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7 Jul 2010

Volume 133, Issue 1, Articles (01xxxx)

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J. Chem. Phys. 133, 014501 (2010); http://dx.doi.org/10.1063/1.3454734 (13 pages)

Beat Vögeli
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Communications: Direct dynamics study of the O(3P)+C2H2 reaction: Contribution from spin nonconserving route

Karunamoy Rajak and Biswajit Maiti

J. Chem. Phys. 133, 011101 (2010); http://dx.doi.org/10.1063/1.3454727 (4 pages) | Cited 1 time

Online Publication Date: 7 July 2010

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The importance of intersystem crossing dynamics for the O(3P)+C2H2 reaction is demonstrated in this work. A direct dynamics trajectory surface hopping method has been employed to study the intersystem crossing effects. Our study reveals that there is a significant contribution from the spin nonconserving route to the chemical dynamics of the O(3P)+C2H2 reaction, despite small spin-orbit coupling constant values (<70 cm−1).
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Fd Collision theories; trajectory models
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back to top Theoretical Methods and Algorithms

Density fitting of intramonomer correlation effects in symmetry-adapted perturbation theory

Edward G. Hohenstein and C. David Sherrill

J. Chem. Phys. 133, 014101 (2010); http://dx.doi.org/10.1063/1.3451077 (12 pages) | Cited 13 times

Online Publication Date: 1 July 2010

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Symmetry-adapted perturbation theory (SAPT) offers insight into the nature of intermolecular interactions. In addition, accurate energies can be obtained from the wave function-based variant of SAPT provided that intramonomer electron correlation effects are included. We apply density-fitting (DF) approximations to the intramonomer correlation corrections in SAPT. The introduction of this approximation leads to an improvement in the computational cost of SAPT by reducing the scaling of certain SAPT terms, reducing the amount of disk I/O, and avoiding the explicit computation of certain types of MO integrals. We have implemented all the intramonomer correlation corrections to SAPT through second-order under the DF approximation. Additionally, leading third-order terms are also implemented. The accuracy of this truncation of SAPT is tested against the S22 test set of Hobza and co-workers [Phys. Chem. Chem. Phys. 8, 1985 (2006)] . When the intramonomer corrections to dispersion are included in SAPT, a mean absolute deviation of 0.3–0.4 kcal mol−1 is observed for the S22 test set when using an aug-cc-pVDZ basis. The computations on the adenine-thymine complexes in the S22 test set with an aug-cc-pVDZ basis represent the largest SAPT computations to date that include this degree of intramonomer correlation. Computations of this size can now be performed routinely with our newly developed DF-SAPT program.
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31.15.xp Perturbation theory
31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.V- Electron correlation calculations for atoms, ions and molecules
34.20.Gj Intermolecular and atom-molecule potentials and forces

Exciton coherence length fluctuations in chromophore aggregates probed by multidimensional optical spectroscopy

František Šanda, Václav Perlík, and Shaul Mukamel

J. Chem. Phys. 133, 014102 (2010); http://dx.doi.org/10.1063/1.3442415 (13 pages) | Cited 2 times

Online Publication Date: 2 July 2010

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The coherent third order optical response of molecular aggregates with fluctuating frequencies, couplings, and transition dipole moments is studied. We derived stochastic nonlinear exciton equations (SNEEs) by combining the quasiparticle picture of excitons with the path integral over stochastic bath paths described by the stochastic Liouville equations. Coherent two-dimensional (2D) spectra are calculated for a tetramer model system whose transition dipole orientations undergo two-state stochastic jumps on an arbitrary timescale. Correspondence between domains of ordered dipoles, which determine the exciton coherence length and the absorption peaks, is established. Signatures of domain coherence length fluctuations are observed in the cross peak dynamics of the 2D spectra in specific pulse polarization configurations.
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74.00.00 Superconductivity

Overlapping resonances in the resistance of superposition states to decoherence

Asoka Biswas, Moshe Shapiro, and Paul Brumer

J. Chem. Phys. 133, 014103 (2010); http://dx.doi.org/10.1063/1.3460262 (7 pages) | Cited 3 times

Online Publication Date: 6 July 2010

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Overlapping resonances are shown to provide new insights into the extent of decoherence experienced by a system superposition state in the regime of strong system-environment coupling. As an example of this general approach, a generic system comprising spin-half particles interacting with a thermalized oscillator environment is considered. We find that (a) among the collection of parametrized Hamiltonians, the larger the overlapping resonances contribution, the greater the maximum possible purity, and (b) for a fixed Hamiltonian, the larger the overlapping resonances contribution, the larger the range of possible values of the purity as one varies the phases in the system superposition states. Systems displaying decoherence free subspaces show the largest overlapping resonances contribution.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Strong correlation in hydrogen chains and lattices using the variational two-electron reduced density matrix method

Anton V. Sinitskiy, Loren Greenman, and David A. Mazziotti

J. Chem. Phys. 133, 014104 (2010); http://dx.doi.org/10.1063/1.3459059 (7 pages) | Cited 4 times

Online Publication Date: 7 July 2010

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The variational two-electron reduced-density-matrix (2-RDM) method, scaling polynomially with the size of the system, was applied to linear chains and three-dimensional clusters of atomic hydrogen as large as H64. In the case of the 4×4×4 hydrogen lattice of 64 hydrogen atoms, a correct description of the dissociation requires about 1018 equally weighted determinants in the wave function, which is too large for traditional multireference methods. The correct energy in the dissociation limit was obtained from the variational 2-RDM method in contrast to Hartree–Fock and single-reference methods. Analysis of the occupation numbers demonstrates that even for 1.0 Å bond distances the presence of strong electron correlation requires a multireference method. Three-dimensional systems exhibit a marked increase in electron correlation from one-dimensional systems regardless of size. The metal-to-insulator transition upon expansion of the clusters was studied using the decay of the 1-RDM off-diagonal elements. The variational 2-RDM method was shown to capture the metal-to-insulator transition and dissociation behavior accurately for all systems.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.xr Self-consistent-field methods
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Phase space structure and dynamics for the Hamiltonian isokinetic thermostat

Peter Collins, Gregory S. Ezra, and Stephen Wiggins

J. Chem. Phys. 133, 014105 (2010); http://dx.doi.org/10.1063/1.3455712 (18 pages)

Online Publication Date: 7 July 2010

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We investigate the phase space structure and dynamics of a Hamiltonian isokinetic thermostat, for which ergodic thermostat trajectories at fixed (zero) energy generate a canonical distribution in configuration space. Model potentials studied consist of a single bistable mode plus transverse harmonic modes. Interpreting the bistable mode as a reaction (isomerization) coordinate, we establish connections with the theory of unimolecular reaction rates, in particular the formulation of isomerization rates in terms of gap times. In the context of molecular reaction rates, the distribution of gap times (or associated lifetimes) for a microcanonical ensemble initiated on the dividing surface is of great dynamical significance; an exponential lifetime distribution is usually taken to be an indicator of “statistical” behavior. Moreover, comparison of the magnitude of the phase space volume swept out by reactive trajectories as they pass through the reactant region with the total phase space volume (classical density of states) for the reactant region provides a necessary condition for ergodic dynamics. We compute gap times, associated lifetime distributions, mean gap times, reactive fluxes, reactive volumes, and total reactant phase space volumes for model thermostat systems with three and four degrees of freedom at three different temperatures. At all three temperatures, the necessary condition for ergodicity is approximately satisfied. At high temperatures a nonexponential lifetime distribution is found, while at low temperatures the lifetime is more nearly exponential. The degree of exponentiality of the lifetime distribution is quantified by computing the information entropy deficit with respect to pure exponential decay. The efficacy of the Hamiltonian isokinetic thermostat is examined by computing coordinate distributions averaged over single long trajectories initiated on the dividing surface.
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82.30.Qt Isomerization and rearrangement
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Uv Stochastic theories of rate constants

A variational formulation of the polarizable continuum model

Filippo Lipparini, Giovanni Scalmani, Benedetta Mennucci, Eric Cancès, Marco Caricato, and Michael J. Frisch

J. Chem. Phys. 133, 014106 (2010); http://dx.doi.org/10.1063/1.3454683 (11 pages) | Cited 11 times

Online Publication Date: 7 July 2010

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Continuum solvation models are widely used to accurately estimate solvent effects on energy, structural and spectroscopic properties of complex molecular systems. The polarizable continuum model (PCM) is one of the most versatile among the continuum models because of the variety of properties that can be computed and the diversity of methods that can be used to describe the solute from molecular mechanics (MM) to sophisticated quantum mechanical (QM) post-self-consistent field methods or even hybrid QM/MM methods. In this contribution, we present a new formulation of PCM in terms of a free energy functional whose variational parameters include the continuum polarization (represented by the apparent surface charges), the solute’s atomic coordinates and—possibly—its electronic density. The problem of finding the optimized geometry of the (polarized) solute, with the corresponding self-consistent reaction field, is recast as the minimization of this free energy functional, simultaneously with respect to all its variables. The numerous potential applications of this variational formulation of PCM are discussed, including simultaneous optimization of solute’s geometry and polarization charges and extended Lagrangian dynamics. In particular, we describe in details the simultaneous optimization procedure and we include several numerical examples.
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31.70.Dk Environmental and solvent effects
31.15.xr Self-consistent-field methods

Linear scaling coupled cluster method with correlation energy based error control

Marcin Ziółkowski, Branislav Jansík, Thomas Kjærgaard, and Poul Jørgensen

J. Chem. Phys. 133, 014107 (2010); http://dx.doi.org/10.1063/1.3456535 (5 pages) | Cited 15 times

Online Publication Date: 7 July 2010

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Coupled cluster calculations can be carried out for large molecular systems via a set of calculations that use small orbital fragments of the full molecular orbital space. The error in the correlation energy of the full molecular system is controlled by the precision in the small fragment calculations. The determination of the orbital spaces for the small orbital fragments is black box in the sense that it does not depend on any user—provided molecular fragmentation, rather orbital spaces are carefully selected and extended during the calculation to give fragment energies of a specified precision. The computational method scales linearly with the size of the molecular system and is massively parallel.
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31.15.bw Coupled-cluster theory
31.15.V- Electron correlation calculations for atoms, ions and molecules

Applying the extended molecule approach to correlated electron transport: Important insight from model calculations

Ioan Bâldea, Horst Köppel, Robert Maul, and Wolfgang Wenzel

J. Chem. Phys. 133, 014108 (2010); http://dx.doi.org/10.1063/1.3455056 (11 pages) | Cited 1 time

Online Publication Date: 7 July 2010

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Theoretical approaches of electric transport in correlated molecules usually consider an extended molecule, which includes, in addition to the molecule itself, parts of electrodes. In the case where electron correlations remain confined within the molecule, and the extended molecule is sufficiently large, the current can be expressed by means of Landauer-type formulas. Electron correlations are embodied into the retarded Green’s function of a sufficiently large but isolated extended molecule, which represents the key quantity that can be accurately determined by means of ab initio quantum-chemical calculations. To exemplify these ideas, we present and analyze numerical results obtained within full configuration interaction calculations for an extended molecule described by the interacting resonant level model. Based on them, we argue that for narrower-band (organic) electrodes, the transport properties can be reliably computed because the extended molecule can be chosen sufficiently small to be tackled within accurate ab initio methods. For wider-band (metallic) electrodes, larger extended molecules have to be considered, in general, but a (semi)quantitative description of the transport should still be possible in the typical cases where electron transport proceeds by off-resonant tunneling. Our numerical results also demonstrate that, contrary to the usual claim, the ratio between the characteristic Coulomb strength and the level width due to molecule-electrode coupling is not the only quantity needed to assess whether electron correlation effects are strong or weak.
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71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
73.40.Gk Tunneling

Cutoff radius effect of the isotropic periodic sum method in homogeneous system. II. Water

Kazuaki Takahashi, Tetsu Narumi, and Kenji Yasuoka

J. Chem. Phys. 133, 014109 (2010); http://dx.doi.org/10.1063/1.3462241 (7 pages) | Cited 7 times

Online Publication Date: 7 July 2010

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Molecular dynamics simulation has been applied for water to compare the isotropic periodic sum (IPS) method [ X. Wu and B. R. Brooks, J. Chem. Phys. 122, 044107 (2005) ] with the Ewald sum based on the diffusion coefficient and liquid structure. The IPS method gives a good estimation for the self-diffusion coefficient at a cutoff radius, rc, greater than 2.2 nm; however, the radial distribution function g(r) has a notable deviation. The peak of this deviation appears at specific intermolecular distances which are near each cutoff radius and decrease in proportion to the inverse of the cube of rc. Thus the deviation becomes insignificant (less than 1%) at rc greater than 2.2 nm. The distance dependent Kirkwood factor Gk(r) was also calculated, and since the truncation of a long-range interaction of the cutofflike method (such as cutoff with or without the switch function and the reaction field) shows serious shortcomings for dipole-dipole correlations in bulk water systems, this was observed by comparing the shape to that of the Ewald sum [ Y. Yonetani, J. Chem. Phys. 124, 204501 (2006) ; D. van der Spoel and P. J. van Maaren, J. Chem. Theory Comput. 2, 1 (2006) ]. The Gk(r) of cutofflike method greatly deviate from that of the Ewald sum. However, the discrepancy of Gk(r) for the IPS method was found to be much less than that of other typical cutofflike methods. In conclusion, the IPS method is an adequately accurate technique for estimating transport coefficients and the liquid structure of water in a homogeneous system at long cutoff distances.
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61.25.Em Molecular liquids
66.10.cg Mass diffusion, including self-diffusion, mutual diffusion, tracer diffusion, etc.

Steady-state simulations using weighted ensemble path sampling

Divesh Bhatt, Bin W. Zhang, and Daniel M. Zuckerman

J. Chem. Phys. 133, 014110 (2010); http://dx.doi.org/10.1063/1.3456985 (12 pages) | Cited 7 times

Online Publication Date: 7 July 2010

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We extend the weighted ensemble (WE) path sampling method to perform rigorous statistical sampling for systems at steady state. A straightforward steady-state implementation of WE is directly practical for simple landscapes, but not when significant metastable intermediates states are present. We therefore develop an enhanced WE scheme, building on existing ideas, which accelerates attainment of steady state in complex systems. We apply both WE approaches to several model systems, confirming their correctness and efficiency by comparison with brute-force results. The enhanced version is significantly faster than the brute force and straightforward WE for systems with WE bins that accurately reflect the reaction coordinate(s). The new WE methods can also be applied to equilibrium sampling, since equilibrium is a steady state.
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05.10.-a Computational methods in statistical physics and nonlinear dynamics
02.50.-r Probability theory, stochastic processes, and statistics
82.60.Hc Chemical equilibria and equilibrium constants
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Branching ratios and anisotropy parameters in ICl photolysis from 400 to 570 nm using slice imaging

Peter C. Samartzis and Theofanis N. Kitsopoulos

J. Chem. Phys. 133, 014301 (2010); http://dx.doi.org/10.1063/1.3455209 (7 pages) | Cited 1 time

Online Publication Date: 7 July 2010

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ICl photolysis in the visible region of the spectrum (400–570 nm) is studied using the slice imaging technique. The Cl(2P1/2)/Cl(2P3/2) branching ratio between the I(2P3/2)+Cl(2P3/2) and I(2P3/2)+Cl(2P1/2) channels is extracted from the iodine photofragment images and it is found to range from 0 to 2.5, rising from 570 to 490 nm and dropping at higher photolysis energies. The I+Cl angular distribution exhibits a similar trend, changing from purely perpendicular at 570 nm to isotropic at 545 nm, fairly parallel at 490 nm and again perpendicular at 440 nm. Following previous work, we discuss these changes in light of avoided curve crossing and determine the crossing probability as a function of wavelength. The angular anisotropy parameter beta of the second channel ranges between 0.6 and 1.4.
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82.50.Hp Processes caused by visible and UV light
82.20.-w Chemical kinetics and dynamics
33.80.Gj Diffuse spectra; predissociation, photodissociation

Structure and vibrational spectra of small water clusters from first principles simulations

Dongdong Kang, Jiayu Dai, Yong Hou, and Jianmin Yuan

J. Chem. Phys. 133, 014302 (2010); http://dx.doi.org/10.1063/1.3462278 (9 pages) | Cited 4 times

Online Publication Date: 7 July 2010

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The structure and vibrational spectra of (H2O)n (n = 2–5) clusters have been studied based on first-principles molecular dynamics simulations. Trends of the cluster structures with the cluster size show that water molecules in cluster are bound more tightly. The vibrational spectra as a function of cluster size and temperature are obtained using Fourier transformation of the velocity autocorrelation function. Results of the clusters in ground state show that when the cluster size increases, the librational peaks shift to blue and the bonded intramolecular OH stretching bands shift to red due to the clusterization and hydrogen-bond strengthening. Meanwhile, there are no significant shifts in the intramolecular bending and free OH stretching modes, indicating that the free hydrogen atoms are insensitive to the local bonding environment. The temperature-dependent vibrational spectra, which exhibit similar behaviors from the dimer to pentamer, show that there are significant broadenings of the spectra with temperature caused by thermal motions. Moreover, different bands shift to different directions, where librational bands shift to red while bonded OH stretching bands shift to blue, although the blueshifts are quite small for the dimer and trimer.
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33.20.Tp Vibrational analysis
36.40.-c Atomic and molecular clusters
33.70.Jg Line and band widths, shapes, and shifts
33.15.Fm Bond strengths, dissociation energies
33.15.Bh General molecular conformation and symmetry; stereochemistry

Possible electronic decay channels in the ionization spectra of small clusters composed of Ar and Xe: A four-component relativistic treatment

Elke Faßhauer, Nikolai V. Kryzhevoi, and Markus Pernpointner

J. Chem. Phys. 133, 014303 (2010); http://dx.doi.org/10.1063/1.3462246 (7 pages) | Cited 1 time

Online Publication Date: 7 July 2010

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Electronic decay of the inner-valence Ar 3s−1 vacancy is energetically forbidden in an isolated argon atom and in all rare gas dimers where argon is present. However, if an argon atom has at least two suitable rare gas atoms in its neighborhood, the Ar 3s−1 vacancy may decay electronically via an electron transfer mediated decay (ETMD) mechanism. An ArXe2 cluster is considered in the present paper as an example of such systems. The single and double ionization spectra of different ArXe2 isomers as well as of homonuclear Ar2 and Xe2 and heteronuclear ArXe clusters have been calculated by means of propagator methods to reveal possible electronic decay channels. A four-component version of the one-particle propagator utilizing the Dirac–Coulomb Hamiltonian was employed to obtain the single ionization potentials of the clusters studied. Hereby electron correlation, scalar relativistic effects, and spin-orbit couplings are described in a consistent manner. A two-particle propagator in its one-component form, in conjunction with effective core potentials to account consistently for correlation and scalar relativistic effects, was used to calculate the double ionization potentials. ETMD is shown to be the only possible electronic decay process of the Ar 3s−1 vacancy in the ArXe2 cluster. In clusters with more Xe atoms, alternative electronic decay mechanisms may appear.
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36.40.Cg Electronic and magnetic properties of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.vn Electron correlation calculations for diatomic molecules
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

The dissociation of NO–Ar(math) from around threshold to 200 cm−1 above threshold

Heather L. Holmes-Ross and Warren D. Lawrance

J. Chem. Phys. 133, 014304 (2010); http://dx.doi.org/10.1063/1.3458911 (12 pages) | Cited 3 times

Online Publication Date: 7 July 2010

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We report an investigation of the dissociation of math state NO–Ar at energies from 23 cm−1 below the dissociation energy to 200 cm−1 above. The NO product rotational distributions show population in states that are not accessible with the energy available for excitation from the NO ground state. This effect is observed at photon energies from below the dissociation energy up to approximately 100 cm−1 above it. Translational energy distributions, extracted from velocity map images of individual rotational levels of the NO product, reveal contributions from excitation of high energy NO–Ar math states at all the excess energies probed, although this diminishes with increasing photon energy and is quite small at 200 cm−1, the highest energy studied. These translational energy distributions show that there are contributions arising from population in vibrational levels up to the math state dissociation energy. We propose that the reason such sparsely populated levels contribute to the observed dissociation is a considerable increase in the transition moment, via the Franck–Condon factor associated with these highly excited states, which arises because of the quite different geometries in the NO–Ar math and math states. This effect is likely to arise in other systems with similarly large geometry changes.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Real-time dissociation dynamics of the Ne2Br2 van der Waals complex

Jordan M. Pio, Molly A. Taylor, Wytze E. van der Veer, Craig R. Bieler, Jose A. Cabrera, and Kenneth C. Janda

J. Chem. Phys. 133, 014305 (2010); http://dx.doi.org/10.1063/1.3456550 (15 pages) | Cited 11 times

Online Publication Date: 7 July 2010

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We have characterized the vibrational predissociation (VP) of the Ne2Br2 van der Waals complex using time- and frequency-resolved pump-probe spectroscopy. After exciting Br2 within the complex to a vibrational level 16 ≤ ν′ ≤ 23 in the B state, we follow the flow of halogen vibrational energy to the van der Waals modes in real time by recording the time-dependent behavior of Ne2Br2 (ν′), the NeBr2 (ν′−m) intermediates, and the Br2 (ν′−n) products. For Ne2Br2 (ν′ = 16–18), the only intermediate observed is NeBr2 (ν′−1), and the majority of the final product is Br2 (ν′−2), indicating the dissociation happens via two sequential direct VP steps. We fit the time-dependent behavior of these species to a sequential mechanism and extracted time constants for each step. For higher ν′ levels, the results show that the dissociation occurs via multiple pathways. Product Br2 from levels lower than (ν′−2) becomes much more important, with products as low as (ν′−5) being observed. For ν′ = 21, we observe both NeBr2 (ν′−1) and (ν′−2) intermediates. The intermediates have significantly different kinetics, with the decay rate of the (ν′−1) transient being nearly twice that of the (ν′−2) transient. Similarly, both Br2 (ν′−2) and (ν′−3) are formed in almost equal amounts, but the (ν′−2) product formation rate is faster than the (ν′−3) rate. The broad vibrational product state distributions and multiple dissociation pathways indicate that intramolecular vibrational energy redistribution becomes increasingly important for ν′>19. We also report vibrational product state distributions for direct excitation to NeBr2 16 ≤ ν′ ≤ 23. For NeBr2, the dominant product channel is Br2 (ν′−1) for all initial ν′ studied, consistent with this complex dissociating primarily via direct VP.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
33.20.Tp Vibrational analysis

Sulfur K-edge photofragmentation of ethylene sulfide

W. C. Stolte and G. Öhrwall

J. Chem. Phys. 133, 014306 (2010); http://dx.doi.org/10.1063/1.3457946 (6 pages) | Cited 1 time

Online Publication Date: 7 July 2010

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We have investigated the photofragmentation properties of the three-membered ring heterocyclic molecule ethylene sulfide or thiirane, C2H4S, by time-of-flight mass spectroscopy. Positive ions have been collected as a function of photon energy around the S K ionization threshold. Branching ratios were derived for all detected ions, which are informative of the decay dynamics and photofragmentation patterns of the core-excited species. We present a new assignment of the spectral features around the S K-edge.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Ta Mass spectra

Photodissociation dynamics of acetylene via the math 1Πu electronic state

Yongwei Zhang, Kaijun Yuan, Shengrui Yu, David H. Parker, and Xueming Yang

J. Chem. Phys. 133, 014307 (2010); http://dx.doi.org/10.1063/1.3456738 (5 pages)

Online Publication Date: 7 July 2010

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Photodissociation of acetylene has been studied using the H-atom Rydberg tagging time-of-flight technique at two excitation wavelengths (148.35 and 151.82 nm) in the vacuum ultraviolet region. Product translational energy distributions have been obtained from the H-atom time-of-flight spectra. Experimental results indicate that the C2H product is mainly populated in the math state. Clear trans-bend ν2 and C–C stretch ν3 vibrational progressions of the C2H(math) product in the product internal energy distribution were observed. The anisotropy parameter obtained from experiment is clearly translational energy dependent for both photolysis wavelengths. The anisotropy parameters at the two photolysis wavelengths were also found to be significantly different from each other, suggesting different dissociation dynamics for the two photolysis wavelengths.
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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)
33.20.Lg Ultraviolet spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Comprehensive description of NMR cross-correlated relaxation under anisotropic molecular tumbling and correlated local dynamics on all time scales

Beat Vögeli

J. Chem. Phys. 133, 014501 (2010); http://dx.doi.org/10.1063/1.3454734 (13 pages) | Cited 1 time

Online Publication Date: 1 July 2010

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A simple general expression for the NMR cross-correlated relaxation rate under anisotropic molecular tumbling is presented for globular proteins. The derivation includes effects of fast and slow motion of the interaction tensors and correlation between them. Expressions suitable for practical analysis are tailored in dependence of standard order parameters of the individual interactions. It is shown that these order parameters must be sensitive to slow motion (slower than molecular tumbling) for detection of slow correlated motion. Such order parameters are those obtained from residual dipolar couplings but not those obtained from T1, T2, and heteronuclear Nuclear Overhauser Enhancement measurements.
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82.56.Pp NMR of biomolecules
87.15.-v Biomolecules: structure and physical properties

Direct observation of low frequency confined acoustic phonons in silver nanoparticles: Terahertz time domain spectroscopy

Sunil Kumar, N. Kamaraju, B. Karthikeyan, M. Tondusson, E. Freysz, and A. K. Sood

J. Chem. Phys. 133, 014502 (2010); http://dx.doi.org/10.1063/1.3456372 (4 pages)

Online Publication Date: 2 July 2010

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Terahertz time domain spectroscopy has been used to study low frequency confined acoustic phonons of silver nanoparticles embedded in poly(vinyl alcohol) matrix in the spectral range of 0.1–2.5 THz. The real and imaginary parts of the dielectric function show two bands at 0.60 and 2.12 THz attributed to the spheroidal and toroidal modes of silver nanoparticles, thus demonstrating the usefulness of terahertz time domain spectroscopy as a complementary technique to Raman spectroscopy in characterizing the nanoparticles.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
63.20.-e Phonons in crystal lattices

On the form of the kinetic energy balance equation in the kinetic variational theory

Yosyp A. Humenyuk and Mykhailo V. Tokarchuk

J. Chem. Phys. 133, 014503 (2010); http://dx.doi.org/10.1063/1.3447747 (5 pages)

Online Publication Date: 6 July 2010

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An alternative balance equation for the kinetic energy density used in the kinetic variational theory (KVT) is proposed. The new equation is consistent with the well-known standard form interpreted in terms of a flux and a source for both a one-component fluid and a mixture. Within the proposed version, the KVT mean-field collision term produces contributions to the heat flux and the source of kinetic energy being absent in the original formulation. It is shown that the introduced modification can affect the KVT thermal conductivity of the mixture while for the one-component fluid it becomes important only in the second and higher orders in gradients.
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51.20.+d Viscosity, diffusion, and thermal conductivity
05.20.Dd Kinetic theory
51.10.+y Kinetic and transport theory of gases

Hydrogen bond, electron donor-acceptor dimer, and residence dynamics in supercritical CO2-ethanol mixtures and the effect of hydrogen bonding on single reorientational and translational dynamics: A molecular dynamics simulation study

Ioannis Skarmoutsos, Elvira Guardia, and Jannis Samios

J. Chem. Phys. 133, 014504 (2010); http://dx.doi.org/10.1063/1.3449142 (13 pages) | Cited 2 times

Online Publication Date: 7 July 2010

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The hydrogen bonding and dynamics in a supercritical mixture of carbon dioxide with ethanol as a cosolvent (Xethanol ∼ 0.1) were investigated using molecular dynamics simulation techniques. The results obtained reveal that the hydrogen bonds formed between ethanol molecules are significantly more in comparison with those between ethanol-CO2 molecules and also exhibit much larger lifetimes. Furthermore, the residence dynamics in the solvation shells of ethanol and CO2 have been calculated, revealing much larger residence times for ethanol molecules in the ethanol solvation shell. These results support strongly the ethanol aggregation effects and the slow local environment reorganization inside the ethanol solvation shell, reported in a previous publication of the authors [ Skarmoutsos et al., J. Chem. Phys. 126, 224503 (2007) ]. The formation of electron donor-acceptor dimers between the ethanol and CO2 molecules has been also investigated and the calculated lifetimes of these complexes have been found to be similar to those corresponding to ethanol-CO2 hydrogen bonds, exhibiting a slightly higher intermittent lifetime. However, the average number of these dimers is larger than the number of ethanol-CO2 hydrogen bonds in the system. Finally, the effect of the hydrogen bonds formed between the individual ethanol molecules on their reorientational and translational dynamics has been carefully explored showing that the characteristic hydrogen bonding microstructure obtained exhibits sufficiently strong influence upon the behavior of them.
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82.30.Nr Association, addition, insertion, cluster formation
61.20.Ja Computer simulation of liquid structure
31.15.xv Molecular dynamics and other numerical methods
82.30.Rs Hydrogen bonding, hydrophilic effects
33.15.Fm Bond strengths, dissociation energies

Temperature dependence of vibrational frequency fluctuation of N3− in D2O

Jumpei Tayama, Akane Ishihara, Motohiro Banno, Kaoru Ohta, Shinji Saito, and Keisuke Tominaga

J. Chem. Phys. 133, 014505 (2010); http://dx.doi.org/10.1063/1.3428672 (11 pages) | Cited 3 times

Online Publication Date: 7 July 2010

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We have studied the temperature dependence of the vibrational frequency fluctuation of the antisymmetric stretching mode of N3− in D2O by three-pulse infrared (IR) photon echo experiments. IR pump-probe measurements were also carried out to investigate the population relaxation and the orientational relaxation of the same band. It was found that the time-correlation function (TCF) of the frequency fluctuation of this mode is well described by a biexponential function with a quasistatic term. The faster decay component has a time constant of about 0.1 ps, and the slower component varies from 1.4 to 1.1 ps in the temperature range from 283 to 353 K. This result indicates that liquid dynamics related to the frequency fluctuation are not highly sensitive to temperature. We discuss the relationship between the temperature dependence of the vibrational frequency fluctuation and that of the molecular motion of the system to investigate the molecular origin of the frequency fluctuation of the solute. We compare the temperature dependence of the frequency fluctuation with that of other dynamics such as dielectric relaxation of water. In contrast to the Debye dielectric relaxation time of D2O, the two time constants of the TCF of the frequency fluctuation do not exhibit strong temperature dependence. We propose a simple theoretical model for the frequency fluctuation in solutions based on perturbation theory and the dipole-dipole interaction between the vibrational mode of the solute and the solvent molecules. This model suggests that the neighboring solvent molecules in the vicinity of the solute play an important role in the frequency fluctuation. We suggest that the picosecond component of the frequency fluctuation results from structural fluctuation of the hydrogen-bonding network in water.
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64.75.Bc Solubility
61.20.Qg Structure of associated liquids: electrolytes, molten salts, etc.
61.25.Em Molecular liquids
78.40.Dw Liquids

Accurate modeling of fluorescence line narrowing difference spectra: Direct measurement of the single-site fluorescence spectrum

Mike Reppert, Virginia Naibo, and Ryszard Jankowiak

J. Chem. Phys. 133, 014506 (2010); http://dx.doi.org/10.1063/1.3455890 (9 pages) | Cited 1 time

Online Publication Date: 7 July 2010

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Accurate lineshape functions for modeling fluorescence line narrowing (FLN) difference spectra (ΔFLN spectra) in the low-fluence limit are derived and examined in terms of the physical interpretation of various contributions, including photoproduct absorption and emission. While in agreement with the earlier results of Jaaniso [Proc. Est. Acad. Sci., Phys., Math. 34, 277 (1985)] and Fünfschilling et al. [J. Lumin. 36, 85 (1986)] , the derived formulas differ substantially from functions used recently [e.g., M. Rätsep et al., Chem. Phys. Lett. 479, 140 (2009) ] to model ΔFLN spectra. In contrast to traditional FLN spectra, it is demonstrated that for most physically reasonable parameters, the ΔFLN spectrum reduces simply to the single-site fluorescence lineshape function. These results imply that direct measurement of a bulk-averaged single-site fluorescence lineshape function can be accomplished with no complicated extraction process or knowledge of any additional parameters such as site distribution function shape and width. We argue that previous analysis of ΔFLN spectra obtained for many photosynthetic complexes led to strong artificial lowering of apparent electron-phonon coupling strength, especially on the high-energy side of the pigment site distribution function.
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78.55.-m Photoluminescence, properties and materials
33.70.Jg Line and band widths, shapes, and shifts
33.50.Dq Fluorescence and phosphorescence spectra
63.20.kd Phonon-electron interactions

Internal motion of an electronically excited molecule in viscoelastic media

Ah-Young Jee, Eunhye Bae, and Minyung Lee

J. Chem. Phys. 133, 014507 (2010); http://dx.doi.org/10.1063/1.3454724 (8 pages) | Cited 2 times

Online Publication Date: 7 July 2010

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The twisting motion of trans-4-[4-(dimethylamino)-styryl]-1-methylpyridinium iodide (4-DASPI) in the excited state was investigated in solutions and various polymers in order to understand dependence of molecular rotor dynamics on viscoelasticity. It was observed that the internal motion of electronically excited 4-DASPI correlates strongly with dynamic viscosity and elastic modulus. Our results also showed that condensed phase dynamics of 4-DASPI are governed by the explicit mode coupling between the rotamerizing coordinate and mechanical properties of viscoelastic media.
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34.50.Gb Electronic excitation and ionization of molecules
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states
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