bluuebunny/crossref_metadata_2025 huggingface dataset:
Primary and secondary dissociation pathways in the ultraviolet photolysis of Cl2O
Authors: Nelson Christine M. , Moore Teresa A. , Okumura Mitchio , Minton Timothy K.
Published: July 2002

The photodissociation of dichlorine monoxide (Cl2O) at 308, 248, and 193 nm was studied by photofragment translational energy spectroscopy. The primary channel upon excitation at 308 and 248 nm was Cl–O bond fission with production of ClO+Cl. A fraction of the ClO photoproducts also underwent spontaneous secondary dissociation at 248 nm. The center-of-mass translational energy distribution for the ClO+Cl channel at 248 nm appeared to be bimodal with a high energy component that was similar in shape to the 308 nm distribution and a second, low energy component with a maximum close to the threshold for the 2Cl+O(3P) channel. Observation of a bimodal distribution suggests that two pathways with different dissociation dynamics lead to ClO+Cl products. The high product internal energy of the second component raises the possibility that ClO is formed in a previously unobserved spin-excited state a 4Σ−. Following excitation at 193 nm, a concerted dissociation pathway leading to Cl2+O was observed in addition to primary Cl–O bond breakage. In both processes, most of the diatomic photofragments were formed with sufficient internal energy that they spontaneously dissociated. The time-of-flight distributions of the Cl2+O products suggest that these fragments are formed in two different channels Cl2(3Π)+O(3P) and Cl2(X 1Σ)+O(1D).
Benchmark studies of electron correlation in six-electron systems
Authors: Fermann Justin T. , Sherrill C. David , Crawford T. Daniel , Schaefer Henry F.
Published: July 2002

Benchmark studies of H6, H+7, and H+5⋅He were initiated in order to develop more efficient theoretical methods for describing the electron correlation energy, due to the relative ease with which the full configuration interaction (full CI) results could be obtained for these six-electron systems. Single-point energies which approach the quality of the full CI results are reported for a variety of coupled-cluster (CC) and configuration interaction (CI) methods using optimized basis sets and full CI optimized geometries. Emphasis is placed on multireference CI (MRCI) methods. By carefully limiting the configurations included in the CI and by using CI natural orbitals, we find it possible to reduce the number of configuration state functions (CSFs) by two orders of magnitude or more with little loss in the correlation energy recovered for the six-electron systems studied here. To judge the applicability of the MRCI methods to the study of potential energy surfaces, the energies of H2O at three geometries are compared to previously published full CI and complete active space self-consistent-field (CASSCF) second-order CI (SOCI) results. Finally, we propose a compact MRCI wave function incorporating limited triple and quadruple excitations. Indirect tests suggest that this new approach should be highly effective.
Theta state and collapse of off-lattice chains in two dimensions
Authors: Torres Ana M. , Rubio Ana M. , Freire Juan J. , Bishop Marvin , Clarke Julian H. R.
Published: July 2002

We have performed a Monte Carlo study of dimensions for two dimensional linear chains of different lengths. These chains are composed of Gaussian units which interact through a 6-12 Lennard-Jones potential. From this study, the theta state for this model has been characterized. Scaling curves have been obtained and different universal exponents, such as the theta point exponent ν, νθ, and the cross-over exponent Φt have been numerically evaluated. The results are compared with theoretical predictions and with the values corresponding to simulations in lattice models. The results for ν and νθ agree with the theory, but our best estimation for the cross-over exponent is closer to the simple mean field estimation.
Scanning tunneling microscopy theory for an adsorbate: Application to adenine adsorbed on a graphite surface
Authors: Ou-Yang Hui , Marcus R. A. , Källebring Bruno
Published: July 2002

An expression is obtained for the current in scanning tunneling microscopy (STM) for a single adsorbate molecule. For this purpose the ‘‘Newns–Anderson’’ treatment (a ‘‘discrete state in a continuum’’ treatment) is used to obtain wave functions and other properties of the adsorbate/substrate system. The current is expressed in terms of the adsorbate–tip matrix elements, and an effective local density of states of the adsorbate/substrate system, at the adsorbate. As an example, the treatment is applied to the STM image of adenine adsorbed on a graphite surface, and the results are compared with experiment. The dependence of the image on the position of adenine with respect to the underlying graphite is considered. A discussion is given of the type of experimental STM data needed for suitable comparison of theory and experiment. In an analysis of the calculations, the role of each atom, its neighbors, next nearest neighbors, etc., in an adsorbed molecule is considered. The need for using in the present calculation more orbitals than only the HOMO and the LUMO of the adsorbate is also noted.
Collisional deactivation of highly vibrationally excited NO2 monitored by time-resolved Fourier transform infrared emission spectroscopy
Authors: Hartland Gregory V. , Qin Dong , Dai Hai-Lung
Published: July 2002

Infrared emission from highly vibrationally excited NO2, prepared by collision induced internal conversion, can be detected with 1 cm−1 spectral and 0.5 μs time resolution over the 800–10 000 cm−1 range by time-resolved Fourier transform emission spectroscopy. The energy distribution of vibrationally excited NO2 during collisional deactivation can be extracted from the emission spectra and shows that the energy loss per collision increases dramatically from <50 cm−1 below 13 000 cm−1 energy to 1300 cm−1 at 20 000 cm−1 energy.
Application of complex L2 functions to the calculation of photodissociation processes
Authors: Mayrhofer Rudolph C. , Bowman Joel M.
Published: July 2002

A general method is presented which uses complex L2 functions to calculate photodissociation cross sections for interactions that exhibit both resonance and direct scattering behavior. The complex L2 functions are eigenfunctions of a complex Hamiltonian containing an imaginary potential in the asymptotic region of the interaction potential. This calculation is related to the time-dependent wave packet propagation method, with an imaginary potential located in the exit channel of the reaction. Three systems are considered to test this complex L2 approach. Two are one-dimensional (nonreactive) examples of diatomic photodissociation which exhibit direct, and resonant and direct scattering dynamics. The third example is the photodetachment spectra of a three-dimensional system FH−2, which has recently been simulated exactly for zero total angular momentum by Manolopoulos [S. E. Bradforth et al., J. Chem. Phys. 99, 6345 (1993)].
Anisotropic stress in a confined chain. Excluded volume effects
Authors: Gao J. , Weiner J. H.
Published: July 2002

When a polymer chain is confined to a box with sides Lx≠Ly=Lz whose dimensions are small compared to the chain radius of gyration, it gives rise to unequal pressure components, Px≠Py=Pz, where Px is the pressure acting on a face perpendicular to the x axis. This problem is treated analytically by Doi and Edwards for an ideal Gaussian chain. We study the effect of excluded volume by molecular dynamics simulation of a confined chain with 1600 bonds and find that it increases the pressure anisotropy by a factor of 6 for reasonable packing fractions. By comparison with the pressure anisotropy in the corresponding confined simple liquid, we conclude that in this system the pressure anisotropy is primarily due to packing entropy rather than to chain conformational entropy.
Laser control of photopredissociation angular distributions and photoionization in Cl2 by two color interference
Authors: Bandrauk André D. , Gauthier Jean-Marc , McCann James F.
Published: July 2002

A coupled equations approach based on a general artificial channel method is used to demonstrate two color laser coherent control of photopredissociation and photoionization in Cl2. Photopredissociation of the 1Πg states of Cl2 is suppressed by a two plus four photon interference process 2ω2=4ω4, whereas the photoionization is controlled by interference between a six photon transition and a four photon transition 6ω4=2ω2+2ω4 using the same 1Πg photopredissociation resonances as intermediates. Photopredissociation angular distributions illustrate the suppression of the corresponding transition in selective rotational states.
Constant temperature molecular dynamics simulations by means of a stochastic collision model. I. Noninteracting particles
Authors: Kast Stefan Michael , Nicklas Kai , Bär Hans-Jürgen , Brickmann Jürgen
Published: July 2002

A new stochastic collision model for molecular dynamics (MD) simulations at constant temperature is presented. It is based on impulsive collisions between system particles and heat bath particles of finite masses. With the algorithm one can switch between Langevin-type and Andersen-type dynamics by changing only one control parameter. The method is implemented in the simple Verlet scheme for the numerical calculation of the Newtonian equations of motion. The case of an ensemble of noninteracting particles subjected to the heat bath is considered. Analytical expressions for stationary probability densities, autocorrelation functions, and diffusion coefficients are derived. The predicitions agree excellently with the results of MD simulations.
Methane in neon: Nearly free rotation in a mismatched guest–host system
Authors: Prager M. , Asmussen B. , Carlile C. J.
Published: July 2002

A nonequilibrium solid solution of 2% CH4 in Ne has been prepared by the matrix isolation technique. The system retains the fcc structure of pure Ne. A broad distribution of methane tunneling frequencies centered at an energy transfer of 0.5 meV is observed by inelastic neutron scattering. All CH4 molecules contribute to this distribution. For geometric reasons, to be consistent with the long range order of the matrix, the guest molecules must appear as trimers or higher multimers. The variety of aggregates with differing local surroundings and the complex level schemes of clusters of interacting molecules produce the broad distribution of tunnel transitions. Annealing leads to a wider potential distribution. In thermal equilibrium methane is insoluble in neon.
The properties of electrons in sodalite saturated with alkali atoms
Authors: Monnier Alain , Srdanov Vojislav , Stucky Galen , Metiu Horia
Published: July 2002

We study the properties of the electrons produced by absorbing sodium atoms into a zeolite which we call dry sodalite. Upon absorption, the sodium atom is ionized and the electron is shared with the other ions present in the sodalite cage. When the dry sodalite is saturated with sodium, each cage has one such electron and the system is periodic. We develop a simple model for the electron interaction with the zeolite framework and calculate the energy bands of the electrons in this periodic system. We find that in the one-electron approximation, the system is a narrow band metal. It is very likely that the electron–electron interactions will transform it into a Mott insulator which is antiferromagnetic at low temperature. Particular attention is paid to understanding how the interactions between the electrons and the zeolite frame affect the electronic properties. Since the properties of the framework can be modified chemically, this understanding can help us determine how to use such modifications to our advantage.
Harmonic corrections to the mean-field phase diagram for block copolymers
Authors: Hamley Ian W. , Bates Frank S.
Published: July 2002

Allowance is made for harmonic corrections within mean-field theory for weakly segregated block copolymers. The theory of Marques and Cates [C. M. Marques and M. E. Cates, Europhys. Lett. 13, 267 (1990)] for the stability of phases near the ordering transition is applied to diblock copolymer melts. A wave-vector dependence of the coefficients in the Landau mean-field free energy is included. We find that the face centered cubic (fcc) and square phases predicted by Marques and Cates to be stable near the ordering transition depending on the structure factor for the system are not stable for diblock copolymers. In addition, hexagonal close packed (hcp) and ordered bicontinuous double diamond (OBDD) structures are found to be unstable with respect to the classical lamellar, cylinder, and body centered cubic phases. The hexagonal intermediate phases and complex bicontinuous structures recently observed near the order–disorder transition are not accounted for in this theory.
Liquid-state theory of the density dependent conformation of nonpolar linear polymers
Authors: Grayce Christopher J. , Yethiraj Arun , Schweizer Kenneth S.
Published: July 2002

We use the two solvation potentials discussed in the previous paper and polymer reference interaction-site model (PRISM) theory to calculate the mutually consistent single-molecule conformation and liquid structure of model linear flexible nonpolar polymers. We focus on the mean end-to-end distance R2 and radius of gyration Rg of the single chain as functions of polymer density. Our calculations are compared to computer simulations to assess their accuracy, and some aspects of the general trends with density predicted are discussed. Two methods of calculating the conformation of the single chain in the solvation potential are used, one is Monte Carlo simulation, the other, introduced in this paper, employs a generating functional. We cannot unambiguously confirm the accuracy of one solvation potential over the other; at low density they give identical results, and the exact values are too few at high density. However, we are inclined to question the accuracy of the hypernetted-chain (HNC) style potential at high densities because it seems to predict a strong contraction of the polymer chain that is not seen experimentally in real polymer melts.
Nonadiabatic chemical reactions in a strong time-dependent electric field: An electron transfer reaction in a polar solvent
Authors: Dakhnovskii Yuri
Published: July 2002

Nonadiabatic chemical reactions of a charge transfer in a polar solvent are considered in the presence of a strong time-dependent electric field. The integro-differential kinetic equation for a transfer probability is obtained in the noninteracting blip approximation for an arbitrary electric field. A kernel of the integral part is expressed through a dielectric loss function of the solvent. In a long time limit a high-temperature solution is obtained. It is found the the rate constant decreases, increases, or oscillates with respect to the intensity and frequency of the field. When the energy of the photon is comparable to the reorganization energy of the solvent, localization of the charge occurs. The localization field parameters are dependent on the heat of the reaction. A strong enhancement of the rate constant is found for barrierless reactions (as much as 18 orders of magnitude) with respect to the intensity of the field. The estimations show that a probable candidate for an observation of these effects is a long-range electron transfer reaction.
Kinetics and dynamics of vibrationally state resolved ion–molecule reactions: 14N+2(v=1 and 2) and 15N+2(v=0, 1, and 2) with 14N2
Authors: Frost Michael J. , Kato Shuji , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Vibrationally state-selected measurements of the kinetics and dynamics of 14N+2(v=1 and 2) and 15N+2(v=0, 1, and 2) in collisions with 14N2 are made using a selected ion flow tube (SIFT), laser induced fluorescence (LIF) technique at thermal energies. Kinetics are measured by monitoring the LIF signal amplitudes of N+2(v) as a function of 14N2 concentration, added after ion injection. By comparison with the known N+2(v=1)+Ar rate, the 15N+2(v=0)+14N2 rate constant is found to be one-half of the Langevin collision rate, or 4.2±0.2×10−10 cm3 molecule−1 s−1. This suggests that the reaction proceeds via an N+4 energized adduct in which charge is shared on a time scale shorter than the adduct lifetime. The removal rates of 14N+2(v=1 and 2) reactions by 14N2 are also found to proceed at one-half of the Langevin collision rate. Thus product channels that remove vibrational energy from the ion upon dissociation of the adduct account for 50% of the collision probability. The removal rates of 15N+2(v=1 and 2) with 14N2 are 21%–26% faster than those for 14N+2(v=1 and 2); this is attributed to the channel that cannot be observed in collisions of 14N+2(v) with 14N2 in which both charge and vibrational energy are transferred between the collision partners. This pathway is also observed directly by the appearance of 14N+2(v=1 and 2) product states. For this channel to proceed, vibrational energy transfer between the two highest frequency modes of the N+4 energized adduct must occur on a time scale comparable to the adduct lifetime.
Measurement of the molecular content of binary nuclei. II. Use of the nucleation rate surface for water–ethanol
Authors: Viisanen Y. , Strey R. , Laaksonen A. , Kulmala M.
Published: July 2002

In a preceding paper we determined the molecular contents of binary nuclei from homogeneous nucleation rate measurements studing ethanol–hexanol as a nearly ideal model system. Here we report the analogous measurements for the nonideal water–ethanol system known from previous investigations to display surface enrichment of ethanol in the nuclei. The nucleation pulse technique applied allows accurate measurements of homogeneous nucleation rates in the range 105<J/cm−3 s−1<109. We determined the homogeneous nucleation rates of mixed droplets in supersaturated water–ethanol vapor mixtures with argon as the carrier gas. The experiments were performed as functions of the water and ethanol gas phase activities, a1 and a2, respectively, ranging from pure water to pure ethanol with ten intermediate activity ratios at T=260 K. The experimental data shape a nucleation rate surface in three-dimensional J–a1–a2 space from which the critical activities for a rate of Jc=107 cm−3 s−1 were determined. The critical activities are compared with the predictions of the classical binary nucleation theory. The observed large discrepancies can be attributed to very water-rich compositions of the nuclei predicted by the theory. The results support previous findings for the water-n-propanol system, where even larger discrepancies were observed. In a next step the individual numbers of ethanol and water molecules in the nuclei are directly determined from the slopes of the experimental nucleation rate surface. The observed variations of the molecular numbers with vapor phase composition differ substantially from that of an ideal mixture. Comparing the measured nucleus compositions to the predictions of a recently proposed explicit cluster model one observes an almost quantitative agreement.
Experimental study of vibrational excitation of allene by slow electron impact: Vibronic coupling in the short-lived negative ion states
Authors: Allan Michael
Published: July 2002

Vibrational electron energy loss spectra, vibrational excitation functions, and angular dependence of vibrational energy losses were measured in relative units for propadiene (allene, H2C=C=CH2) in an incident energy range up to 16 eV. Resonant excitation via the 2 eV resonance is not very selective; symmetric and antisymmetric C–C–C stretch, CH2 twist and scissoring, CH stretch, and C–C–C bending are all excited. The antisymmetric C–C–C stretch and CH2 twist are excited by Jahn–Teller activity of the degenerate 2E resonance, the bending by vibronic coupling with higher lying resonances. The essential features of the excitation are qualitatively rationalized by a Hartree–Fock (HF)/6-31G* anion potential energy surface. Unspecific excitation of high vibrational levels, accompanied by detachment of slow electrons, is also observed to result from attachment of 2 eV electrons, and is rationalized as a consequence of temporary trapping of part of the nuclear wave packet on the bound (not autodetaching) part of the anion potential surface. Very broad resonance features are observed in the 2–16 eV range, mainly in the excitation functions of the C–H stretch and the CH2 scissoring vibrations. A moderately broad resonance peak at 11.5 eV, observed in the excitation of the symmetric and antisymmetric C–C–C stretch vibrations, is assigned to two overlapping σ* shape resonances. Absolute elastic cross sections are given for reference.
Atomic oxygen in crystalline Kr and Xe. II. Adiabatic potential energy surfaces
Authors: Danilychev A. V. , Apkarian V. A.
Published: July 2002

The potential energy surfaces of atomic oxygen, O(3P,1D,1S), trapped in crystalline Kr and Xe are developed based on known angularly anisotropic pair interactions. The electrostatic limit, with the neglect of exchange and spin–orbit interactions, is assumed. Using a classical statistical treatment for the simulation of spectra, the surfaces are shown to reproduce the experimental O(1S→1D) emissions in substitutional and interstitial sites of crystalline Kr. The surfaces are also in accord with charge transfer emission spectra of O/Xe solids. With lattice relaxation, the Xe–O(1D)–Xe insertion site becomes the global minimum, and can therefore act as a stable trap site. This is in accord with experimental observations of a third trapping site in Xe. To rationalize the recently reported long-range mobility of O atoms in these solids [A. V. Danilychev and V. A. Apkarian, J. Chem. Phys. 99, 8617 (1993)], the topology of various electronic surfaces are presented. It is shown that the minimum energy paths connecting interstices on the triplet and singlet surfaces are quite different. The triplet path is strongly modulated and proceeds along body diagonals of the unit cell. The singlet path is more gently modulated and proceeds along face diagonals. These features are consistent with the postulated thermal mobility as proceeding via triplet-singlet conversion. However, on a quantitative basis, the electrostatic surfaces fail to support the model. The site specific crossing energies, including lattice relaxation, are calculated to range between 1.2 and 1.7 eV in Xe and Kr, which is an order of magnitude larger than the observed experimental activation energies of migration. Inclusion of spin–orbit and charge transfer mixing in these surfaces, absent in the present treatment, should reduce this discrepancy.
Ultrafast charge transfer in an electron donor–acceptor complex
Authors: Wynne Klaas , Galli C. , Hochstrasser R. M.
Published: July 2002

Ultrafast pump–probe measurements on the electron donor–acceptor complex of tetracyanoethylene with hexamethylbenzene in polar and nonpolar solvents are reported. Ground state coherence in the complex stretching mode at 165 cm−1 excited by impulsive stimulated Raman scattering is observed as well as decay of the ground state bleaching signal due to return electron transfer to the ground state. The experimental electron-transfer rates are compared with nonadiabatic and adiabatic electron-transfer theories using a previously published analysis of all the vibrational modes active in the reaction. It is shown that a breakdown of the Born–Oppenheimer approximation can give rise to a coupling that leads to the observed electron-transfer reaction. The non-Born–Oppenheimer matrix element is estimated using information obtained from the absorption and Raman spectra. Using this coupling, good agreement is found between the experimentally observed and theoretically predicted rates. Caveats of the various theories, the reliability of the normal mode analysis, and aspects of electron transfer that theory should address are discussed.
Frequency-dependent polarizabilities of O2 and van der Waals coefficients of dimers containing O2
Authors: Hettema H. , Wormer P. E. S. , Jo/rgensen P. , Jensen H. J. Aa. , Helgaker T.
Published: July 2002

We report frequency-dependent dipole and quadrupole polarizabilities of the oxygen molecule in its 3Σ−g ground state. These properties are obtained by means of the multiconfiguration self-consistent field method and used for the computation of the van der Waals coefficients of (O2)2 and the O2–Rg dimers, where Rg is He, Ne, Ar, and Kr. The required frequency-dependent polarizabilities of the rare gases were computed earlier by means of second-order many body perturbation theory.
Real-space renormalization for Heisenberg models on two-dimensional lattices
Authors: Cheranovski V. O. , Schmalz T. G. , Klein D. J.
Published: July 2002

Real-space renormalization methods which simply shift and rescale the interaction parameters in the nearest-neighbor Heisenberg model are investigated for two-dimensional lattices of equivalent sites. Results are presented for the hexagonal, square-planar, triangular, and Kagome lattices via three different renormalization techniques. The first, which has been studied for some time, uses perturbation theory to evaluate the renormalized interactions. The second uses the variational method to improve on the perturbative results. The third method is based on a cluster expansion and is found to give much improved agreement with numerical results form Monte Carlo calculations, but it does not provide a variational bound to the exact solution. The dependence of all three methods on the size and shape of the renormalized block of sites is also investigated.
Vibronic analysis of overlapping resonances and the third-harmonic-generation spectrum of β-carotene
Authors: Soos Z. G. , Mukhopadhyay D.
Published: July 2002

Vibronic contributions to third-harmonic-generation (THG) are obtained in the Condon approximation for displaced harmonic oscillators in the region of overlapping three- and two-photon resonances, where enhanced THG intensity and explicit dependence on the relative signs of the displacements are found. The THG intensity and phase of β-carotene are then modeled in terms of four electronic states, including overlapping 2 1Ag and 1 1Bu resonances whose displacements are taken from two-photon and linear spectra, respectively, and a high-energy Ag state based on a Pariser–Parr–Pople (PPP) sum rule for transition dipoles. Relations between THG and other spectra show the limitations of three-state models and provide useful constraints on the excited-state structure of related conjugated systems such as polyenes and β-carotene.
Molecular electronics: Disordered molecular wires
Authors: Kemp M. , Mujica V. , Ratner M. A.
Published: July 2002

We present results for the effect of diagonal disorder on the conductance of molecular wires, using a simple one-dimensional tight-binding picture. We show that diagonal disorder affects three aspects of the conductance: (i) It produces large conductance fluctuations from wire to wire. (ii) It reduces the conductance in situations where the transfer would be resonant, and enhances the conductance when transfer is nonresonant. (iii) It gives near-exponential conductance decays with wire length.
Static polarizabilities and dipole moment derivatives for the closed shell coupled cluster singles and doubles wave function
Authors: Kobayashi Rika , Koch Henrik , Jo/rgensen Poul
Published: July 2002

Analytic expressions are presented for the static polarizabilities and dipole moment derivatives for the CCSD wave function as energy derivatives with respect to a field. Sample calculations are given for the isomers of cyanogen (NCCN, CNCN, CNNC).
The vacuum-ultraviolet photodissociation of NH2(X̃ 2B1)→NH(A 3Π)+H
Authors: Biehl H. , Schönnenbeck G. , Stuhl F. , Staemmler V.
Published: July 2002

The photodissociation of NH2→NH(A 3Π)+H was investigated by photolyzing NH2 in a flow system with tunable synchrotron radiation from 200 to 105 nm and other vuv light sources. The NH photofragments were analyzed by their triplet emission at 336 nm. Additionally, ab initio configuration interaction calculations were performed for the electronic states of NH2 involved in the photodissociation process. Vertical excitation energies, bending potentials for the excited states, Franck–Condon factors, and transition moments were calculated in order to interpret the experimental observations. The following picture evolves for the dynamics of the NH2 photodissociation: At about 7.8 eV, NH2 is excited to the 2 2A1(A′) state, which possesses the same bending angle as the X̃ 2B1 ground state. The upper state correlates with the fragments NH(A 3Π)+H. Since the bending angle is not changed, the NH(A) radicals are formed with little rotational excitation. However, the symmetric stretch becomes excited at the beginning of the dissociation leaving the NH(A) fragment with vibrational excitation. Because of symmetry conservation, the formation of the Π(A′) component of NH(A) is preferred. In the region of ∼9 eV, transitions to the 1 2A2 and/or 3 2B1 states (both have A″ symmetry in Cs) occur. The bending potentials of both states have minima for linear configurations. Therefore, the structure of the excitation spectrum is determined by a progression in the bending motion and a preferred population of high rotational NH(A) levels is observed. Vibrational excitation is small suggesting that the unbroken NH bond stays unchanged during the dissociation process. According to symmetry conservation, the Π(A″) component of NH(A) is preferably formed.
Ab initio molecular orbital study of potential energy surface for the NH+NO2 reaction
Authors: Mebel A. M. , Morokuma K. , Lin M. C.
Published: July 2002

The reaction of NH with NO2, which can produce N2O+OH and HNO+NO by two distinct reaction paths, has been studied by ab initio molecular orbital calculations. The first reaction path taking place by initial N–N association forms an intermediate HNNO2, 1, which undergoes H-migration yielding NN(O)OH, 3, before reaching the N2O+OH product. The transition state 2 for the rate-determining 1→3 rearrangement, with the activation barrier of 30 kcal/mol at the G2-level of calculation, lies below the energy of the reactants. The O migration for the HNNO2 1 intermediate to produce HNO+NO is inaccessible at low temperatures due to the presence of a high migration barrier. The second path via initial N–O association forms an intermediate HNONO, 9, which is expected to dissociate readily to HNO+NO via a loose transition state lying 24 kcal/mol below the reactants. Since the initial N–N and N–O association reactions effectively occur with no barriers, the overall activation energy for NH+NO2 is expected to be negligible or slightly negative as was found experimentally.
Computer simulation studies of a model system for liquid crystals consisting of semiflexible molecules
Authors: Nicklas K. , Bopp P. , Brickmann J.
Published: July 2002

A simple steric model of so-called rfr (rigid–flexible–rigid) molecules is developed. It consists of two rigid spherocylindrical fragments linked by a flexible alkyl chain. Molecular dynamics simulations of 128 model molecules are carried out in a periodic box, and the influence of density and alkyl chain length on the molecular order and the self-diffusion is studied. Different liquid crystalline phases are found which are stable within the time scale of the simulations. In agreement with experimental results the nematic order parameters and the stability of the phase, as indicated by the location of the isotropic/liquid crystalline phase transition, show an odd–even effect with respect to the number of methylene groups in the alkyl chain. A separation of rigid and flexible fragments is observed leading to smectic liquid crystalline phases.
Interdiffusion in dilute polymer mixtures. A subtle concentration effect
Authors: Jian T. , Anastasiadis S. H. , Rizos A. K. , Fytas G. , Akcasu A. Z.
Published: July 2002

Dynamic light scattering has been used to investigate the diffusional dynamics in very dilute polystyrene/poly(propylene oxide), PS/PPO, polymer blends. Compared to previous investigations in the field, this system is more suitable for this type of investigation due to the significant refractive index difference between the two components and the fact that the matrix (PPO) dynamics do not interfere with the measurements. The tracer diffusion coefficient of PS thus obtained in the limit of infinite dilution scales as N−0.8±0.04PS with the PS degree of polymerization, i.e., behavior intermediate between the limits of nondraining Zimm and free-draining Rouse behavior. The effect of the addition of a third component even at tracer concentrations on the diffusion dynamics was investigated both experimentally and theoretically in the framework of the dynamic random phase approximation. Similarities and differences between theory and experiment were found that are rather due to a modification of hydrodynamic interactions.
Infrared vibrational photodissociation spectra of Ar+2 ions
Authors: Duffy Liam M. , Feinberg Thomas N. , Baer Tomas
Published: July 2002

The infrared photodissociation spectra of Ar dimer ions have been obtained at several wavelengths with a line tunable CO2 laser. The dimer ions were produced by high energy electron impact ionization near or at the nozzle orifice of a supersonic expansion. When the electron beam is focused several millimeters from the nozzle, and the laser polarization is parallel to the dimer ion beam, the product Ar+ kinetic energy spectra exhibit structure, with a spacing of 117 cm−1. However, when the ions are produced by focusing the electron beam directly onto the nozzle, the structure largely disappears. Analysis of the results indicates that the transition is a bound to continuum transition, and that the observed spacing is associated with vibrational levels of the first excited Ar+2 I(3/2)g state.
The H+3 molecule ion: A two-particle density study of electron correlation
Authors: Banyard K. E. , Sanders J.
Published: July 2002

Changes in the one- and two-particle densities arising from Coulomb correlation have been examined, in detail, for the ground state of H+3. This electron deficient system was described in its equilateral triangular conformation by a wave function which recovered approximately 99% of the correlation energy. Contour diagrams are presented for densities evaluated in the molecular plane and also in a plane perpendicular to the molecule. For the two-particle distribution, correlation effects are highlighted in different regions of the system by considering a series of fixed positions for a ‘‘test’’ or ‘‘reference’’ electron. In addition, for each location of the test particle, planar Coulomb holes are determined which emphasize the relative importance of the radial and angular components of correlation. Contrary to an earlier study, it is demonstrated that Coulomb correlation reduces the density within the H+3 nuclear triangle. The present findings are compared with a similar analysis for the H2 molecule in position (real) space, already reported.
A redetermination of the dissociation energy of MgO+
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry
Published: July 2002

The binding energy for the X 2Π ground state of MgO+ has been determined using the coupled-cluster singles and doubles approach with a perturbational estimate of the triples and the modified coupled-pair functional approach in conjunction with very large basis sets. Our best value of 2.32±0.06 eV is in excellent agreement with our previous result and with the photodissociation result of Freiser and co-workers, but is smaller than the recent value determined using the guided-ion beam method.
The structure of water at a planar wall: An integral equation approach with the central force model
Authors: Vossen M. , Forstmann F.
Published: July 2002

The structure of liquid water in contact with a planar wall is calculated for the central force model by which water is modeled as a stoichiometric mixture of partially charged hydrogen and oxygen atoms interacting via effective spherical pair potentials. The density profiles near the wall are obtained from the Wertheim–Lovett–Mou–Buff equation extended to mixtures with coulombic interactions. The correlations in the bulk liquid are calculated by adding bridge functions of appropriate reference hard sphere systems to the hypernetted chain equation. We have obtained structural (stoichiometry, hydrogen bonds) and thermodynamic properties in good agreement with computer simulations. We also expose the water to electric fields due to a surface charge on the wall.
Franck–Condon modeling of the structure of the S→S2 transition of trans, trans-, cis, trans-, and cis, cis-octatetraene
Authors: Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

The Franck–Condon structure of the lowest lying intense polyenic electronic transition of trans, trans-, cis, trans-, and cis, cis-octatetraene is investigated through model calculations. To avoid a possible bias in the parameters of the model, the starting inputs are obtained ab initio. The molecular orbital (MO) procedure consists in first optimizing the structures of S0 and S2 and then calculating the vibrational frequencies at the stationary points on the potential energy surface of the three isomers. Together with the minima associated with the three isomers, we find one more saddle point in S0 and two more in S2. These three saddle points correspond to planar S0 cis, cis-octatetraene, planar S2 cis, cis-octatetraene, and planar S2 cis, trans-octatetraene. The displacement, between the surfaces, of the harmonic oscillators associated with the normal modes, are obtained and used to simulate the Franck–Condon activity of the S0→S2 transitions of the three isomers. Such displacements are calculated in two schemes, the first uses the variation of the equilibrium position of the vibrational oscillators in the two states involved in the transition and requires full geometry optimization of all the geometrical parameters of the two states; the second, approximate, scheme requires only a single point calculation on the excited state surface and is therefore far less demanding. A simple scaling procedure, proposed before for hexatriene, is used to improve the agreement between theory and experiment. The model calculations of the Franck–Condon structure simulate very well the S0→S2 absorption spectra of trans, trans- and cis, trans-octatetraene. It is further proposed that the large homogeneous linewidth in the S0→S2 transitions of polyenic systems is a function of the nonplanarity of these molecules in S2.
Optogalvanic transients in a neon radio frequency discharge
Authors: Kumar D. , Zinn R. R. , McGlynn S. P.
Published: July 2002

The time dependence of the optogalvanic signals induced by 1sj→2pk (Paschen notation) pulsed laser excitations of neon in a low power ∼30 MHz radio frequency (rf) discharge at ∼5 Torr has been investigated. The method of Kumar and McGlynn [Chem. Phys. Lett. 176, 536 (1991)] has been improved and is used to separate the temporal profiles of the optogalvanic signals into two components: one attributable to ionization rate changes and the other to acoustic effects. This separation simplifies the identification of the kinetic processes which produce optogalvanic signals. Selective laser excitation of 2pk states, ones which possess quite different decay branching ratios to the metastable and nonmetastable 1sj states, reveals time dependencies due to unique perturbations of specific 1sj populations. No dimer mediated nor, indeed, any effects associable with 2pk state populations contribute to the profiles. Competitive processes which increase or decrease the ionization rates are identified at t≥1 μs. The extensive broadening of the acoustic and ionization components is caused by radiation trapping of 1s2→1S0 and 1s4→1S0 photons.
The normal to local mode transition in AB2 triatomic molecules: The susceptibility of eigenstates to symmetry breaking perturbations
Authors: Schmid Günter Maximilian , Coy Stephen L. , Field Robert W. , Silbey Robert J.
Published: July 2002

We argue that the physical significance of the normal to local mode transition in triatomic molecules AB2 lies in an enhanced susceptibility of eigenstates to symmetry breaking perturbations. States of local character have higher susceptibilities than states of normal character. The quantum mechanical condition for the normal to local mode transition is formulated. The issue of stability is addressed from a classical, semiclassical, and quantum mechanical point of view. The question of instability of the unperturbed quantum system is decided individually for each eigenstate. The relevance for experiments is outlined.
Initiation of a phase transition by preexisting nuclei
Authors: Shneidman Vitaly A. , Hänggi Peter
Published: July 2002

Preexisting nuclei are often invoked in situations where the observed kinetics of a phase transition is much faster than one would expect from the classical theory of homogeneous nucleation. We examine analytically the time-dependent effects produced by such nuclei. Corrections to the number of nucleated particles, the time lag, and the transient rate of homogeneous nucleation are derived. For a high nucleation barrier the results are expected to be asymptotically accurate, both for the continuous (‘‘Zeldovich–Frenkel’’) and the discrete (‘‘Becker–Döring’’) nucleation models. This is confirmed by comparison with exact expressions available for a parabolic barrier in the continuous case, and with numerical solutions of the discrete Master equation. The probability formulation of the nucleation problem is also considered and the distribution of waiting times to detect the first ‘‘successful’’ nucleus is obtained.
The S↔S1 transition of trans-β-methyl styrene
Authors: Drescher W. , Kendler S. , Zilberg S. , Zingher E. , Zuckermann H. , Haas Y.
Published: July 2002

The vibronic structure of the fluorescence excitation and emission spectra of trans-β-methyl styrene have been measured and analyzed, using an ab initio calculation. Good agreement between the experimental and calculated data, which indicate that the molecule is planar in both the ground and the S1 states, is obtained.
Determination of a chemisorption bond strength by direct measurement of the threshold energy for collision-induced desorption: Ammonia on Pt{111}
Authors: Szulczewski Gregory , Levis Robert J.
Published: July 2002

Direct measurement of the threshold desorption energy for collision-induced desorption is used to determine a chemisorption bond energy. In this experiment the absolute cross section for desorption is measured using x-ray photoelectron spectroscopy as a function of the impact energy between a translationally energetic Ar atom and an ammonia molecule chemisorbed to a Pt{111} single crystal. For this adsorbate-surface system the threshold desorption energy is found to be 1.8±0.17 eV and is independent of the angle of incidence of the Ar beam. Using a classical energy transfer mechanism this threshold energy is found to correspond to a bond energy of 1.4±0.13 eV. This represents an upper limit for the bond strength assuming maximum energy transfer between Ar and NH3 and no internal excitation of the adsorbate at the desorption threshold.
The charge transfer excited state of the I−⋅CH3I SN2 reaction intermediate: Photoinduced intracluster dissociative attachment
Authors: Cyr Donna M. , Bailey Christopher G. , Serxner David , Scarton M. Georgina , Johnson Mark A.
Published: July 2002

We describe the reaction dynamics which occur upon excitation of the I−⋅CH3I ion–molecule complex to the first optically allowed excited electronic state. Photoelectron spectroscopy of I−⋅CH3I confirms the identification of the species as essentially charge localized, where the observed vibrational fine structure is found to be consistent with small distortions of the CH3I neutral upon complexation to form a stable intermediate in the SN2 identity reaction. A narrow photofragmentation band lies just below the vertical electron detachment energy and is assigned to the I−⋅CH3I→I...[CH3I]− charge transfer excited state. The recoil energy imparted to the I− fragment is only about 10% of the available energy, indicating that most of the energy is lost to the methyl group as expected for an impulsive dissociation. The I− photoproduct is preferentially ejected along the electric vector of the laser with an anisotropy parameter β of +0.5±0.2. This requires that photoabsorption occurs to a repulsive state which dissociates on a time scale at least comparable to the rotational period of the complex. While the recoil characteristics suggest a direct transition to a repulsive excited state, which would be expected for a charge-transfer excitation between the I− and CH3I moieties, the I− photofragment action spectrum displays vibronic structure similar to that found in the photoelectron spectrum. These results indicate that the photoexcitation mechanism is not direct charge transfer, but mediated by a weakly bound negative ion state. We develop a picture of the photofragmentation dynamics in the context of the diabatic (charge-localized) states recently invoked to interpret the structure of the (ground state) SN2 potential surface.
Ab initio structure and wave packet dynamics of ICN photodissociation
Authors: Qian Jiwen , Tannor David J. , Amatatsu Yoshiaki , Morokuma Keiji
Published: July 2002

We present a time-dependent quantum mechanical calculation of ICN photodissociation in the Ã continuum, using the ab initio potential surfaces of Morokuma and co-workers [S. Yabushita and K. Morokuma, Chem. Phys. Lett. 175, 518 (1990); Y. Amatatsu, S. Yabushita, and K. Morokuma, J. Chem. Phys. 100, 4894 (1994)]. Five excited state potential energy surfaces are included in this model, 3Π0+, 1Π1 (A′,A″), and 3Π1 (A′,A″), which are accessed, respectively, by parallel, perpendicular, and perpendicular transitions from the ground state. The calculated absorption spectrum, β parameters, the I/I* branching ratio, and the rotational product distribution are in good agreement with experiment. The I/I* branching ratio for photodissociation from vibrationally excited states of ICN has been calculated. The results are in good agreement with the recent measurements at different vibrational temperatures by Kash and Butler [P. W. Kash and L. J. Butler, J. Chem. Phys. 96, 8923 (1992)] at 249 nm but, interestingly, predict opposite trends at higher and lower excitation energies.
Molecular dynamics simulation studies of solvent cage effects on photodissociation in condensed phases
Authors: Wang Weining , Nelson Keith A. , Xiao Li , Coker David F.
Published: July 2002

Molecular dynamics simulations have been applied to study solvent cage effects on photodissociation. The results show that when the solvent cages are sufficiently rigid, such as in Lennard-Jones glasses at low temperatures and high densities, or when solvent masses are substantially greater than solute masses, photoexcitation of a solute molecule to a repulsive excited state could lead to coherent oscillatory quasibound motion. This type of motion should be observable through femtosecond time-resolved spectroscopy.
Neutron inelastic scattering studies of UF4 and NpF4
Authors: Kern S. , Lander G. H. , Soderholm L. , Loong C.-K. , Trouw F. , West M. , Hoisington D. , Cort B. , Welp U.
Published: July 2002

Neutron inelastic scattering experiments have been performed on UF4 and NpF4. We had anticipated observing a number of sharp magnetic transitions in these experiments because the tetravalent metal ions are expected to have localized f states. However, very few transitions were observed, three and two in UF4 and NpF4, respectively. Except for the lowest-lying level (at 1.5 meV) in UF4 the transitions are wider than the instrumental resolution, suggesting a strong interaction between the electronic states and lattice vibrations. The levels observed do not correspond to those predicted by a crystal-field scheme recently proposed from optical spectroscopy. However, the paucity of observed levels in our experiments excludes us proposing an alternative scheme. The 1.5 meV energy level in UF4 is in good agreement with that deduced from specific heat and magnetic susceptibility data.