with single and double excitations is applied to the calculation of optical properties of large polyaromatic hydrocarbons. Dipole polarizabilities are reported for benzene, pyrene, and the oligoacenes sequence n = 2–6. Dynamic polarizabilities were calculated on polyacences as large as pentacene for a single frequency and for benzene and pyrene at many frequencies. The basis set effect was studied for benzene using a variety of basis sets in the Pople [Theor. Chim. Acta 28, 213 (1973)] and Dunning [J. Chem. Phys. 90, 1007 (1989)] families up to aug-cc-pVQZ and the Sadlej pVTZ basis [ Collect. Czech. Chem. Commun. 53, 1995 (1998)] , which was used exclusively for the largest molecules. Geometries were optimized using HF, B3LYP, PBE0, and MP2 and compared to experiment to measure method dependence and the possible role of bond-length alternation. Finally, the polarizability results were compared to four common density functionals (B3LYP, BLYP, PBE0, PBE).
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31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods
31.15.E- Density-functional theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Dj Interatomic distances and angles
33.15.Bh General molecular conformation and symmetry; stereochemistry

Efficient evaluation of analytic vibrational frequencies in Hartree-Fock and density functional theory for periodic nonconducting systems

Artur F. Izmaylov and Gustavo E. Scuseria

J. Chem. Phys. 127, 144106 (2007); http://dx.doi.org/10.1063/1.2790024 (9 pages) | Cited 11 times

Online Publication Date: 12 October 2007

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We report a method for the efficient evaluation of analytic energy second derivatives with respect to in-phase nuclear coordinate displacements within Hartree-Fock and Kohn-Sham density functional theories using Gaussian orbitals and periodic boundary conditions. The use of an atomic orbital formulation for all computationally challenging steps allows us to adapt the direct space fast multipole method for the Coulomb-type infinite summations. Our implementation also exploits the local character of the exact Hartree-Fock exchange in nonconducting systems. Exchange-correlation contributions are computed using extensive screening and fast numerical quadratures. We benchmark our scheme for in-phase vibrational frequencies of a trans-polyacetylene chain, a two-dimensional boron nitride sheet, and bulk diamond with the 6-31G** basis set and various density functionals. A study of computational scaling with the size of the unit cell for trans-polyacetylene reveals subquadratic scaling for our scheme.
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31.15.xr Self-consistent-field methods
31.15.E- Density-functional theory
33.15.Mt Rotation, vibration, and vibration-rotation constants

Electron transport through single conjugated organic molecules: Basis set effects in ab initio calculations

San-Huang Ke, Harold U. Baranger, and Weitao Yang

J. Chem. Phys. 127, 144107 (2007); http://dx.doi.org/10.1063/1.2770718 (6 pages) | Cited 9 times

Online Publication Date: 12 October 2007

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We investigate electron transport through single conjugated molecules—including benzenedithiol, oligophenylene ethynylenes of different lengths, and a ferrocene-containing molecule sandwiched between two gold electrodes with different contact structures—by using a single-particle Green function method combined with density functional theory calculation. We focus on the effect of the basis set in the ab initio calculation. It is shown that the position of the Fermi energy in the transport gap is sensitive to the molecule-lead charge transfer which is affected by the size of basis set. This can dramatically change, by orders of magnitude, the conductance for long molecules, though the effect is only minor for short ones. A resonance around the Fermi energy tends to pin the position of the Fermi energy and suppress this effect. The result is discussed in comparison with experimental data.
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72.80.Le Polymers; organic compounds (including organic semiconductors)
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Topological effects for nonsymmetrical configurations: The C2H2+ as a case study

G. J. Halász, Á. Vibók, and M. Baer

J. Chem. Phys. 127, 144108 (2007); http://dx.doi.org/10.1063/1.2779035 (7 pages) | Cited 6 times

Online Publication Date: 12 October 2007

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During the last decade the study of topological effects formed by molecular systems became a routine but it was always carried out for configurations that were limited by symmetry conditions. To be more specific this applied to the Jahn-Teller (JT) effect formed by molecular configurations of planar symmetry [see, e.g., Baer et al., Faraday Discuss. 127, 337 (2004) ] and the Renner-Teller effect formed by configurations of axial symmetry [see, e.g., Halász et al., J. Chem. Phys. 126, 154309 (2007) ]. In this article we consider for the first time molecular configurations that avoid any symmetry conditions or, in other words, are characterized by the C1 point group. We report on a detailed study of topological effects formed by such a molecular system. The study concentrates on both, the two-state (Abelian) case and the multistate (non-Abelian) case. It is shown that the theory that was originally developed to treat topological effects due the JT intersection and also applies for the study of topological effects in the most general case. The study is accompanied with numerical results.
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33.15.Bh General molecular conformation and symmetry; stereochemistry
31.30.-i Corrections to electronic structure
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Kinetic energy density study of some representative semilocal kinetic energy functionals

David García-Aldea and J. E. Alvarellos

J. Chem. Phys. 127, 144109 (2007); http://dx.doi.org/10.1063/1.2774974 (10 pages) | Cited 14 times

Online Publication Date: 12 October 2007

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There is a number of explicit kinetic energy density functionals for noninteracting electron systems that are obtained in terms of the electron density and its derivatives. These semilocal functionals have been widely used in the literature. In this work, we present a comparative study of the kinetic energy density of these semilocal functionals, stressing the importance of the local behavior to assess the quality of the functionals. We propose a quality factor that measures the local differences between the usual orbital-based kinetic energy density distributions and the approximated ones, allowing us to ensure if the good results obtained for the total kinetic energies with these semilocal functionals are due to their correct local performance or to error cancellations. We have also included contributions coming from the Laplacian of the electron density to work with an infinite set of kinetic energy densities. For all but one of the functionals, we have found that their success in the evaluation of the total kinetic energy is due to global error cancellations, whereas the local behavior of their kinetic energy density becomes worse than that corresponding to the Thomas-Fermi functional.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.45.-d Collective effects
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Secondary decomposition of C3H5 radicals formed by the photodissociation of 2-bromopropene

Haiyan Fan, Stephen T. Pratt, and James A. Miller

J. Chem. Phys. 127, 144301 (2007); http://dx.doi.org/10.1063/1.2775445 (8 pages) | Cited 1 time

Online Publication Date: 8 October 2007

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The photodissociation of 2-bromopropene at 193 nm produces C3H5 radicals with a distribution of internal energies that spans the threshold for the secondary decomposition of the 2-propenyl radicals into C3H4+H. Just above this threshold, the decomposition rate is on the nanosecond time scale, and in the present study, time-resolved velocity-map ion imaging is used to gain insight into this process. The results provide information on the energy dependence of the secondary dissociation process. In addition, comparison of the results with theoretical predictions of the energy dependence of the dissociation rate provides information on the branching between fragment rotational and vibrational energies in the primary photodissociation process.
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82.50.Bc Processes caused by infrared radiation
82.20.Pm Rate constants, reaction cross sections, and activation energies
34.50.Ez Rotational and vibrational energy transfer
33.80.Gj Diffuse spectra; predissociation, photodissociation

Quantum dynamics of the CH3 fragment: A curvilinear coordinate system and kinetic energy operators

Chris Evenhuis, Gunnar Nyman, and Uwe Manthe

J. Chem. Phys. 127, 144302 (2007); http://dx.doi.org/10.1063/1.2779034 (11 pages) | Cited 12 times

Online Publication Date: 8 October 2007

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A curvilinear coordinate system for AB3 fragments is given. The corresponding exact kinetic energy operator is derived and a series of simpler, progressively more approximate kinetic energy operators are suggested. The operators are tailored for quantum dynamics simulations using the multiconfigurational time-dependent Hartree approach. It is outlined how these fragment coordinates can be utilized to set up coordinate systems for larger systems such as AB3C or AB3CD. Calculations of the vibrational levels of CH3 and quantum dynamics studies investigate the accuracy of the different kinetic energy operators suggested.
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82.20.Ej Quantum theory of reaction cross section
82.20.Hf Product distribution

On the properties of microsolvated molecules in the ground (S0) and excited (S1) states: The anisole-ammonia 1:1 complex

Malgorzata Biczysko, Giovanni Piani, Massimiliano Pasquini, Nicola Schiccheri, Giangaetano Pietraperzia, Maurizio Becucci, Michele Pavone, and Vincenzo Barone

J. Chem. Phys. 127, 144303 (2007); http://dx.doi.org/10.1063/1.2767265 (11 pages) | Cited 9 times

Online Publication Date: 8 October 2007

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State-of-the-art spectroscopic and theoretical methods have been exploited in a joint effort to elucidate the subtle features of the structure and the energetics of the anisole-ammonia 1:1 complex, a prototype of microsolvation processes. Resonance enhanced multiphoton ionization and laser-induced fluorescence spectra are discussed and compared to high-level first-principles theoretical models, based on density functional, many body second order perturbation, and coupled cluster theories. In the most stable nonplanar structure of the complex, the ammonia interacts with the delocalized π electron density of the anisole ring: hydrogen bonding and dispersive forces provide a comparable stabilization energy in the ground state, whereas in the excited state the dispersion term is negligible because of electron density transfer from the oxygen to the aromatic ring. Ground and excited state geometrical parameters deduced from experimental data and computed by quantum mechanical methods are in very good agreement and allow us to unambiguously determine the molecular structure of the anisole-ammonia complex.
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82.30.Nr Association, addition, insertion, cluster formation
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.50.Dq Fluorescence and phosphorescence spectra
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment
33.15.Fm Bond strengths, dissociation energies
34.70.+e Charge transfer

The photodissociation dynamics of ozone at 226 and 248 nm: O(3PJ) atomic angular momentum polarization

M. Brouard, A. Goman, S. J. Horrocks, A. J. Johnsen, F. Quadrini, and W.-H. Yuen

J. Chem. Phys. 127, 144304 (2007); http://dx.doi.org/10.1063/1.2790890 (14 pages) | Cited 7 times

Online Publication Date: 8 October 2007

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Speed distributions, and spatial anisotropy and atomic angular momentum polarization parameters have been determined for the O(3PJ) products following the photodissociation of ozone at 248 and 226 nm using velocity map ion imaging. The data have been interpreted in terms of two dissociation mechanisms that give rise to fast and slow products. In both cases, excitation is believed to occur to the math state. Consistent with previous interpretations, the speed distributions, translational anisotropy parameters, and angular momentum polarization moments support the assignment of the major pathway to curve crossing from the math to the repulsive math surface, generating fast fragments in a wide range of vibrational states. For the slow fragments, it is proposed that following excitation to the math state, the system crosses onto the math state. The crossing seam is only accessible to molecules that are highly vibrationally excited and therefore possess modest recoil speeds. Once on the math state, the wavepacket is thought to funnel through a conical intersection to the ground state. The velocity distributions, spatial anisotropy parameters, spin-orbit populations and polarization data each lend support to this mechanism.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis

Vibrational dynamics of excited electronic states of molecular iodine

A. Scaria, V. Namboodiri, J. Konradi, and A. Materny

J. Chem. Phys. 127, 144305 (2007); http://dx.doi.org/10.1063/1.2790438 (6 pages) | Cited 5 times

Online Publication Date: 9 October 2007

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Femtosecond degenerate four-wave-mixing spectroscopy following an initial pump laser pulse was used to observe the wave packet dynamics in excited electronic states of gas phase iodine. The focus of the investigation was on the ion pair states belonging to the first tier dissociating into the two ions I−(1S)+I+(3P2). By a proper choice of the wavelengths of the initial pump and degenerate four-wave-mixing pulses, we were able to observe the vibrational dynamics of the B 3Πu+ state of molecular iodine as well as the ion pair states accessible from there by a one-photon transition. The method proves to be a valuable tool for exploring higher lying states that cannot be directly accessed from the ground state due to selection rule exclusion or unfavorable Franck-Condon overlap.
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33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.80.Be Level crossing and optical pumping

Semiclassical initial value representation treatment of a hydrogen bonded complex of rigid water molecules from a single trajectory in Cartesian coordinates

Bilkiss B. Issack and Pierre-Nicholas Roy

J. Chem. Phys. 127, 144306 (2007); http://dx.doi.org/10.1063/1.2786456 (4 pages) | Cited 2 times

Online Publication Date: 9 October 2007

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A semiclassical initial value representation approach for molecular systems in Cartesian coordinates is combined with a recently proposed time averaging technique [ J. Chem. Phys. 118, 7174 (2003) ]. It is shown that a single trajectory can yield the zero-point energy of the water dimer with good accuracy for the model chosen when compared to fully constrained Cartesian semiclassical calculations. The convergence with respect to the number of averaging time origins is discussed.
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31.15.xg Semiclassical methods

Optical control of ground-state atomic orbital alignment: Cl(2P3/2) atoms from HCl(v = 2,J = 1) photodissociation

Dimitris Sofikitis, Luis Rubio-Lago, Marion R. Martin, Davida J. Ankeny Brown, Nathaniel C.-M. Bartlett, Andrew J. Alexander, Richard N. Zare, and T. Peter Rakitzis

J. Chem. Phys. 127, 144307 (2007); http://dx.doi.org/10.1063/1.2772272 (7 pages) | Cited 3 times

Online Publication Date: 10 October 2007

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H35Cl(v = 0,J = 0) molecules in a supersonic expansion were excited to the H35Cl(v = 2,J = 1,M = 0) state with linearly polarized laser pulses at about 1.7 μm. These rotationally aligned J = 1 molecules were then selectively photodissociated with a linearly polarized laser pulse at 220 nm after a time delay, and the velocity-dependent alignment of the 35Cl(2P3/2) photofragments was measured using 2+1 REMPI and time-of-flight mass spectrometry. The 35Cl(2P3/2) atoms are aligned by two mechanisms: (1) the time-dependent transfer of rotational polarization of the H35Cl(v = 2,J = 1,M = 0) molecule to the 35Cl(2P3/2) nuclear spin [which is conserved during the photodissociation and thus contributes to the total 35Cl(2P3/2) photofragment atomic polarization] and (2) the alignment of the 35Cl(2P3/2) electronic polarization resulting from the photoexcitation and dissociation process. The total alignment of the 35Cl(2P3/2) photofragments from these two mechanisms was found to vary as a function of time delay between the excitation and the photolysis laser pulses, in agreement with theoretical predictions. We show that the alignment of the ground-state 35Cl(2P3/2) atoms, with respect to the photodissociation recoil direction, can be controlled optically. Potential applications include the study of alignment-dependent collision effects.
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32.30.-r Atomic spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.-m Photochemistry

Electron and ion reactions with hexamethyldisiloxane and pentamethyldisiloxane

S. Carles, J. L. Le Garrec, and J. B. A. Mitchell

J. Chem. Phys. 127, 144308 (2007); http://dx.doi.org/10.1063/1.2774984 (7 pages) | Cited 2 times

Online Publication Date: 10 October 2007

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The dissociative recombination of electrons with the hexamethyldisiloxane (HMDSO) cation ((CH3)3Si–O–Si(CH3)3)+ and the pentamethyldisiloxane cation ((CH3)3Si–O–Si(CH2)2)+ as well as the ion-molecule reaction between Ar+ and HMDSO have been studied at 300 K using a flowing afterglow Langmuir probe-mass spectrometer apparatus. The rate constants for these reactions, measured directly for the first time, are, respectively, α1 = 1.8×10−6, α2 = 3.6×10−6 cm3/s, and k = 2.0×10−9 cm3/s with uncertainties of ±30%. In addition, the electronic attachment to neutral HMDSO was also studied and an upper limit value of the rate constant was determined to be β = 3.3×10−11 cm3/s.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Pm Rate constants, reaction cross sections, and activation energies

Neutral photodissociation of superexcited states of molecular iodine

P. O’Keeffe, D. Stranges, and P. L. Houston

J. Chem. Phys. 127, 144309 (2007); http://dx.doi.org/10.1063/1.2777160 (7 pages)

Online Publication Date: 10 October 2007

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The formation of high-n Rydberg atoms from the neutral dissociation of superexcited states of I2 formed by resonant two-photon excitation of molecular iodine using an ArF laser has been investigated. The high-n Rydberg atoms I* are formed by predissociation of the optically excited molecular Rydberg states I2*[R(B 2Σg+)] converging on the I2+(B 2Σg+) state of the ion. Measurement of the kinetic energy release of the Rydberg I* fragments allowed the identification of the asymptotic channels as I*[R(3PJ)]+I(2P3/2), where the I*[R(3PJ)] are Rydberg atoms converging on the I+(3PJ) states of the ion with J = 2, 1, and 0. In the case of the I*[R(3P2)] fragments, the average Rydberg lifetime is observed to be 325±25 μs. Based on experiments on the variation of the Rydberg atom signal with the field ionizing strength, the distribution of Rydberg levels peaks at about 25–50 cm−1 below the ionization limit.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Infrared spectra of the OCS trimer

Mahin Afshari, M. Dehghani, Z. Abusara, N. Moazzen-Ahmadi, and A. R. W. McKellar

J. Chem. Phys. 127, 144310 (2007); http://dx.doi.org/10.1063/1.2793782 (5 pages) | Cited 4 times

Online Publication Date: 11 October 2007

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Infrared spectra of the barrel-shaped trimer (OCS)3, previously known from its microwave spectrum, are reported for the first time. The observations are carried out in a supersonic slit-jet expansion of a He+OCS gas mixture which is probed with a tunable diode laser. Three rotationally resolved bands associated with the ν1 fundamental vibration of OCS (2062.20 cm−1) are observed, at about 2047, 2053, and 2077 cm−1. Small perturbations are noted in the 2077 cm−1 band and may also be present in the 2053 cm−1 band, which is weak and hence more difficult to analyze precisely. Employing a variety of evidence, we suggest a plausible assignment for the nature of the OCS vibrations in each of the three bands.
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33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants

Ab initio calculation of temperature effects in the optical response of open-shell sodium clusters

Marie Lopez del Puerto, Murilo L. Tiago, and James R. Chelikowsky

J. Chem. Phys. 127, 144311 (2007); http://dx.doi.org/10.1063/1.2755720 (10 pages) | Cited 2 times

Online Publication Date: 11 October 2007

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Many properties of atomic clusters have been found to be size dependent, e.g., the optical response. There are, however, factors other than size that can also play an important role in determining the properties of nanoscale systems. Temperature, in particular, has been shown to have a strong effect on the optical response of open-shell sodium clusters. We incorporate the temperature effect on the optical absorption spectra by combining pseudopotentials, Langevin molecular dynamics, and time-dependent density functional theory. We have done calculations for several open-shell sodium clusters, Na4+, Na7+, and Na11+, for which experimental data are available for comparison. We find that the positions of the lower energy peaks of the calculated spectra correspond very well to the peaks in the experimental spectra, although the local density approximation tends to overestimate the gap of the smaller clusters by up to 0.2 eV and underestimate the gap of the largest cluster by 0.4 eV. We fit the width of the peaks in the lower-temperature calculations to the corresponding experimental result to obtain the instrumental linewidth. We then use this same width for the high-temperature calculations and find very good agreement with experiment. Finally, we analyze the transitions that contribute to the observed peaks in the absorption spectra and we plot the effective valence charge density for specific transitions for each cluster. We find that for the two smaller clusters the absorption spectra are dominated by transitions from the occupied levels to a few (three for Na4+ and five for Na7+) empty levels, although the contribution from transitions to other empty levels can still be significant. In contrast, the absorption spectra for Na11+ come from a greater mixture of transitions as evidenced in the analysis as well as in the plot of the effective valence charge density.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
78.30.Er Solid metals and alloys
78.40.Kc Metals, semimetals, and alloys
71.20.Dg Alkali and alkaline earth metals
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