Volume 125, Issue 16, 28 October 2006
 COMMUNICATIONS


Bulk photochromism in a tungstatephosphate glass: A new optical memory material?
View Description Hide DescriptionIn 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 3dimensional (3D) and holographic optical storage.

Solvent effects on the threephoton absorption crosssection of a highly conjugated fluorene derivative
View Description Hide DescriptionHerein, we report the study of the threephoton absorption crosssection dependence on solvents parameters for a highly conjugated organic dye, 2,(4,(1E,E)2,(9,9didecyl9Hfluorene2,7diyl) bis(ethene2,1diyl)bis(4,1phenylene))dibenzo[d]thiazole (). The threephoton absorption crosssection was measured for this organic dye in solution in four different solvents with polarity function, between 0.162 and 0.247. The experiments show how the solvent’s reorientation of the electrons and polarity contribute to the 3PA crosssection. Multiphotonabsorption experiments of AA in all four different solvents were performed with a tunable OPG pumped by a 25 picosecond laser.
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 ARTICLES

 Theoretical Methods and Algorithms

Crooks equation for steered molecular dynamics using a NoséHoover thermostat
View Description Hide DescriptionThe 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 NoséHoover 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 nonGaussian thereby implying, according to Crooks equation, that also the work distribution of the unfolding process must be inherently nonGaussian. The clearly asymmetric behavior of the forward and backward irreversible work distributions is a signature of a nonMarkovian regime for the folding/unfolding of decaalanine.

Periodic boundary condition induced breakdown of the equipartition principle and other kinetic effects of finite sample size in classical hardsphere molecular dynamics simulation
View Description Hide DescriptionWe examine consequences of the nonBoltzmann nature of probability distributions for oneparticle 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 ( or microcanonical ensemble) and periodic boundaries ( or molecular dynamics ensemble), we describe three consequences of the centerofmass 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 , and the ratio of average collision energy to average kinetic energy is increased by a factor of . Simulations in one, two, and three dimensions confirm the analytic results for arbitrary dimension.

Inclusion of inversion symmetry in centroid molecular dynamics: A possible avenue to recover quantum coherence
View Description Hide DescriptionInversion symmetry is included in the operator formulation of the centroid molecular dynamics (CMD). This work involves the development of a symmetryadapted CMD (SACMD), here particularly for symmetrization and antisymmetrization projections. A symmetryadapted 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 onedimensional symmetric quartic and symmetric doublewell potentials. Two SACMD 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 SACMD 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.

Forwardbackward semiclassical initial value series representation of quantum correlation functions
View Description Hide DescriptionThe forwardbackward (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 FBSCIVR has proven elusive. In this paper, we provide a new derivation of a forwardbackward phase space SCIVR expression (FBPSSCIVR) derived previously by Thompson and Makri [Phys. Rev. E59, R4729 (1999)]. This enables us to represent quantum correlation functions exactly in terms of a series whose leading order term is the FBPSSCIVR expression. Numerical examples for systems with over of freedom are presented for the spin boson problem. Comparison of the FBPSSCIVR 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 FBPSSCIVR term already provides an excellent approximation.

Extracting electron transfer coupling elements from constrained density functional theory
View Description Hide DescriptionConstrained density functional theory(DFT) is a useful tool for studying electron transfer(ET)reactions. It can straightforwardly construct the chargelocalized diabatic states and give a direct measure of the innersphere reorganization energy. In this work, a method is presented for calculating the electronic coupling matrix element based on constrained DFT. This method completely avoids the use of groundstateDFT 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 KohnSham wave functions for the diabatic states in a careful way. Test calculations on the and the benzeneCl atom systems show that the new prescription yields reasonable agreement with the standard generalized MullikenHush method. We then proceed to produce the diabatic and adiabatic potential energy curves along the reaction pathway for intervalence ET in the tetrathiafulvalenediquinone (QTTFQ) anion. While the unconstrained DFT curve has no reaction barrier and gives , which qualitatively disagrees with experimental results, the calculated from constrained DFT is about and the generated ground state has a barrier height of , successfully predicting to be a class II mixedvalence compound.

Relativistic calculation of nuclear magnetic shielding tensor including twoelectron spinorbit interactions
View Description Hide DescriptionA relativistic calculation of nuclear magnetic shieldingtensor including twoelectron spinorbit interactions is performed. In order to reduce the computational load in evaluating the twoelectron relativistic integrals, the charge density is approximated by a linear combination of the squares of type spatial basis functions. Including the twoelectron spinorbit interaction effect is found to improve the calculation results.

A truncated version of reduced multireference coupledcluster method with singles and doubles and noniterative triples: Application to and (, 2, and 4)
View Description Hide DescriptionA perturbatively truncated version of the reduced multireference coupledcluster 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, singlereference (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 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 with , 2, and 4. In particular, 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 , while the energy barrier separating the linear and bent structures of is smaller than .

Minmap bias Monte Carlo for chain molecules: Biased Monte Carlo sampling based on bijective minimumtominimum mapping
View Description Hide DescriptionA 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 minmap bias Monte Carlo acts in combination with any other proper, bare MC. It carries over the bare MC move from the original configuration space , where trial move acceptance may be low, to a different configuration space, , where acceptance is higher. The bare MC move is then performed in and the resulting configuration is finally mapped back to . Mappings between and 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. Minmap bias MC is found to alleviate the pervasive problem of very low acceptance rates encountered when using an explicit molecular description.

The nature of the calculation of the pressure in molecular simulations of continuous models from volume perturbations
View Description Hide DescriptionWe consider some fundamental aspects of the calculation of the pressure from simulations by performing volume perturbations. The method, initially proposed for hardcore 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 volumeperturbation 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 vaporliquid interface.

The Morse oscillator under timedependent external fields
View Description Hide DescriptionA method to solve the equations for the Morse oscillator under intense timedependent 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.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Inelastic scattering matrix elements for the nonadiabatic collision
View Description Hide DescriptionInelastic scattering matrix elements for the nonadiabatic collision are calculated using the time dependent channel packet method (CPM). The calculation employs , , and adiabatic electronic potential energy surfaces determined by numerical computation at the multireference configurationinteraction level [M. H. Alexander, J. Chem. Phys.99, 6041 (1993)]. The and , adiabatic electronic potential energy surfaces are transformed to yield diabatic electronic potential energy surfaces that, when combined with the total 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 rotor quantum numbers , 2, 4, 6 and B electronic states , , . 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 bond length is constrained to a constant value of and state to state scattering matrix elements corresponding to a total angular momentum of are discussed for and , over a range of total energy between 0.0 and .

Femtosecond coherent antiStokes Ramanscattering polarization beat spectroscopy of complex in solid krypton
View Description Hide DescriptionTimeresolved coherent antiStokes Ramanscattering (CARS) measurements are carried out to study the interaction between xenon atom and iodine molecule in a solid krypton matrix. Interference between the CARSpolarizations 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 complex. The harmonic frequency of the complex is found to be redshifted by when compared to the free , 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.

Statetostate reactive differential cross sections for the reaction on five different potential energy surfaces employing a new quantum wavepacket computer code: DIFFREALWAVE
View Description Hide DescriptionStatetostate differential cross sections have been calculated for the hydrogen exchange reaction,, 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 BalintKurti [J. Chem. Phys.108, 950 (1998)]. The calculations are parallelized over the helicity quantum number (i.e., the quantum number for the bodyfixed component of the total angular momentum) and wavepackets for each set are assigned to different processors, similar in spirit to the Corioliscoupled processors approach of Goldfield and Gray [Comput. Phys. Commun.84, 1 (1996)]. Calculations for have been performed to obtain converged statetostate differential cross sections in the energy range from . The calculations employ five different potential energy surfaces, the BKMP2 surface and a hierarchical family of four new ab initiosurfaces [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 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.

Energies and spatial features for the rotationless bound states of : A cationic core from helium cluster ionization
View Description Hide DescriptionAb 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 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.

Time dependent quantum dynamics study of the ionmolecule reaction and isotopic variants
View Description Hide DescriptionThe 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 reaction and its isotopic variants with 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 ), involving a much lower computational cost than the quantum exact methods and without a significant loss of accuracy in the cross sections.

Reaction of niobium and tantalum neutral clusters with low pressure, unsaturated hydrocarbons in a pickup cell: From dehydrogenation to MetCar formation
View Description Hide DescriptionNeutral niobium and tantalumclusters ( and ) are generated by laser ablation and supersonic expansion into a vacuum and are reacted in a pickup cell with various low pressure unsaturated hydrocarbons (acetylene, ethylene, propylene, 1butene, 1,3butadiene, benzene, and toluene) under nearly single collision conditions. The bare metal clusters and their reaction products are ionized by a laser and detected by a time of flight mass spectrometer. Partially and fully dehydrogenated products are observed for small and large neutral metal clusters, respectively, with 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 and are particularly abundant, indicating that the MetCar molecule can be formed from the neutral metal cluster upon two collisions with toluene molecules. The dehydrogenation results for low pressurereactions are compared with those available from previous studies employing flow tube (high pressure) reactors. Low pressure and high pressurecluster ion reactions are also compared with the present neutral metal clusterreactions.Reactions of unsaturated hydrocarbons and metal surfaces are discussed in terms of the present neutral cluster results.

Photodissociation dynamics of allyl bromide at 234, 265, and
View Description Hide DescriptionThe photodissociation dynamics of allyl bromide was investigated at 234, 265, and . A twodimensional photofragment ion velocity imaging technique coupled with a resonanceenhanced multiphoton ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent and Br atoms. The Br fragments show a bimodal translational energy distribution, while the 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 or state to the repulsive state, and to the direct dissociation from and states, while the low kinetic energy bromine atoms stem from internal conversion from the lowest state to state.

Theoretical study of the dication
View Description Hide DescriptionThe potential energy and spinorbit coupling functions of 11 lowest electronic states of 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 logphaseamplitude methods. The states governing the second step in the sequential pathway of the overall threebody Coulomb explosion of have been determined.