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7 Apr 2008

Volume 128, Issue 13, Articles (13xxxx)

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back to top Theoretical Methods and Algorithms

Adaptively biased molecular dynamics for free energy calculations

Volodymyr Babin, Christopher Roland, and Celeste Sagui

J. Chem. Phys. 128, 134101 (2008); http://dx.doi.org/10.1063/1.2844595 (7 pages) | Cited 20 times

Online Publication Date: 1 April 2008

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We present an adaptively biased molecular dynamics (ABMD) method for the computation of the free energy surface of a reaction coordinate using nonequilibrium dynamics. The ABMD method belongs to the general category of umbrella sampling methods with an evolving biasing potential and is inspired by the metadynamics method. The ABMD method has several useful features, including a small number of control parameters and an O(t) numerical cost with molecular dynamics time t. The ABMD method naturally allows for extensions based on multiple walkers and replica exchange, where different replicas can have different temperatures and/or collective variables. This is beneficial not only in terms of the speed and accuracy of a calculation, but also in terms of the amount of useful information that may be obtained from a given simulation. The workings of the ABMD method are illustrated via a study of the folding of the Ace-GGPGGG-Nme peptide in a gaseous and solvated environment.
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87.19.Pp Biothermics and thermal processes in biology
87.10.Tf Molecular dynamics simulation
87.15.Cc Folding: thermodynamics, statistical mechanics, models, and pathways

Correlation holes for the helium dimer

M. Piris, X. Lopez, and J. M. Ugalde

J. Chem. Phys. 128, 134102 (2008); http://dx.doi.org/10.1063/1.2883959 (7 pages) | Cited 15 times

Online Publication Date: 1 April 2008

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We have investigated the radial electron pair probability distributions (REPPDs) of the helium dimer within the Piris natural orbital functional (PNOF) theory. The analytical formulas to evaluate intracule densities, Fermi, Coulomb, and total correlation holes using our reconstruction functional PNOF-2 [ J. Chem. Phys. 126, 214103 (2007) ] are derived. The Löwdin’s Coulomb holes from PNOF-2 and full configuration interaction calculations are analyzed showing a very similar behavior. New definitions of the Coulomb and Fermi holes based on the cumulant expansion of the two-particle reduced density matrix are presented. The holes are defined in terms of the exact one-particle reduced density matrix and the two-particle cumulant without any reference to the Hartree–Fock state. Through these definitions, we analyze separately the contribution of each component to the total REPPD at several values of the internuclear distance. A straight connection between the Coulomb hole and dispersion interactions is observed.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.15.vn Electron correlation calculations for diatomic molecules
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations

Two-surface Monte Carlo with basin hopping: Quantum mechanical trajectory and multiple stationary points of water cluster

Pradipta Bandyopadhyay

J. Chem. Phys. 128, 134103 (2008); http://dx.doi.org/10.1063/1.2899020 (3 pages) | Cited 4 times

Online Publication Date: 1 April 2008

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The efficiency of the two-surface monte carlo (TSMC) method depends on the closeness of the actual potential and the biasing potential used to propagate the system of interest. In this work, it is shown that by combining the basin hopping method with TSMC, the efficiency of the method can be increased by several folds. TSMC with basin hopping is used to generate quantum mechanical trajectory and large number of stationary points of water clusters.
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61.20.Ja Computer simulation of liquid structure
61.25.Em Molecular liquids

Linear-scaling fixed-node diffusion quantum Monte Carlo: Accounting for the nodal information in a density matrix-based scheme

Jörg Kussmann and Christian Ochsenfeld

J. Chem. Phys. 128, 134104 (2008); http://dx.doi.org/10.1063/1.2884920 (8 pages) | Cited 2 times

Online Publication Date: 1 April 2008

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A reformulation of the fixed-node diffusion quantum Monte Carlo method (FN-DQMC) in terms of the N-particle density matrix is presented, which allows us to reduce the computational effort to linear for the evaluation of the local energy. The reformulation is based on our recently introduced density matrix-based approach for a linear-scaling variational QMC method [J. Kussmann et al., Phys. Rev. B. 75, 165107 (2007)] . However, within the latter approach of using the positive semi-definite N-particle trial density (ρNT(R) = ∣ΨT(R)∣2), the nodal information of the trial function is lost. Therefore, a straightforward application to the FN-DQMC method is not possible, in which the sign of the trial function is usually traced in order to confine the random walkers to their nodal pockets. As a solution, we reformulate the FN-DQMC approach in terms of off-diagonal elements of the N-particle density matrix ρNT(R;R′), so that the nodal information of the trial density matrix is obtained. Besides all-electron moves, a scheme to perform single-electron moves within N-PDM QMC is described in detail. The efficiency of our method is illustrated for exemplary calculations.
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02.70.Ss Quantum Monte Carlo methods
03.65.Yz Decoherence; open systems; quantum statistical methods
31.15.bt Statistical model calculations (including Thomas-Fermi and Thomas-Fermi-Dirac models)

Auxiliary density perturbation theory

Roberto Flores-Moreno and Andreas M. Köster

J. Chem. Phys. 128, 134105 (2008); http://dx.doi.org/10.1063/1.2842103 (10 pages) | Cited 9 times

Online Publication Date: 2 April 2008

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A new approach, named auxiliary density perturbation theory, for the calculation of second energy derivatives is presented. It is based on auxiliary density functional theory in which the Coulomb and exchange-correlation potentials are expressed by auxiliary function densities. Different to conventional coupled perturbed Kohn–Sham equations the perturbed density matrix is obtained noniteratively by solving an inhomogeneous equation system with the dimension of the auxiliary function set used to expand the auxiliary function density. A prototype implementation for the analytic calculation of molecular polarizabilities is presented. It is shown that the polarizabilities obtained with the newly developed auxiliary density perturbation approach match quantitative with the ones from standard density functional theory if augmented auxiliary function sets are used. The computational advantages of auxiliary density perturbation theory are discussed, too.
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33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.xp Perturbation theory
31.15.eg Exchange-correlation functionals (in current density functional theory)

Optimization methods for finding minimum energy paths

Daniel Sheppard, Rye Terrell, and Graeme Henkelman

J. Chem. Phys. 128, 134106 (2008); http://dx.doi.org/10.1063/1.2841941 (10 pages) | Cited 81 times

Online Publication Date: 2 April 2008

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A comparison of chain-of-states based methods for finding minimum energy pathways (MEPs) is presented. In each method, a set of images along an initial pathway between two local minima is relaxed to find a MEP. We compare the nudged elastic band (NEB), doubly nudged elastic band, string, and simplified string methods, each with a set of commonly used optimizers. Our results show that the NEB and string methods are essentially equivalent and the most efficient methods for finding MEPs when coupled with a suitable optimizer. The most efficient optimizer was found to be a form of the limited-memory Broyden-Fletcher-Goldfarb-Shanno method in which the approximate inverse Hessian is constructed globally for all images along the path. The use of a climbing-image allows for finding the saddle point while representing the MEP with as few images as possible. If a highly accurate MEP is desired, it is found to be more efficient to descend from the saddle to the minima than to use a chain-of-states method with many images. Our results are based on a pairwise Morse potential to model rearrangements of a heptamer island on Pt(111), and plane-wave based density functional theory to model a rollover diffusion mechanism of a Pd tetramer on MgO(100) and dissociative adsorption and diffusion of oxygen on Au(111).
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82.20.-w Chemical kinetics and dynamics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Dissipation of classical energy in nonlinear quantum systems

Andrey Pereverzev, Yuriy V. Pereverzev, and Oleg V. Prezhdo

J. Chem. Phys. 128, 134107 (2008); http://dx.doi.org/10.1063/1.2844597 (8 pages) | Cited 3 times

Online Publication Date: 3 April 2008

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We show using two simple nonlinear quantum systems that the infinite set of quantum dynamical variables, as introduced in quantized Hamilton dynamics [ O. V. Prezhdo and Y. V. Pereverzev, J. Chem. Phys. 113, 6557 (2000) ], behave as a thermostat with respect to the finite number of classical variables. The coherent classical component of the evolution decays by coupling to the chaotic quantum reservoir. The classical energy, understood as the part of system energy expressible through the average values of coordinates and momenta, is transferred to the quantum energy expressible through the higher moments of coordinates and momenta and other quantum variables. At long times, the classical variables reach equilibrium, and the classical energy fluctuates around the equilibrium value. These phenomena are illustrated with the exactly solvable Jaynes–Cummings model and a nonlinear oscillator.
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03.65.Sq Semiclassical theories and applications
05.45.Mt Quantum chaos; semiclassical methods
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency

Projected gradient algorithms for Hartree-Fock and density matrix functional theory calculations

Eric Cancès and Katarzyna Pernal

J. Chem. Phys. 128, 134108 (2008); http://dx.doi.org/10.1063/1.2888550 (8 pages) | Cited 6 times

Online Publication Date: 3 April 2008

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We present projected gradient algorithms designed for optimizing various functionals defined on the set of N-representable one-electron reduced density matrices. We show that projected gradient algorithms are efficient in minimizing the Hartree-Fock or the Müller-Buijse-Baerends functional. On the other hand, they converge very slowly when applied to the recently proposed BBk (k = 1,2,3) functionals [ O. Gritsenko et al., J. Chem. Phys. 122, 204102 (2005) ]. This is due to the fact that the BBk functionals are not proper functionals of the density matrix.
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31.15.xr Self-consistent-field methods
31.15.E- Density-functional theory

Boltzmann bias grand canonical Monte Carlo

G. Garberoglio

J. Chem. Phys. 128, 134109 (2008); http://dx.doi.org/10.1063/1.2883683 (10 pages) | Cited 6 times

Online Publication Date: 4 April 2008

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We derive an efficient method for the insertion of structured particles in grand canonical Monte Carlo simulations of adsorption in very confining geometries. We extend this method to path integral simulations and use it to calculate the isotherm of adsorption of hydrogen isotopes in narrow carbon nanotubes (two-dimensional confinement) and slit pores (one-dimensional confinement) at the temperatures of 20 and 77 K, discussing its efficiency by comparison to the standard path integral grand canonical Monte Carlo algorithm. We use this algorithm to perform multicomponent simulations in order to calculate the hydrogen isotope selectivity for adsorption in narrow carbon nanotubes and slit pores at finite pressures. The algorithm described here can be applied to the study of adsorption of real oligomers and polymers in narrow pores and channels.
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68.43.-h Chemisorption/physisorption: adsorbates on surfaces
61.46.Fg Nanotubes

Benchmarks for electronically excited states: CASPT2, CC2, CCSD, and CC3

Marko Schreiber, Mario R. Silva-Junior, Stephan P. A. Sauer, and Walter Thiel

J. Chem. Phys. 128, 134110 (2008); http://dx.doi.org/10.1063/1.2889385 (25 pages) | Cited 104 times

Online Publication Date: 7 April 2008

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A benchmark set of 28 medium-sized organic molecules is assembled that covers the most important classes of chromophores including polyenes and other unsaturated aliphatic compounds, aromatic hydrocarbons, heterocycles, carbonyl compounds, and nucleobases. Vertical excitation energies and one-electron properties are computed for the valence excited states of these molecules using both multiconfigurational second-order perturbation theory, CASPT2, and a hierarchy of coupled cluster methods, CC2, CCSD, and CC3. The calculations are done at identical geometries (MP2/6-31G*) and with the same basis set (TZVP). In most cases, the CC3 results are very close to the CASPT2 results, whereas there are larger deviations with CC2 and CCSD, especially in singlet excited states that are not dominated by single excitations. Statistical evaluations of the calculated vertical excitation energies for 223 states are presented and discussed in order to assess the relative merits of the applied methods. CC2 reproduces the CC3 reference data for the singlets better than CCSD. On the basis of the current computational results and an extensive survey of the literature, we propose best estimates for the energies of 104 singlet and 63 triplet excited states.
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31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory
33.15.Bh General molecular conformation and symmetry; stereochemistry

Self-adaptive enhanced sampling in the energy and trajectory spaces: Accelerated thermodynamics and kinetic calculations

Yi Qin Gao

J. Chem. Phys. 128, 134111 (2008); http://dx.doi.org/10.1063/1.2901037 (8 pages) | Cited 6 times

Online Publication Date: 7 April 2008

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Here, we introduce a simple self-adaptive computational method to enhance the sampling in energy, configuration, and trajectory spaces. The method makes use of two strategies. It first uses a non-Boltzmann distribution method to enhance the sampling in the phase space, in particular, in the configuration space. The application of this method leads to a broad energy distribution in a large energy range and a quickly converged sampling of molecular configurations. In the second stage of simulations, the configuration space of the system is divided into a number of small regions according to preselected collective coordinates. An enhanced sampling of reactive transition paths is then performed in a self-adaptive fashion to accelerate kinetics calculations.
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82.20.Wt Computational modeling; simulation
82.60.-s Chemical thermodynamics
82.20.Pm Rate constants, reaction cross sections, and activation energies
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

State-selected imaging of HCCO radical photodissociation dynamics

Cunshun Huang, Armando D. Estillore, and Arthur G. Suits

J. Chem. Phys. 128, 134301 (2008); http://dx.doi.org/10.1063/1.2831788 (6 pages) | Cited 2 times

Online Publication Date: 1 April 2008

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We present a dc sliced ion imaging study of HCCO radical photodissociation to CH and CO at 230 nm. The measurements were made using a two-color reduced Doppler probe strategy. The CO rotational distribution was consistent with a Boltzmann distribution at 3500 K. Using the dc slice ion imaging approach, we obtained CO images for various rotational levels of CO (v = 0). The results are largely consistent with earlier work, albeit with a significant 0.9 eV peak seen previously in the translational energy distributions absent in our state-selected imaging study.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis

Transport of O+ through argon gas

Daniel M. Danailov, Larry A. Viehland, Rainer Johnsen, Timothy G. Wright, and Alan S. Dickinson

J. Chem. Phys. 128, 134302 (2008); http://dx.doi.org/10.1063/1.2898523 (9 pages) | Cited 6 times

Online Publication Date: 1 April 2008

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New experimental and theoretical results are presented that address the movement of O+ ions through argon gas. On the experimental front, improved ion mobility results are presented. These results confirm the presence of the oft-cited mobility minimum as a function of electrostatic field strength at room temperature. On the theoretical side, high-level ab initio potential energy curves are calculated for the Ar–O+ system and, from these, transport properties are calculated and compared to experiment. A crossing between the lowest 2Π curve and the ground state 4Σ− curve near the minimum of each potential becomes an avoided crossing on the inclusion of spin-orbit coupling. It is shown that the more appropriate potential for the description of the motion of O+(4S∘) through Ar at the energies of interest is the diabatic potential, neglecting fine structure. By using an improved 4Σ− potential, agreement with the mobility measurements is obtained for low and intermediate electrostatic field strengths, although small discrepancies remain for high field strengths. The appropriate choice of diabatic or adiabatic potentials is also considered for related systems of interest: He–O+, Ne–O+, and Rg–O− (Rg = He,Ne,Ar).
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51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)
31.15.A- Ab initio calculations
31.50.Bc Potential energy surfaces for ground electronic states
31.50.-x Potential energy surfaces

Steering dissociation of Br2 molecules with two femtosecond pulses via wave packet interference

Yong-Chang Han, Kai-Jun Yuan, Wen-Hui Hu, Tian-Min Yan, and Shu-Lin Cong

J. Chem. Phys. 128, 134303 (2008); http://dx.doi.org/10.1063/1.2844792 (9 pages) | Cited 3 times

Online Publication Date: 1 April 2008

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The dissociation dynamics of Br2 molecules induced by two femtosecond pump pulses are studied based on the calculation of time-dependent quantum wave packet. Perpendicular transition from X 1Σg+ to A 3Π1u+ and 1Π1u+ and parallel transition from X 1Σg+ to B 3Π0u+, involving two product channels Br (2P3/2)+Br (2P3/2) and Br (2P3/2)+Br* (2P1/2), respectively, are taken into account. Two pump pulses create dissociating wave packets interfering with each other. By varying laser parameters, the interference of dissociating wave packets can be controlled, and the dissociation probabilities of Br2 molecules on the three excited states can be changed to different degrees. The branching ratio of Br*/(Br+Br*) is calculated as a function of pulse delay time and phase difference.
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82.37.Np Single molecule reaction kinetics, dissociation, etc.
31.50.Df Potential energy surfaces for excited electronic states

Including anharmonicity in the calculation of rate constants. II. The OH+H2→H2O+H reaction

Alan D. Isaacson

J. Chem. Phys. 128, 134304 (2008); http://dx.doi.org/10.1063/1.2834934 (14 pages) | Cited 1 time

Online Publication Date: 1 April 2008

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A recently developed method for calculating anharmonic vibrational energy levels at nonstationary points along a reaction path that is based on second-order perturbation theory in curvilinear coordinates is combined with variational transition state theory with semiclassical multidimensional tunneling approximations to calculate thermal rate constants for the title reaction. Two different potential energy surfaces were employed for these calculations, an improved version of the author’s surface 5 and the WSLFH surface of Wu et al. [J. Chem. Phys. 113, 3150 (2000) ]. We present detailed comparisons of rate constants computed for the two surfaces with and without anharmonicity and with various approximations for incorporating tunneling along the reaction path. The results for this system are quite sensitive to the surface employed, the choice of coordinates (curvilinear versus rectilinear), and the inclusion of anharmonicity. A comparison with experiment provides information on the accuracy of these surfaces.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
34.20.Gj Intermolecular and atom-molecule potentials and forces
31.15.xp Perturbation theory
31.15.xt Variational techniques

Properties and long range interactions of the calcium atom

J. Mitroy and J.-Y. Zhang

J. Chem. Phys. 128, 134305 (2008); http://dx.doi.org/10.1063/1.2841470 (11 pages) | Cited 5 times

Online Publication Date: 2 April 2008

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The properties of a number of states of calcium are determined from a large basis configuration interaction calculation. The main focus is on the polarizabilities of the low lying states (the 4s2 math, 4s3d math, 4s4p math, and 4s5s math states) and the dispersion interactions of those states with the calcium ground state, the hydrogen atom, and the rare gases.
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32.30.-r Atomic spectra
32.70.Cs Oscillator strengths, lifetimes, transition moments

Electron emission from laser-heated fullerene dianions: Probing the repulsive Coulomb barrier

Bruno Concina, Marco Neumaier, Oliver Hampe, and Manfred M. Kappes

J. Chem. Phys. 128, 134306 (2008); http://dx.doi.org/10.1063/1.2842145 (9 pages) | Cited 5 times

Online Publication Date: 2 April 2008

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Electron detachment from fullerene dianions Cm2− (m = 76,84) stored in a room temperature Penning trap was probed upon pulsed laser excitation at wavelengths of 355 and 532 nm. The fraction of Cm2− surviving trapping times exceeding tens of milliseconds under UHV conditions, as well as the fraction of singly charged anions Cm− generated were recorded as a function of the laser fluence. Analysis by means of Poisson statistics yields absolute absorption cross sections and the number of photons necessary to induce the detachment. The cross sections obtained are in good agreement with the literature values. By describing the electron detachment as a statistical unimolecular process, we deduce effective activation energies from the number of photons required. These energies are compared to the sum of the second electron affinity and the Coulomb barrier height as calculated from an electrostatic charging model.
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36.40.-c Atomic and molecular clusters
33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Spin-orbit effects in the photoabsorption of WAu12 and MoAu12: A relativistic time dependent density functional study

M. Stener, A. Nardelli, and G. Fronzoni

J. Chem. Phys. 128, 134307 (2008); http://dx.doi.org/10.1063/1.2884003 (9 pages) | Cited 5 times

Online Publication Date: 2 April 2008

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The electronic structure of both WAu12 and MoAu12 has been calculated at the density functional theory (DFT) level, employing the zero order regular approximation at the scalar relativistic level and including a spin-orbit coupling. The effect of the inclusion of the spin-orbit coupling is discussed, and the differences assigned to the nature of the encaged atom (W or Mo) are identified. Then, the excitation spectra of both clusters are calculated at the time-dependent DFT level, also in this case at both scalar relativistic and spin-orbit levels. The inclusion of spin-orbit coupling is mandatory for an accurate description in the low energy region. At higher energy, where the density of states is higher, the convoluted intensity can be properly described already at the scalar relativistic level. The consequences of the spin-orbit coupling on the excitation spectrum of the clusters indicate that while in WAu12 the lowest excitations are essentially shifted in energy with respect to the scalar relativistic results, in MoAu12, a dramatic splitting in many lines is actually predicted, revealing a quite different behavior of the two clusters.
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71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
71.20.Be Transition metals and alloys
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Dissociative recombination of OPCl+ and OPCl2+: Pushing the upper mass limit at CRYRING

V. Zhaunerchyk, R. D. Thomas, W. D. Geppert, M. Hamberg, M. Kamińska, E. Vigren, M. Larsson, A. J. Midey, and A. A. Viggiano

J. Chem. Phys. 128, 134308 (2008); http://dx.doi.org/10.1063/1.2884924 (7 pages)

Online Publication Date: 3 April 2008

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The dissociative recombination of OPCl+ and OPCl2+ has been studied at the storage ring CRYRING. The rate constants as a function of electron temperature have been derived to be 7.63×10−7(Te/300)−0.89 and >1.2×10−6(Te/300)−1.22 cm3 s−1, respectively. The lower limit quoted for the latter rate constant reflects the experimental inability to detect all of the reaction products. The branching fractions from the reaction have been measured for OPCl+ at ≈ 0 eV interaction energy and are determined to be N(O+P+Cl) = (16±7)%, N(O+PCl) = (16±3)% and N(OP+Cl) = (68±5)%. These values have been obtained assuming that the rearrangement channel forming P+ClO is negligible, and ab initio calculations using GAUSSIAN03 are presented for the ion structures and energetics to support such an assumption. Finally, the limitations to using heavy ion storage rings such as CRYRING for studies into the dissociative recombination of large singly charged molecular ions are discussed.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Photofragmentation of SiF4 upon Si 2p and F 1s core excitation: Cation and anion yield spectroscopy

M. N. Piancastelli, W. C. Stolte, R. Guillemin, A. Wolska, and D. W. Lindle

J. Chem. Phys. 128, 134309 (2008); http://dx.doi.org/10.1063/1.2851135 (6 pages) | Cited 4 times

Online Publication Date: 4 April 2008

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We have studied the fragmentation dynamics of core-excited SiF4 by means of soft-x-ray photoexcitation and partial positive and negative ion yield measurements around the Si L2,3-shell and F K-shell ionization thresholds. All detectable ionic fragments are reported and we observe significant differences between the various partial ion yields near the Si 2p threshold. The differences are similar to our previous results from CH3Cl showing more extended fragmentation in correspondence to transitions to Rydberg states. At variance with smaller systems, we observe negative ion production in the shape resonance region. This can be related to the possibility in a relatively large system to dissipate positive charge over several channels.
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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)
32.80.Ee Rydberg states
82.37.Vb Single molecule photochemistry

Molecular beam resonant two-photon ionization study of caffeine and its hydrated clusters

Doory Kim, Hyung Min Kim, Key Young Yang, Seong Keun Kim, and Nam Joon Kim

J. Chem. Phys. 128, 134310 (2008); http://dx.doi.org/10.1063/1.2844806 (6 pages) | Cited 4 times

Online Publication Date: 4 April 2008

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We investigated electronically excited states of caffeine and its 1:1 complex with water by using resonant two-photon ionization (R2PI) and UV-UV hole-burning techniques. Strong vibronic coupling between a pair of close-lying π-π* and n-π* transitions is proposed to be responsible for the broad spectral feature observed. By comparing the experimental results with those of theoretical calculations, both the O-bonded and N-bonded forms were suggested to be initially produced for the 1:1 complex. The electronic transitions of the O-bonded complex were blueshifted in the R2PI spectrum. For the N-bonded complex, the excited state undergoes an ultrafast decay process, followed by dissociation on a repulsive potential energy surface, which gives rise to a characteristically anomalous cluster distribution in nanosecond experiments.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
33.70.Jg Line and band widths, shapes, and shifts

Product state resolved excitation spectroscopy of He–, Ne–, and Ar–Br2 linear isomers: Experiment and theory

Jordan M. Pio, Wytze E. van der Veer, Craig R. Bieler, and Kenneth C. Janda

J. Chem. Phys. 128, 134311 (2008); http://dx.doi.org/10.1063/1.2885047 (10 pages) | Cited 14 times

Online Publication Date: 7 April 2008

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Valence excitation spectra for the linear isomers of He–, Ne–, and Ar–Br2 are reported and compared to a two-dimensional simulation using the currently available potential energy surfaces. Excitation spectra from the ground electronic state to the region of the inner turning point of the Rg–Br2 (B,ν′) stretching coordinate are recorded while probing the asymptotic Br2 (B,ν′) state. Each spectrum is a broad continuum extending over hundreds of wavenumbers, becoming broader and more blueshifted as the rare gas atom is changed from He to Ne to Ar. In the case of Ne–Br2, the threshold for producing the asymptotic product state reveals the X-state linear isomer bond energy to be 71±3 cm−1. The qualitative agreement between experiment and theory shows that the spectra can be correctly regarded as revealing the one-atom solvent shifts and also provides new insight into the one-atom cage effect on the halogen vibrational relaxation. The measured spectra provide data to test future ab initio potential energy surfaces in the interaction of rare gas atoms with the halogen valence excited state.
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33.70.Jg Line and band widths, shapes, and shifts
31.50.Df Potential energy surfaces for excited electronic states
31.50.Bc Potential energy surfaces for ground electronic states
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

The homogeneous nucleation of 1-pentanol in a laminar flow diffusion chamber: The effect of pressure and kind of carrier gas

D. Brus, A.-P. Hyvärinen, J. Wedekind, Y. Viisanen, M. Kulmala, V. Ždímal, J. Smolík, and H. Lihavainen

J. Chem. Phys. 128, 134312 (2008); http://dx.doi.org/10.1063/1.2901049 (7 pages) | Cited 6 times

Online Publication Date: 7 April 2008

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The influence of total pressure and kind of carrier gas on homogeneous nucleation rates of 1-pentanol was investigated using experimental method of laminar flow diffusion chamber in this study. Two different carrier gases (helium and argon) were used in the total pressure range from 50 to 400 kPa. Nucleation temperatures ranged from 265 to 290 K for 1-pentanol-helium and from 265 to 285 K for 1-pentanol-argon. Nucleation rates varied between 101 and 106 cm−3 s−1 for 1-pentanol-helium and between 102 and 105 cm−3 s−1 for 1-pentanol-argon. Both positive and slight negative pressure effects were observed depending on temperature and carrier gas. The trend of pressure effect was found similar for both carrier gases. Error analysis on thermodynamic properties was conducted, and the lowering of surface tension due to adsorption of argon on nucleated droplets was estimated. A quantitative overview of pressure effect is provided.
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66.10.-x Diffusion and ionic conduction in liquids
65.20.Jk Studies of thermodynamic properties of specific liquids
68.03.Cd Surface tension and related phenomena
68.03.Fg Evaporation and condensation of liquids

Interactions of the 3pπu c1Πu(v = 2) Rydberg-complex member in isotopic N2

M. O. Vieitez, T. I. Ivanov, C. A. de Lange, W. Ubachs, A. N. Heays, B. R. Lewis, and G. Stark

J. Chem. Phys. 128, 134313 (2008); http://dx.doi.org/10.1063/1.2883955 (11 pages) | Cited 3 times

Online Publication Date: 7 April 2008

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The 3pπu c1Πu−X 1Σg+(2,0) Rydberg and b′ 1Σu+−X 1Σg+(7,0) valence transitions of 14N2, 14N15N, and 15N2 are studied using laser-based 1 extreme ultraviolet (XUV)+1′ UV two-photon-ionization spectroscopy, supplemented by synchrotron-based photoabsorption measurements in the case of 14N2. For each isotopomer, effective rotational interactions between the c(v = 2) and b′(v = 7) levels are found to cause strong Λ-doubling in c(v = 2) and dramatic P/R-branch intensity anomalies in the b′−X(7,0) band due to the effects of quantum interference. Local perturbations in energy and predissociation line width for the c(v = 2) Rydberg level are observed and attributed to a spin-orbit interaction with the crossing, short-lived C 3Πu(v = 17) valence level.
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33.80.Be Level crossing and optical pumping
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.20.Lg Ultraviolet spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.70.Jg Line and band widths, shapes, and shifts
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

On the computation and contribution of conductivity in molecular ionic liquids

C. Schröder, M. Haberler, and O. Steinhauser

J. Chem. Phys. 128, 134501 (2008); http://dx.doi.org/10.1063/1.2868752 (10 pages) | Cited 25 times

Online Publication Date: 1 April 2008

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In this study we present the results of the molecular dynamics simulation of the ionic liquids: 1-butyl-3-methyl-imidazolium tetrafluoroborate and trifluoromethylacetate as well as 1-ethyl-3-methyl-imidazolium dicyanamide. Ionic liquids are characterized by both a molecular dipole moment and a net charge. Thus, in contrast to a solution of simple ions in a (non-) polar solvent, rotational and translational effects influence the very same molecule. This study works out the theoretical framework necessary to compute the conductivity spectrum and its low frequency limit of ionic liquids. Merging these computed conductivity spectra with previous simulation results on the dielectric spectra of ionic liquids yields the spectrum of the generalized dielectric constant, which may be compared to experiments. This spectrum was calculated for the three ionic liquids over six orders of magnitude in frequency ranging from 10 MHz to 50 THz. The role of rotation and translation and their coupling term on the generalized dielectric constant is discussed in detail with a special emphasis on the zero-frequency limit. Thereby, the frequency dependence of the cross correlation between the collective rotational dipole moment and the current is discussed.
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66.10.Ed Ionic conduction
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
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