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28 Oct 2006

Volume 125, Issue 16, Articles (16xxxx)

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Bulk photochromism in a tungstate-phosphate glass: A new optical memory material?

Gaël Poirier, Marcelo Nalin, Lucila Cescato, Younes Messaddeq, and Sidney J. L. Ribeiro

J. Chem. Phys. 125, 161101 (2006); http://dx.doi.org/10.1063/1.2364476 (3 pages) | Cited 10 times

Online Publication Date: 30 October 2006

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In this work, we present a new photochromic tungstate based glass which have both absorption coefficient and refractive index modified under laser exposure. The photosensitive effect is superficial under ultraviolet (UV) irradiation but occurs in the entire volume of the glass under visible irradiation. The effect can be obtained in any specific point inside the volume using an infrared femtosecond laser. In addition, the photosensitive phenomenon can be erased by specific heat treatment. This glass can be useful to substitute actual data storage supports and is a promising material for 3-dimensional (3D) and holographic optical storage.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.47.-p Spectroscopy of solid state dynamics
42.70.Ce Glasses, quartz

Solvent effects on the three-photon absorption cross-section of a highly conjugated fluorene derivative

Ion Cohanoschi, Kevin D. Belfield, Carlos Toro, and Florencio E. Hernández

J. Chem. Phys. 125, 161102 (2006); http://dx.doi.org/10.1063/1.2370750 (4 pages)

Online Publication Date: 30 October 2006

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Herein, we report the study of the three-photon absorption cross-section dependence on solvents parameters for a highly conjugated organic dye, 2,2′-(4,4′-(1E,1′E)-2,2′-(9,9-didecyl-9H-fluorene-2,7-diyl) bis(ethene-2,1-diyl)bis(4,1-phenylene))dibenzo[d]thiazole (A-π-π-π-A). The three-photon absorption cross-section was measured for this organic dye in solution in four different solvents with polarity function, Δf between 0.162 and 0.247. The experiments show how the solvent’s reorientation of the electrons and polarity contribute to the 3PA cross-section. Multiphoton-absorption experiments of A-π-π-π-A in all four different solvents were performed with a tunable OPG pumped by a 25 picosecond Nd-YAG laser.
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61.25.Em Molecular liquids
78.55.-m Photoluminescence, properties and materials
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back to top Theoretical Methods and Algorithms

Crooks equation for steered molecular dynamics using a Nosé-Hoover thermostat

Piero Procacci, Simone Marsili, Alessandro Barducci, Giorgio F. Signorini, and Riccardo Chelli

J. Chem. Phys. 125, 164101 (2006); http://dx.doi.org/10.1063/1.2360273 (9 pages) | Cited 17 times

Online Publication Date: 23 October 2006

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The Crooks equation [Eq. (10) in J. Stat. Phys. 90, 1481 (1998) ], originally derived for microscopically reversible Markovian systems, relates the work done on a system during an irreversible transformation to the free energy difference between the final and the initial state of the transformation. In the present work we provide a theoretical proof of the Crooks equation in the context of constant volume, constant temperature steered molecular dynamics simulations of systems thermostated by means of the Nosé-Hoover method (and its variant using a chain of thermostats). As a numerical test we use the folding and unfolding processes of decaalanine in vacuo at finite temperature. We show that the distribution of the irreversible work for the folding process is markedly non-Gaussian thereby implying, according to Crooks equation, that also the work distribution of the unfolding process must be inherently non-Gaussian. The clearly asymmetric behavior of the forward and backward irreversible work distributions is a signature of a non-Markovian regime for the folding/unfolding of decaalanine.
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05.70.Ln Nonequilibrium and irreversible thermodynamics
05.70.Ce Thermodynamic functions and equations of state
87.15.Cc Folding: thermodynamics, statistical mechanics, models, and pathways
87.19.Pp Biothermics and thermal processes in biology

Periodic boundary condition induced breakdown of the equipartition principle and other kinetic effects of finite sample size in classical hard-sphere molecular dynamics simulation

Randall B. Shirts, Scott R. Burt, and Aaron M. Johnson

J. Chem. Phys. 125, 164102 (2006); http://dx.doi.org/10.1063/1.2359432 (9 pages) | Cited 8 times

Online Publication Date: 24 October 2006

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We examine consequences of the non-Boltzmann nature of probability distributions for one-particle kinetic energy, momentum, and velocity for finite systems of classical hard spheres with constant total energy and nonidentical masses. By comparing two cases, reflecting walls (NVE or microcanonical ensemble) and periodic boundaries (NVEPG or molecular dynamics ensemble), we describe three consequences of the center-of-mass constraint in periodic boundary conditions: the equipartition theorem no longer holds for unequal masses, the ratio of the average relative velocity to the average velocity is increased by a factor of [N/(N−1)]1/2, and the ratio of average collision energy to average kinetic energy is increased by a factor of N/(N−1). Simulations in one, two, and three dimensions confirm the analytic results for arbitrary dimension.
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61.20.Ja Computer simulation of liquid structure
61.20.Gy Theory and models of liquid structure

Inclusion of inversion symmetry in centroid molecular dynamics: A possible avenue to recover quantum coherence

Yoonjung Huh and Pierre-Nicholas Roy

J. Chem. Phys. 125, 164103 (2006); http://dx.doi.org/10.1063/1.2358989 (10 pages) | Cited 1 time

Online Publication Date: 25 October 2006

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Inversion symmetry is included in the operator formulation of the centroid molecular dynamics (CMD). This work involves the development of a symmetry-adapted CMD (SA-CMD), here particularly for symmetrization and antisymmetrization projections. A symmetry-adapted quasidensity operator, as defined by Blinov and Roy [J. Chem. Phys. 115, 7822 (2001)] , is employed to obtain the centroid representation of quantum mechanical operators. Numerical examples are given for a single particle confined to one-dimensional symmetric quartic and symmetric double-well potentials. Two SA-CMD simulations are performed separately for both projections, and centroid position autocorrelation functions are obtained. For each projection, the quality of the approximation as well as the accuracy are similar to those of regular CMD. It is shown that individual trajectories from two separate SA-CMD simulations can be properly combined to recover trajectories for Boltzmann statistics. Position autocorrelation functions are compared to the exact quantum mechanical ones. This explicit account of inversion symmetry provides a qualitative improvement on the conventional CMD approach and allows the recovery of some quantum coherence.
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03.65.Fd Algebraic methods
03.65.Yz Decoherence; open systems; quantum statistical methods

Forward-backward semiclassical initial value series representation of quantum correlation functions

Eva Martin-Fierro and Eli Pollak

J. Chem. Phys. 125, 164104 (2006); http://dx.doi.org/10.1063/1.2358985 (13 pages) | Cited 20 times

Online Publication Date: 25 October 2006

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The forward-backward (FB) approximation as applied to semiclassical initial value representations (SCIVR’s) has enabled the practical application of the SCIVR methodology to systems with many degrees of freedom. However, to date a systematic representation of the exact quantum dynamics in terms of the FB-SCIVR has proven elusive. In this paper, we provide a new derivation of a forward-backward phase space SCIVR expression (FBPS-SCIVR) derived previously by Thompson and Makri [Phys. Rev. E 59, R4729 (1999) ]. This enables us to represent quantum correlation functions exactly in terms of a series whose leading order term is the FBPS-SCIVR expression. Numerical examples for systems with over 50 degrees of freedom are presented for the spin boson problem. Comparison of the FBPS-SCIVR with the numerically exact results of Wang [J. Chem. Phys. 113, 9948 (2000) ] obtained using a multiconfigurational time dependent method shows that the leading order FBPS-SCIVR term already provides an excellent approximation.
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03.65.Sq Semiclassical theories and applications

Extracting electron transfer coupling elements from constrained density functional theory

Qin Wu and Troy Van Voorhis

J. Chem. Phys. 125, 164105 (2006); http://dx.doi.org/10.1063/1.2360263 (9 pages) | Cited 41 times

Online Publication Date: 26 October 2006

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Constrained density functional theory (DFT) is a useful tool for studying electron transfer (ET) reactions. It can straightforwardly construct the charge-localized diabatic states and give a direct measure of the inner-sphere reorganization energy. In this work, a method is presented for calculating the electronic coupling matrix element (Hab) based on constrained DFT. This method completely avoids the use of ground-state DFT energies because they are known to irrationally predict fractional electron transfer in many cases. Instead it makes use of the constrained DFT energies and the Kohn-Sham wave functions for the diabatic states in a careful way. Test calculations on the Zn2+ and the benzene-Cl atom systems show that the new prescription yields reasonable agreement with the standard generalized Mulliken-Hush method. We then proceed to produce the diabatic and adiabatic potential energy curves along the reaction pathway for intervalence ET in the tetrathiafulvalene-diquinone (Q-TTF-Q) anion. While the unconstrained DFT curve has no reaction barrier and gives Hab ≈ 17 kcal/mol, which qualitatively disagrees with experimental results, the Hab calculated from constrained DFT is about 3 kcal/mol and the generated ground state has a barrier height of 1.70 kcal/mol, successfully predicting (Q-TTF-Q)− to be a class II mixed-valence compound.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Kh Potential energy surfaces for chemical reactions

Relativistic calculation of nuclear magnetic shielding tensor including two-electron spin-orbit interactions

Y. Ootani, H. Yamaguti, H. Maeda, and H. Fukui

J. Chem. Phys. 125, 164106 (2006); http://dx.doi.org/10.1063/1.2361292 (4 pages) | Cited 6 times

Online Publication Date: 27 October 2006

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A relativistic calculation of nuclear magnetic shielding tensor including two-electron spin-orbit interactions is performed. In order to reduce the computational load in evaluating the two-electron relativistic integrals, the charge density is approximated by a linear combination of the squares of s-type spatial basis functions. Including the two-electron spin-orbit interaction effect is found to improve the calculation results.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
33.25.+k Nuclear resonance and relaxation

A truncated version of reduced multireference coupled-cluster method with singles and doubles and noniterative triples: Application to F2 and Ni(CO)n (n = 1, 2, and 4)

Xiangzhu Li and Josef Paldus

J. Chem. Phys. 125, 164107 (2006); http://dx.doi.org/10.1063/1.2361295 (12 pages) | Cited 26 times

Online Publication Date: 27 October 2006

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A perturbatively truncated version of the reduced multireference coupled-cluster method with singles and doubles and noniterative triples RMR CCSD(T) is described. In the standard RMR CCSD method, the effect of all triples and quadruples that are singles or doubles relative to references spanning a chosen multireference (MR) model space is accounted for via the external corrections based on the MR CISD wave function. In the full version of RMR CCSD(T), the remaining triples are then handled via perturbative corrections as in the standard, single-reference (SR) CCSD(T) method. By using a perturbative threshold in the selection of MR CISD configuration space, we arive at the truncated version of RMR CCSD(T), in which the dimension of the MR CISD problem is significantly reduced, thus leaving more triples to be treated perturbatively. This significantly reduces the computational cost. We illustrate this approach on the F2 molecule, in which case the computational cost of the truncated version of RMR CCSD(T) is only about 10%–20% higher than that of the standard CCSD(T), while still eliminating the failure of CCSD(T) in the bond breaking region of geometries. To demonstrate the capabilities of the method, we have also used it to examine the structure and binding energy of transition metal complexes Ni(CO)n with n = 1, 2, and 4. In particular, Ni(CO)2 is shown to be bent rather than linear, as implied by some earlier studies. The RMR CCSD(T) binding energy differs from the SR CCSD(T) one by 1–2 kcal/mol, while the energy barrier separating the linear and bent structures of Ni(CO)2 is smaller than 1 kcal/mol.
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31.15.bw Coupled-cluster theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Fm Bond strengths, dissociation energies
31.15.xp Perturbation theory

Min-map bias Monte Carlo for chain molecules: Biased Monte Carlo sampling based on bijective minimum-to-minimum mapping

Manuel Laso, Nikos Ch. Karayiannis, and Matthias Müller

J. Chem. Phys. 125, 164108 (2006); http://dx.doi.org/10.1063/1.2359442 (13 pages)

Online Publication Date: 27 October 2006

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A novel Monte Carlo (MC) simulation scheme based on Theodorou’s bijective mapping strategy [ D. N. Theodorou, J. Chem. Phys. 124, 034109 (2006) ] is introduced. This min-map bias Monte Carlo acts in combination with any other proper, bare MC. It carries over the bare MC move from the original configuration space Ω(0), where trial move acceptance may be low, to a different configuration space, Ω(1), where acceptance is higher. The bare MC move is then performed in Ω(1) and the resulting configuration is finally mapped back to Ω(0). Mappings between Ω(0) and Ω(1) entail weighted selection of trial configurations, the bias of which is subsequently removed in the overall acceptance criterion. The new method is applied, in conjunction with continuum configurational bias as bare MC scheme, to the simulation of explicit hydrogen linear alkanes in the canonical ensemble. Min-map bias MC is found to alleviate the pervasive problem of very low acceptance rates encountered when using an explicit molecular description.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.15.Bh General molecular conformation and symmetry; stereochemistry

The nature of the calculation of the pressure in molecular simulations of continuous models from volume perturbations

Enrique de Miguel and George Jackson

J. Chem. Phys. 125, 164109 (2006); http://dx.doi.org/10.1063/1.2363381 (11 pages) | Cited 22 times

Online Publication Date: 30 October 2006

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We consider some fundamental aspects of the calculation of the pressure from simulations by performing volume perturbations. The method, initially proposed for hard-core potentials by Eppenga and Frenkel [Mol. Phys.52, 1303 (1984)] and then extended to continuous potentials by Harismiadis et al. [J. Chem. Phys. 105, 8469 (1996)] , is based on the numerical estimate of the change in Helmholtz free energy associated with the perturbation which, in turn, can be expressed as an ensemble average of the corresponding Boltzmann factor. The approach can be easily generalized to the calculation of components of the pressure tensor and also to ensembles other than the canonical ensemble. The accuracy of the method is assessed by comparing simulation results obtained from the volume-perturbation route with those obtained from the usual virial expression for several prototype fluid models. Monte Carlo simulation data are reported for bulk fluids and for inhomogeneous systems containing a vapor-liquid interface.
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61.20.Ja Computer simulation of liquid structure
65.20.-w Thermal properties of liquids
68.03.-g Gas-liquid and vacuum-liquid interfaces

The Morse oscillator under time-dependent external fields

Emanuel F. de Lima and José E. M. Hornos

J. Chem. Phys. 125, 164110 (2006); http://dx.doi.org/10.1063/1.2364502 (10 pages) | Cited 7 times

Online Publication Date: 31 October 2006

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A method to solve the equations for the Morse oscillator under intense time-dependent external fields is presented. Exact analytical formulas for the dipole matrix elements are calculated by the use of the hypergeometric algebra. The continuum is described by an expansion using Laguerre functions. The full algorithm for the calculation of wave functions can be controlled by the convergence of series and by the errors of a first order integration method. We apply our technique to the selective preparation of high overtones by femtosecond laser pulses. The population of the target state is optimized as a function of the intensity and frequency. Introducing a second simultaneous laser, we study the effects of relative frequency and phase over the target state population and dissociation channels. The calculations exhibit a rich interference pattern showing the enhancement and the suppression of the target population by varying the laser parameters.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Inelastic scattering matrix elements for the nonadiabatic collision B(math)+H2(math,j)↔B(math)+H2(math,j′)

David E. Weeks, Thomas A. Niday, and Sang H. Yang

J. Chem. Phys. 125, 164301 (2006); http://dx.doi.org/10.1063/1.2222369 (14 pages)

Online Publication Date: 23 October 2006

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Inelastic scattering matrix elements for the nonadiabatic collision B(math)+H2(math,j)↔B(math)+H2(math,j′) are calculated using the time dependent channel packet method (CPM). The calculation employs 1 math, 2 math, and 1 math adiabatic electronic potential energy surfaces determined by numerical computation at the multireference configuration-interaction level [ M. H. Alexander, J. Chem. Phys. 99, 6041 (1993)]. The 1 math and 2 math, adiabatic electronic potential energy surfaces are transformed to yield diabatic electronic potential energy surfaces that, when combined with the total B+H2 rotational kinetic energy, yield a set of effective potential energy surfaces [ M. H. Alexander et al., J. Chem. Phys. 103, 7956 (1995)]. Within the framework of the CPM, the number of effective potential energy surfaces used for the scattering matrix calculation is then determined by the size of the angular momentum basis used as a representation. Twenty basis vectors are employed for these calculations, and the corresponding effective potential energy surfaces are identified in the asymptotic limit by the H2 rotor quantum numbers j = 0, 2, 4, 6 and B electronic states math, ja = 1/2, 3/2. Scattering matrix elements are obtained from the Fourier transform of the correlation function between channel packets evolving in time on these effective potential energy surfaces. For these calculations the H2 bond length is constrained to a constant value of req = 1.402 a.u. and state to state scattering matrix elements corresponding to a total angular momentum of J = 1/2 are discussed for j = 0↔j′ = 0,2,4 and math↔math, math over a range of total energy between 0.0 and 0.01 a.u.
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34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.50.Gh Surface crossings, non-adiabatic couplings
34.50.-s Scattering of atoms and molecules
34.50.Fa Electronic excitation and ionization of atoms (including beam-foil excitation and ionization)
31.15.V- Electron correlation calculations for atoms, ions and molecules

Femtosecond coherent anti-Stokes Raman-scattering polarization beat spectroscopy of I2–Xe complex in solid krypton

Tiina Kiviniemi, Toni Kiljunen, and Mika Pettersson

J. Chem. Phys. 125, 164302 (2006); http://dx.doi.org/10.1063/1.2358987 (16 pages) | Cited 5 times

Online Publication Date: 23 October 2006

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Time-resolved coherent anti-Stokes Raman-scattering (CARS) measurements are carried out to study the interaction between xenon atom and iodine molecule in a solid krypton matrix. Interference between the CARS polarizations of the “free” and complexed iodine molecules is observed, while the quantum beats of the complex are not detected due to low concentration. Vibrational analysis based on the polarization beats yields accurate molecular constants for the I2–Xe complex. The harmonic frequency of the I2–Xe complex is found to be redshifted by 0.90 cm−1 when compared to the free I2, whereas the anharmonicity is approximately the same. The dephasing rate of the complex is found to be somewhat higher than that of the free iodine molecule in solid Kr, showing that the complexation affects dephasing, although not dramatically. Molecular dynamics simulations are carried out to find the conformation of the complex, and wave packet simulations are used to reproduce the CARS signal to confirm the assignments of the observed beatings as quantum and polarization beats. The results show that the polarization beats are a useful tool for investigating weak interactions in condensed phase.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
31.15.xv Molecular dynamics and other numerical methods
33.15.Mt Rotation, vibration, and vibration-rotation constants

State-to-state reactive differential cross sections for the H+H2→H2+H reaction on five different potential energy surfaces employing a new quantum wavepacket computer code: DIFFREALWAVE

Marlies Hankel, Sean C. Smith, Robert J. Allan, Stephen K. Gray, and Gabriel G. Balint-Kurti

J. Chem. Phys. 125, 164303 (2006); http://dx.doi.org/10.1063/1.2358350 (12 pages) | Cited 28 times

Online Publication Date: 23 October 2006

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State-to-state differential cross sections have been calculated for the hydrogen exchange reaction, H+H2→H2+H, using five different high quality potential energy surfaces with the objective of examining the sensitivity of these detailed cross sections to the underlying potential energy surfaces. The calculations were performed using a new parallel computer code, DIFFREALWAVE. The code is based on the real wavepacket approach of Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998) ]. The calculations are parallelized over the helicity quantum number Ω′ (i.e., the quantum number for the body-fixed z component of the total angular momentum) and wavepackets for each J,Ω′ set are assigned to different processors, similar in spirit to the Coriolis-coupled processors approach of Goldfield and Gray [Comput. Phys. Commun. 84, 1 (1996) ]. Calculations for J = 0–24 have been performed to obtain converged state-to-state differential cross sections in the energy range from 0.4 to 1.2 eV. The calculations employ five different potential energy surfaces, the BKMP2 surface and a hierarchical family of four new ab initio surfaces [ S. L. Mielke, et al., J. Chem. Phys. 116, 4142 (2002) ]. This family of four surfaces has been calculated using three different hierarchical sets of basis functions and also an extrapolation to the complete basis set limit, the so called CCI surface. The CCI surface is the most accurate surface for the H3 system reported to date. Our calculations of differential cross sections are the first to be reported for the A2, A3, A4, and CCI surfaces. They show that there are some small differences in the cross sections obtained from the five different surfaces, particularly at higher energies. The calculations also show that the BKMP2 performs well and gives cross sections in very good agreement with the results from the CCI surface, displaying only small divergences at higher energies.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Wt Computational modeling; simulation
82.20.Ej Quantum theory of reaction cross section

Energies and spatial features for the rotationless bound states of math(math): A cationic core from helium cluster ionization

Emanuele Scifoni, Franco A. Gianturco, Sergy Yu. Grebenshchikov, and Reinhard Schinke

J. Chem. Phys. 125, 164304 (2006); http://dx.doi.org/10.1063/1.2358986 (9 pages) | Cited 2 times

Online Publication Date: 24 October 2006

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Ab initio quantum calculations have been carried out on the helium ionic trimer. The potential energy surface is accurately fitted, especially in the vicinity of the three equivalent minima. The spectrum of bound states for the zero angular momentum is computed and analyzed in detail. Energies and wave functions reveal several interesting features related to the fact that He3+ represents one of the few homonuclear ionic trimers that are linear in their ground vibrational state. At low energies, the triply degenerate eigenfunctions are localized at the potential minimum. With growing excitation energy, however, the wave functions exhibit stronger spatial delocalization.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Wa Charged clusters
31.15.A- Ab initio calculations
31.50.Bc Potential energy surfaces for ground electronic states
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants

Time dependent quantum dynamics study of the O++H2(v = 0,j = 0)→OH++H ion-molecule reaction and isotopic variants (D2,HD)

Rodrigo Martínez, José Daniel Sierra, Stephen K. Gray, and Miguel González

J. Chem. Phys. 125, 164305 (2006); http://dx.doi.org/10.1063/1.2359727 (7 pages) | Cited 6 times

Online Publication Date: 24 October 2006

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The time dependent real wave packet method using the helicity decoupling approximation was used to calculate the cross section evolution with collision energy (excitation function) of the O++H2(v = 0,j = 0)→OH++H reaction and its isotopic variants with D2 and HD, using the best available ab initio analytical potential energy surface. The comparison of the calculated excitation functions with exact quantum results and experimental data showed that the present quantum dynamics approach is a very useful tool for the study of the selected and related systems, in a quite wide collision energy interval (approximately 0.0–1.1 eV), involving a much lower computational cost than the quantum exact methods and without a significant loss of accuracy in the cross sections.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Ej Quantum theory of reaction cross section
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Tr Kinetic isotope effects including muonium

Reaction of niobium and tantalum neutral clusters with low pressure, unsaturated hydrocarbons in a pickup cell: From dehydrogenation to Met-Car formation

S.-G. He, Y. Xie, F. Dong, and E. R. Bernstein

J. Chem. Phys. 125, 164306 (2006); http://dx.doi.org/10.1063/1.2360278 (10 pages) | Cited 11 times

Online Publication Date: 24 October 2006

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Neutral niobium and tantalum clusters (Nbn and Tan) are generated by laser ablation and supersonic expansion into a vacuum and are reacted in a pickup cell with various low pressure ( ∼ 1 mTorr) unsaturated hydrocarbons (acetylene, ethylene, propylene, 1-butene, 1,3-butadiene, benzene, and toluene) under nearly single collision conditions. The bare metal clusters and their reaction products are ionized by a 193 nm laser and detected by a time of flight mass spectrometer. Partially and fully dehydrogenated products are observed for small (n ⩽ m) and large (n ≥ m) neutral metal clusters, respectively, with m ranging from 2 to 5 depending on the particular hydrocarbon. In addition to primary, single collision products, sequential addition products that are usually fully dehydrogenated are also observed. With toluene used as the reactant gas, carbon loss products are observed, among which Nb8C12 and Ta8C12 are particularly abundant, indicating that the Met-Car molecule M8C12 can be formed from the neutral metal cluster upon two collisions with toluene molecules. The dehydrogenation results for low pressure reactions are compared with those available from previous studies employing flow tube (high pressure) reactors. Low pressure and high pressure cluster ion reactions are also compared with the present neutral metal cluster reactions. Reactions of unsaturated hydrocarbons and metal surfaces are discussed in terms of the present neutral cluster results.
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36.40.Jn Reactivity of clusters
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution
82.80.Dx Analytical methods involving electronic spectroscopy

Photodissociation dynamics of allyl bromide at 234, 265, and 267 nm

Lei Ji, Ying Tang, Rongshu Zhu, Zhengrong Wei, and Bing Zhang

J. Chem. Phys. 125, 164307 (2006); http://dx.doi.org/10.1063/1.2360280 (7 pages) | Cited 6 times

Online Publication Date: 24 October 2006

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The photodissociation dynamics of allyl bromide was investigated at 234, 265, and 267 nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br* (math) and Br (math) atoms. The Br fragments show a bimodal translational energy distribution, while the Br* fragments reveal one translational energy distribution. The vertical excited energies and the mixed electronic character of excited states were calculated at ab initio configuration interaction method. It is presumed that the high kinetic energy bromine atoms are attributed to the predissociation from math or math state to the repulsive math state, and to the direct dissociation from math and math states, while the low kinetic energy bromine atoms stem from internal conversion from the lowest math state to math state.
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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)
33.80.Eh Autoionization, photoionization, and photodetachment
31.15.A- Ab initio calculations
31.15.vj Electron correlation calculations for atoms and ions: excited states
33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)

Theoretical study of the CS2+ dication

T. Šedivcová, V. Špirko, and J. Fišer

J. Chem. Phys. 125, 164308 (2006); http://dx.doi.org/10.1063/1.2358982 (5 pages) | Cited 3 times

Online Publication Date: 24 October 2006

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The potential energy and spin-orbit coupling functions of 11 lowest electronic states of CS2+ dication have been calculated using internally contracted multireference configuration method. Using these functions, the positions and widths of the corresponding vibronic levels have been evaluated by means of the stabilization and log-phase-amplitude methods. The states governing the second step in the sequential pathway CS23+→S++CS2+→S++C++S+ of the overall three-body Coulomb explosion of CS23+ have been determined.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Kh Potential energy surfaces for chemical reactions

Quasiclassical trajectories on a finite element density functional potential energy surface: The C++H2O reaction revisited

Jesús R. Flores

J. Chem. Phys. 125, 164309 (2006); http://dx.doi.org/10.1063/1.2359726 (10 pages) | Cited 6 times

Online Publication Date: 24 October 2006

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A new method for the representation of potential energy surfaces (PESs) based on the p version of the finite element method is presented and applied to the PES of the [COH2]+ system in order to study the C++H2O→[COH]++H reaction through the quasiclassical trajectory method. Benchmark ab initio computations have been performed on the most relevant stationary points of the PES through a procedure that incorporates basis set extrapolations, the contribution of the core correlation energy, and scalar relativistic corrections. The electronic structure method employed to compute the many points needed to construct the PES is a hybrid density functional approach of the B3LYP type with geometry-dependent parameters, which improves dramatically the performance with respect of the B3LYP method. The trajectory computations shed light on the behavior of the COH2+ complex formed in the collision. At a fixed relative translational energy of 0.62 eV, which corresponds to the crossed beam experiments [ D. M. Sonnenfroh et al., J. Chem. Phys. 83, 3985 (1985) ], the complex dissociates significantly into the reactants (37%). However, the behavior for a thermal sampling at T = 300 K is significantly different because only 9% of the trajectories where capture occurs lead to dissociation into the reactants. The latter kind of behavior is coherent with the view that simple ion-molecule reactions proceed quite often at the capture rate provided it is corrected by the fraction of the electronic states which, being nearly degenerate for the reactants, become attractive at short distances. For both T = 300 K and crossed beam conditions, the trajectory computations indicate that COH2+ is the critical intermediate, in agreement with a recent work [ Y. Ishikawa et al., Chem. Phys. Lett. 370, 490 (2003) ] and in contrast with the interpretation of the crossed beam experiments. Besides, virtually all trajectories generate COH++H (>99%), but a significant proportion of the isoformyl cation is formed with enough vibrational energy as to surmount the COH+–HCO+ isomerization barrier, about 37% at T = 300 K.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Hf Product distribution
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Qt Isomerization and rearrangement

An accurate analytic potential function for ground-state N2 from a direct-potential-fit analysis of spectroscopic data

Robert J. Le Roy, Yiye Huang, and Calvin Jary

J. Chem. Phys. 125, 164310 (2006); http://dx.doi.org/10.1063/1.2354502 (12 pages) | Cited 41 times

Online Publication Date: 24 October 2006

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Two types of combined-isotopologue analysis have been performed on an extensive spectroscopic data set for ground-state N2 involving levels up to v = 19, which is bound by half the well depth. Both a conventional Dunham-type analysis and a direct-potential-fit (DPF) analysis represent the data within (on average) the estimated experimental uncertainties. However, the Dunham-type parameters do not yield realistic predictions outside the range of the data used in the analysis, while the potential function obtained from the DPF treatment yields quantum mechanical accuracy over the data region and realistic predictions of the energies and properties of unobserved higher vibrational levels. Our DPF analysis also introduces a compact new analytic potential function form which incorporates the two leading inverse-power terms in the long-range potential.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
31.30.Gs Hyperfine interactions and isotope effects
34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Mt Rotation, vibration, and vibration-rotation constants

A complete active space self-consistent field study of the photochemistry of nitrosamine

Daniel Peláez, Juan F. Arenas, Juan C. Otero, and Juan Soto

J. Chem. Phys. 125, 164311 (2006); http://dx.doi.org/10.1063/1.2360259 (11 pages) | Cited 2 times

Online Publication Date: 24 October 2006

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Photodissociation mechanisms of nitrosamine (NH2NO) have been studied at the complete active space self-consistent field level of theory in conjunction with atomic-natural-orbital-type basis sets. In addition, the energies of all the critical points and the potential energy curves connecting them have been recomputed with the multiconfigurational second-order perturbation method. Ground state minimum of nitrosamine has a C1 nonplanar structure with the hydrogen atoms of the amino moiety out of the plane defined by the N–N–O bonds. Electronic transitions to the three lowest states are allowed by selection rules: (i) S0→S3 (7.41 eV) has an oscillator strength of f = 0.0006 and it is assigned as an (npO)0→(πNO*)2 transition, (ii) S0→S2 (5.86 eV) has an oscillator strength of f = 0.14 and it is assigned as an npN→πNO* transition, and (iii) S0→S1 (2.98 eV) has an oscillator strength of f = 0.002 and it is assigned as an npO→πNO* transition. It is found that N–N bond cleavage is the most likely process in all the photochemical relevant states, namely, S1 (1 math), S2 (2 math), and T1 (1 math). While S1 and T1 yield exclusively homolytic dissociation: NH2NO→NH2 (1 math)+NO(X math), on S2 the latter process constitutes the major path, but two additional minor channels are also available: adiabatic homolytic dissociation: NH2NO→NH2 (1 math)+NO(X math), and adiabatic oxygen extrusion: NH2NO→NH2N (1 math)+O(math). The excited species NH2 (1 math) experiences a subsequent ultrafast decay to the ground state, the final products in all cases the fragments being in their lowest electronic state. We have not found a unimolecular mechanism connecting excited states with the ground state. In addition, homolytic dissociation in the ground state, tautomerizations to NHNOH and NHNHO, and intersystem crossings to T1 are considered. The most favorable process on this state is the isomerization to NHNOH.
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82.50.-m Photochemistry
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Hf Product distribution
82.30.Qt Isomerization and rearrangement

Quantum dynamics study of the dissociative photodetachment of HOCO−

Shesheng Zhang, Dmitry M. Medvedev, Evelyn M. Goldfield, and Stephen K. Gray

J. Chem. Phys. 125, 164312 (2006); http://dx.doi.org/10.1063/1.2360945 (8 pages) | Cited 8 times

Online Publication Date: 24 October 2006

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Six-dimensional wave packet calculations are carried out to study the behavior of HOCO subsequent to the photodetachment of an electron from the negative anion, HOCO−. It is possible to form stable and/or long-lived HOCO complexes, as well as the dissociative products OH+CO and H+CO2. A variety of observables are determined: the electron kinetic energy (eKE) distributions associated with the OH+CO and H+CO2 channels, the correlated eKE and product translational energy distribution for the OH+CO channel, and product branching ratios. Most of our results are in good accord with the experimental results of Clements, Continetti, and Francisco [J. Chem. Phys. 117, 6478 (2002)] , except that the calculated eKE distribution for the H+CO2 channel is noticeably colder than experiment. Reasons for this discrepancy are suggested.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Hf Product distribution
82.20.Ej Quantum theory of reaction cross section

HF in clusters of molecular hydrogen: II. Quantum solvation by H2 isotopomers, cluster rigidity, and comparison with CO-doped parahydrogen clusters

Francesco Sebastianelli, Yael S. Elmatad, Hao Jiang, and Zlatko Bačić

J. Chem. Phys. 125, 164313 (2006); http://dx.doi.org/10.1063/1.2363989 (10 pages) | Cited 7 times

Online Publication Date: 25 October 2006

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We present a comprehensive theoretical study of the quantum solvation of the HF molecule by small clusters of the H2 isotopomers, p‐H2, HD, and o‐D2, with up to 13 hydrogen solvent molecules. This complements our earlier work on the HF-doped parahydrogen clusters [ H. Jiang and Z. Bačić, J. Chem. Phys. 122, 244306 (2005) ]. The ground-state properties of the clusters are calculated exactly using the diffusion Monte Carlo method. Detailed information is obtained regarding the size and isotopomer dependences of the energetics, vibrationally averaged structures, and their rigidity. The rigidity of these clusters is investigated further by analyzing the distributions of their principal moments of inertia from the diffusion Monte Carlo simulations. The clusters are found to be rather rigid, especially when compared with the pure parahydrogen clusters of the same size. Extensive comparison is made with the quantum Monte Carlo results for the CO-doped parahydrogen clusters and significant differences are observed in the size evolution of certain properties, notably the chemical potential.
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36.40.Jn Reactivity of clusters
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Tr Kinetic isotope effects including muonium
82.20.Xr Quantum effects in rate constants (tunneling, resonances, etc.)
82.30.Nr Association, addition, insertion, cluster formation
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