bluuebunny/crossref_metadata_2025 huggingface dataset:
Symmetry and rotational orientation effects in dissociative adsorption of diatomic molecules on metals: H2 and HD on Cu(111)
Authors: Dai Jiqiong , Sheng Jia , Zhang John Z. H.
Published: July 2002

Following two previous quantum dynamics studies [J. Chem. Phys. 97, 6784 (1992); 99, 1373 (1993)], we present in this paper a more thorough investigation of the symmetry and rotational orientation effects in dissociative chemisorption of diatomic molecules on metals. Specifically, we extended our theoretical studies to calculate the sticking coefficients for H2 and its isotopomer HD on Cu from all angular momentum states (up to j=8). Our calculation shows a strong dependence of the dissociation probability P(jm) on both j and m rotation quantum numbers, and the increases of P(jm) are closely correlated with the increase of the quantum number m in a given j manifold. Also the dissociation of the diatomic rotational states whose quantum numbers satisfy  j+m=odd is forbidden at low energies for the homonuclear H2 due to the selection rule. The present study provides further evidence that the effect of diatomic rotation on adsorption mainly arises from the effect of rotational orientation (m dependence) as found in previous studies. This m dependence predicts that at low kinetic energies, the degeneracy-averaged dissociation probability of hydrogen on Cu increases monotonically as the rotation quantum number j increases. However, at high kinetic energies, the adsorption probability first decreases as j increases from 0 to about 4–5 before increasing as j further increases above 4–5. The latter behavior is consistent with a recent experimental measurement by Michelsen et al. of the mean kinetic energy of the rotational states of D2 desorbed from Cu(111).
Ab initio Hartree–Fock self-consistent-field molecular dynamics study of structure and dynamics of Li8
Authors: Jellinek J. , Bonac̆ić-Koutecký V. , Fantucci P. , Wiechert M.
Published: July 2002

An ab initio Hartree–Fock-based molecular dynamics scheme is formulated and applied to study structural and dynamical properties of the Li8 cluster. Three new locally stable isomers of the cluster—of a C3v, a Cs, and a D2d symmetry, respectively—are identified. The main emphasis is on exploring the cluster dynamics as a function of its energy. The simulations indicate that the cluster undergoes a solid-to-liquid-like transition as its energy is increased. An analysis of the transition is given at the level of detail which approximates that of studies utilizing semiempirical potentials. In particular, isomer-specific mechanisms of the phenomenon are described. The computational efficiency of the scheme is achieved through code optimizations and use of parallel processing. The approach is critically evaluated and directions of future work are outlined.
Cluster impact chemistry. High-energy collisions of I2ArN clusters with a Pt surface
Authors: Schek Israel , Raz Tamar , Levine R. D. , Jortner Joshua
Published: July 2002

In this paper, we explore cluster–surface impact induced dissociation of an I2 molecule initially embedded within an I2ArN (N=11–553) cluster, which collides with a Pt surface. Molecular dynamics simulations of high-energy I2ArN–Pt surface collisions (with initial center of mass velocities v=0.2–10 km s−1 and initial kinetic energies E0K=1 eV−1.2×104 eV) provide information on the yields and time scales for energy acquisition by the cluster and by the surface and energy deposition to the guest molecule via the formation of an intracluster microscopic shock wave, as well as on the I2 dissociation dynamics. The intracluster shock wave is characterized by a temporal peak in the cluster potential energy and in the saturation of the cluster temperature, with the sum of the yields for potential and kinetic energy deposition into the cluster being 0.5–0.6. The cluster residence time (τ=50–800 fs over our velocity and cluster size domain) coincides (within 20%) with the time scale for the cluster energy acquisition, decreasing linearly with v−1 and obeying a dynamic size equation τ∝(N+2.9)1/3. The characteristic time tp for energy deposition to the I2 molecule via a local mechanism involving pair interactions is also close to τ.

The initial cluster kinetic energy dependence of the dissociation yields YD of I2 reveals a gradual increase of YD towards unity above a threshold at the energy Et. For smaller (N=11,53) clusters, Et/N is close to the dissociation energy of bare I2, while for larger clusters Et exhibits an exponential N dependence. Cluster impact dissociation of I2 in I2ArN results in higher YD values (≳0.4) than the high-energy collision of bare I2 with the Pt surface for which YD saturates at 0.35. The I2 dissociation times 〈τD〉, which were characterized by averaging over the first passage times for the attainment of the turning point of the I–I intramolecular Morse potential for reactive trajectories, fall in the range 170–800 fs, exhibiting a marked inverse kinetic energy dependence, revealing an increase with increasing cluster size and obeying the rough relation 〈τD〉≂2τ, i.e., being proportional to the cluster radius. Energy acquisition and dissociation times are comparable to or even shorter than the vibrational time [τ(I2)=156 fs] of the I2 molecule, opening up a new research area of thermal femtosecond chemistry.
Intramolecular association in flexible hard chain molecules
Authors: Ghonasgi Dhananjay , Perez Victor , Chapman Walter G.
Published: July 2002

Due to the flexibility of associating polymer and protein molecules, intramolecular association can have a significant affect on the thermodynamic properties and structure of associating polymer and protein solutions. The equilibrium state is determined by the minimization of the appropriate free energy with respect to intermolecular association between like and unlike species and intramolecular association. As a first step to understanding this competition between intramolecular and intermolecular hydrogen bonding, we have conducted a molecular simulation study of flexible hard chain molecules that intramolecularly associate in the absence of intermolecular association. To explain the simulation results, we have developed a new simple and accurate theory of intramolecular association. By considering the limit of total bonding, we have also developed an accurate equation of state for hard rings. The theory is in good agreement with new molecular simulation results for intramolecularly associating hard chains, rigid hard rings, and bent triatomics.
Intermolecular vibrations of o-xylene⋅Ar in the S and S1 states: Experiment and quantum three dimensional calculations
Authors: Droz Thierry , Leutwyler Samuel , Mandziuk Margaret , Bačić Zlatko
Published: July 2002

A combined experimental and theoretical study of the intermolecular vibrations of the o-xylene⋅Ar van der Waals complex is reported for both the S0 and S1 electronic states. Two-color resonant two-photon ionization and fluorescence emission spectra of the vdW mode region of supersonic jet-cooled o-xylene⋅Ar exhibit five bands within 70 cm−1 of the electronic origin, which arise from low-frequency large-amplitude intermolecular vibrations. Accurate quantum 3D calculations of vdW vibrational levels were performed, based on the 3D discrete variable representation. Apart from the restriction to the J=0 state the calculated eigenstates are exact for the intermolecular potential energy surface (PES) employed. The PES is represented as a sum of Lennard-Jones (LJ) pair potentials, and the direct comparison between theory and experiment enabled calibration of the LJ parameters. Very good agreement was achieved for both the S0 and S1 states of o-xylene⋅Ar. The quantum 3D calculations provide a quantitative description of the vdW level structure up to ≊70 cm−1 above the vdW ground state. The low-energy eigenfunctions have nodal patterns analogous to the 2,3-dimethylnaphthalene⋅Ar complex. However, in the energy range 40–60 cm−1 the vdW mode eigenfunctions change over to 2D radial-oscillator-type wave functions, similar to those of benzene⋅Ar, but switch back to Cartesian type above 60 cm−1. The S1 state vdW levels of 2,3-dimethylnaphthalene [M. Mandziuk, Z. Bačić, T. Droz, and S. Leutwyler, J. Chem. Phys. 100, 52 (1994)] were recalculated with the present parameters, and the agreement between experimental and calculated frequencies is improved.
The application of transition state theory to gas–surface reactions in Langmuir systems
Authors: Pitt Ian G. , Gilbert Robert G. , Ryan Keith R.
Published: July 2002

Transition state theory (TST) has long been applied to gas–surface reactions for the purpose of characterizing and predicting desorption rate coefficients, but current perceptions of its use exclude TST from meaningful description of the sticking coefficient for partially covered surfaces. This paper re-examines the application of TST to partially covered surfaces, and in particular to Langmuir systems with barrierless adsorption. An analogy is drawn in the present work between barrierless adsorption processes and gas-phase radical–radical recombination reactions that reveals a simple method by which TST can describe kinetics in Langmuir systems. Construction of a model ‘‘hard-wall’’ Hamiltonian that reflects the exclusion of adsorbates from occupied sites enables the kinetics in Langmuir systems to be described by constructing a transition state surface that does not permit adsorption at occupied sites. This method gives analytic expressions for the TST sticking coefficient (and desorption rate coefficient) in two important limiting cases for the gas–surface dynamics, and resolves previous difficulties in the TST description of Langmuir systems.
Rotationally resolved electronic spectra of the ‘‘half-sandwich’’ organometallic radical, CaC5H5
Authors: Cerny Timothy M. , Williamson James M. , Miller Terry A.
Published: July 2002

Rotationally resolved spectra have been recorded and analyzed for both spin–orbit components of the 000 and 410 (ν4= Ca–C5H5 stretch) vibrational bands of the Ã 2E1←X̃ 2A1 transition of the organometallic free radical CaC5H5. The radicals were prepared with a laser ablation/photolysis technique, then probed in a supersonic free-jet expansion. Intensity variations from transitions originating from various K″ levels confirm the C5v nature of the system with the Ca atom centered over the cyclopentadienyl ring. A complete structure could not be uniquely deduced from the rotational constants determined in the global fit of the two vibrational bands but reasonable assumptions for the C5H5 ring enable the Ca–C5H5 ring distance to be determined to be 2.333(+13−23) Å in the ground state which shortens by 0.060(1) Å in the excited state. Other fitted molecular constants indicate that Jahn–Teller distortion effects are unmeasurably small and that the electronic orbital angular momentum in the excited state is essentially unquenched with ζt=0.9678(3).
Fluorescence excitation spectroscopy and dynamics of the ArAlH(X 1Σ+,A 1Π) van der Waals complex
Authors: Hwang Eunsook , Dagdigian Paul J.
Published: July 2002

The laser fluorescence excitation spectrum of the ArAlH van der Waals complex, in the vicinity of the A 1Π–X 1Σ+ (0,0) band of diatomic AlH, is reported. This species was prepared in a pulsed free jet by 193 nm multiphoton dissociation of trimethylaluminum seeded in Ar/He. Both rotationally resolved and diffuse bands of the ArAlH complex were observed. Rotational analyses were carried out for most of the sharp bands; both perpendicular (P′=1←P″=0) and parallel (P′=0←P″=0) transitions were found. The bending and van der Waals stretch vibrational quantum numbers were assigned with the help of the calculated ArAlH(X,A) vibrational energies reported in the accompanying paper [M. Yang, S. Gregurick, M. H. Alexander, and P. J. Dagdigian, J. Chem. Phys. 102, 2413 (1995)]. Unambiguous assignments for all the observed sharp bands (except one very weak band), including four hot bands originating from the first excited bend–stretch level of the ArAlH(X) complex, have been made.
Recombination and relaxation of molecular ions in size-selected clusters: Monte Carlo and molecular dynamics simulations of I−2 (CO2)n
Authors: Papanikolas John M. , Maslen Paul E. , Parson Robert
Published: July 2002

The equilibrium structures and the recombination dynamics of I−2 molecular ions embedded in clusters of 3–17 CO2 molecules are studied by Monte Carlo and molecular dynamics simulations. The potential model incorporates, in a self-consistent manner, a description of the I−2 electronic structure that depends on both the I−2 bond length and the solvent degrees of freedom. The influence of the solvent upon the I−2 electronic structure is treated by means of a single effective solvent coordinate, in a manner reminiscent of the theory of electron transfer reactions. This causes the electronic charge to localize on a single I atom when the I–I bond is long or when the solvent cage has become highly asymmetric. The primary focus is the I−2 vibrational relaxation that follows recombination. Simulations of I−2(CO2)16 and I−2(CO2)9 yield vibrational relaxation times of less than 3 ps, even faster than the experimentally observed absorption recovery time of 10–40 ps. It is suggested that the latter time scale is determined by electronic as well as vibrational relaxation mechanisms.
Description of excitation processes induced by short laser pulses in low-dimensional effective spaces of Floquet states
Authors: Dietz Klaus , Schmidt Christoph , Warken Markus
Published: July 2002

The time evolution of atoms or molecules under the influence of a pulse of a single mode laser is most effectively described in the space of quasistationary Floquet states. Separating in this way the fast and slow time scales involved one finds that the time evolution is determined by the singularities in the quasienergy complex plane in strict analogy to the stationary case. Employing the Λ transformation we calculate quasienergies and the corresponding Floquet states perturbatively. We show, for the case of an H2 molecule, that Λ-transformed many body perturbation theory is an excellent tool for the calculation of quasienergies and quasistationary states even for very intense laser fields. We then show that the evolution of the system in an effective two-level subsystem allows for a quantitative calculation of transition probabilities.
A molecular dynamics study of the coupling of torsional motions to self-diffusion in liquid n-hexane
Authors: Travis Karl P. , Brown David , Clarke Julian H. R.
Published: July 2002

Dynamic coupling of molecular flexibility with self-diffusion has been studied for a model of liquid n-hexane using the technique of frozen distribution sampling (FDS) [Travis et al., J. Chem. Phys. 98, 1524 (1993)]. FDS facilitates comparison of transport properties between a liquid of flexible molecules and a liquid in which the molecular shape fluctuations are frozen in an equilibrium distribution; thus the two liquids have identical equilibrium configurational properties. The diffusion coefficient of model hexane was found to increase by a factor of ∼20 on raising the temperature from 200 and 1000 K; coupling with torsional motions enhances the diffusion coefficient by about 20% over the whole of this range. At each temperature a quantitative measure of the coupling was obtained by making comparisons with self-diffusion in a liquid for which the potential restricting torsional motion is removed—called flexane [D. Brown and J. H. R. Clarke, J. Chem. Phys. 86, 1542 (1987)]. Using FDS it was confirmed that the enhancement of the diffusion coefficient in flexane as compared to hexane arises from differences in the short time dynamics and not from differences in the static structure of the two liquids. For hexane a coupling parameter is defined which shows that the extent of dynamic coupling increases from ∼10% at 200 K and appears to have reached a maximum by 1000 K. The coupling mechanism is discussed using power spectra obtained from the relative velocities of the methylene (and methyl) groups relative to the molecular centers of mass. It is concluded that torsional oscillations mainly involving the outer dihedral angles provide the probable mechanism for the coupling.
Fluctuation phenomena in structurally symmetric polymer blends
Authors: Singh Chandralekha , Schweizer Kenneth S. , Yethiraj Arun
Published: July 2002

Polymer reference interaction site model theory with the new molecular closures is employed to numerically and analytically study structurally and interaction potential symmetric binary blends. Both the compressibility and free energy routes to the thermodynamics are studied and the issue of thermodynamic consistency is addressed. A variety of non-Flory–Huggins effects, or ‘‘fluctuation phenomena,’’ are found. These include nonuniversal renormalization of the critical temperature and effective chi-parameter from their mean field values, composition-dependent chi-parameters, and nonlinear dependence of the inverse osmotic compressibility on inverse temperature. All these fluctuation effects depend on degree of polymerization, N, chain length asymmetry, polymer density, range and precise form of the attractive tail potentials, chain stiffness, and proximity to the phase boundary. Some of the fluctuation effects are intrinsic, i.e., survive in the long chain N→∞ limit, while others are finite size effects which arise from chain-connectivity-induced coupled local density and long wavelength concentration fluctuations. Due to the multiple sources of the fluctuation effects, even asymptotic finite size effects can appear ‘‘intrinsic’’ over extended ranges of N. Comparison with lattice Monte Carlo simulations of Deutsch and Binder shows good agreement with the theoretical predictions. All the fluctuation effects can be understood in simple terms by examining the enthalpy of mixing and local interchain correlations. The key physical process is thermally driven local interchain rearrangements corresponding to the formation of diffuse interfaces and clusters or droplets. Analytic results are derived using the Gaussian thread model, which provides a simple physical understanding of the origin of the numerically determined fluctuation effects. In the long chain limit the predictions for the thread blend are shown to be exactly thermodynamically consistent which is a unique circumstance for liquid state theories. The relation of the blend fluctuation stabilization process to the corresponding diblock copolymer problem is briefly discussed.
On the interpretation of ‘‘ripple’’ polymer interdiffusion experiments in terms of models for bulk single-chain dynamics
Authors: Grayce Christopher J. , Szamel Grzegorz , Schweizer Kenneth S.
Published: July 2002

The short-time ripple in the deuterium fraction profile observed during interdiffusion of deuterium-labeled polystyrenes, reported by Russell et al. [Nature (London) 365, 235 (1993)] and Agrawal et al. [Macromolecules 27, 4407 (1994)] has been called ‘‘direct evidence of reptation.’’ We suggest that this may overstate the present case. Models of polymer motion which lack the essential features of reptation, in particular the Rouse and polymer mode-coupling models, are shown here to also predict the qualitative results of these ‘‘ripple’’ interdiffusion experiments. We suggest that for ripple interdiffusion experiments to be able to place stringent restrictions on models for bulk polymer dynamics they must be done with higher resolution of the ripple, and be combined with a more detailed understanding of the equilibrium structure of polymer fluids near interfaces.
Ab initio molecular dynamics study of antimony clusters
Authors: Sundararajan V. , Kumar Vijay
Published: July 2002

We present an ab initio molecular dynamics study of the atomic and electronic structure of SbN (N=2–8 and 12) clusters within the local density approximation and pseudopotential representation of the electron–ion interaction. Simulated annealing calculations have been done for 6-, 7-, 8-, and 12-atom clusters. While for Sb4 a bent rhombus is about 2 eV higher in energy than a regular tetrahedron, we find that it plays an important role in the structure of larger clusters. For Sb8 we obtain two weakly interacting tetrahedra to be of lowest energy. However, this is nearly degenerate with a bent rhombus interacting with a distorted tetrahedron. Further, our calculations suggest a bent rhombus based structure for Sb12 cluster indicating the observation of Sb4n clusters in Sb vapor condensation cell to be due to abundance of Sb4 clusters. A large gap is found to exist between the highest and the next occupied Kohn–Sham eigenvalues of the lowest energy isomers of 3-, 5-, and 7-atom clusters. This is in agreement with the abundance of cations of these clusters in the laser ablation experiments.
The HeCl2 potential: Atom–atom and ab initio compared to experiment
Authors: Huang Steve S. , Bieler Craig R. , Janda Kenneth C. , Tao Fu-Ming , Klemperer William , Casavecchia Piergiorgio , Volpi Gian Gualberto , Halberstadt Nadine
Published: July 2002

Two forms for the HeCl2 potential are compared to the available experimental data. First, an atom–atom form that incorporates the recently measured anisotropic He–Cl potential is used. The anisotropy of this potential is slight, and its strengths and weaknesses are similar to previous potentials in which the He–Cl interaction was treated as isotropic. In particular, the fit to the scattering data is poor. Second, a fit to ab initio points calculated using Mo/ller–Plesset perturbation theory to fourth order was performed. The resulting potential is much more anisotropic than any potential previously proposed and tested for HeCl2. This potential fits the rotationally resolved excitation spectra as well as do previous empirical potentials, and is consistent with certain features of the total differential scattering data with which previous potentials were not. Although the ab initio potential has a global minimum in the linear configuration, the probability distribution of the ground vibrational level still maximizes in the perpendicular configuration, accounting for the good fit to the rotationally resolved spectrum. We conclude that noble gas–halogen potentials are much more anisotropic than previously believed, and we suggest several experiments that could help to confirm this anisotropy.
Electric field dependence of magnetic properties: Multiconfigurational self-consistent field calculations of hypermagnetizabilities and nuclear shielding polarizabilities of N2, C2H2, HCN, and H2O
Authors: Rizzo Antonio , Helgaker Trygve , Ruud Kenneth , Barszczewicz Andrzej , Jaszuński Michal/ , Jo/rgensen Poul
Published: July 2002

Multiconfigurational self-consistent field (MCSCF) response is used to study the electric field dependence of magnetizabilities and nuclear shielding constants for N2, C2H2, HCN, and H2O. London perturbation-dependent atomic orbitals are used to ensure gauge origin independence. The computed magnetizabilities and shielding derivatives show a strong electron correlation dependence. The N2 results confirm the conclusions of previous ab initio studies. For the other molecules, this is the first study of the above magnetic properties beyond the SCF approximation.
Locating transition structures by mode following: A comparison of six methods on the Ar8 Lennard-Jones potential
Authors: Jensen Frank
Published: July 2002

Six different methods for walking from a minimum on a potential energy surface to a transition structure are tested on the Lennard-Jones surface for a cluster of eight argon atoms. The six methods consist of two Newton–Raphson-type algorithms using augmented Hessians, two methods for following gradient extremals, one following the intrinsic reaction coordinate on the image potential, and a constrained optimization technique. Only if the lowest mode of a given symmetry is followed can these methods locate transition structures in a stable manner. Optimizations along the higher modes display erratic or no convergence. The analysis shows that this is due to two factors: Hessian eigenvectors in general provide a poor direction for the uphill walk, and the presence of bifurcations along the path.
Performance of a fully close-coupled wave packet method for the H2+LiF(001) model problem
Authors: Kroes G. J. , Snijders J. G. , Mowrey R. C.
Published: July 2002

We have investigated the performance of a fully close-coupled wave packet method and its symmetry-adapted version for a model problem of H2 scattering from LiF(001). The computational cost of the fully close-coupled methods scales linearly with the number of rotation-diffraction states present in the basis set, provided that the sparseness of the potential coupling matrix is taken into account. For normal incidence, the symmetry adapted version is faster than the conventional close-coupling wave packet method by almost an order of magnitude. An extension of the method to more realistic molecule-surface problems is considered.
Fourier transform spectroscopy of the CO-stretching band of C-13 methanol in the torsional ground state
Authors: Mukhopadhyay I. , Lees R. M. , Lewis-Bevan W. , Johns J. W. C.
Published: July 2002

The Fourier transform spectrum of the CO-stretching fundamental band of 13CH3OH has been investigated at 0.002 cm−1 resolution. For the n=0 torsional ground state, 35 subbands containing some 2300 lines have been identified for K values from 0 to 10 for the three torsional symmetries A, E1, and E2 up to J values typically well over 30. The assigned transitions have been fitted to J(J+1) power-series expansions to obtain subband origins and sets of phenomenological state-specific parameters describing the J dependence of the subbands. These parameters reproduce the observed wave numbers to within the experimental uncertainty of ±0.0005 cm−1 for unblended lines, except when particularly large asymmetry shifts or perturbations due to Fermi or Coriolis resonances are present. The subband origins have been fitted to effective molecular constants for the excited CO-stretching state and a torsional barrier of 378.7(8) cm−1 is found, representing a 1.3% increase over the ground state value. A wide variety of global or J-localized perturbations has been seen, and most of the interacting states have been identified. New assignments have been obtained for far-infrared laser lines optically pumped by the 10R(2) and 9P(12) CO2 laser lines.
Scattering of H2 by LiF(001) studied using a new model potential. I. Prediction of large differences in diffraction of cold beams of para-H2 and normal-H2
Authors: Kroes Geert-Jan , Mowrey R. C.
Published: July 2002

The close-coupling wave packet (CCWP) method has been used for performing calculations on rotationally and diffractionally inelastic scattering of H2 from LiF(001), using a model potential. The scattering from the initial j=0, 1, and 2 states was investigated at normal incidence for a collision energy of 0.1 eV. If the quadrupole-ionic lattice interaction is included in the potential model, large probabilities (up to 0.3) are obtained for reorientational (mj changing) transitions in the scattering from the initial j=1 and j=2 states. This is in contrast with results of previous theoretical work which used model potentials not including the electrostatic interaction and found much smaller probabilities for Δmj transitions. Inclusion of the quadrupole-ionic lattice interaction in the model also leads to the prediction of large differences between the diffraction of H2 in its j=0 rotational state and diffraction of j=1 H2. It should be possible to check this result by diffraction experiments employing cold beams of para-H2 and normal-H2. In addition, the calculations show large differences between the diffraction of ‘‘helicoptering’’ H2 (j=1 or 2, ‖mj‖=j) and ‘‘cartwheeling’’ H2 (j=1 or 2, ‖mj‖=0).
Statistical interpretation of topographies and dynamics of multidimensional potentials
Authors: Kunz Ralph E. , Berry R. Stephen
Published: July 2002

A statistically based method of characterizing the topography of a multidimensional potential surface classifies not only local minima and saddles but entire basins containing many minima, and divides separating basins and monotonic sequences of local minima within each basin. The data, so classified, fold readily into the formalisms of chemical kinetic isomerization theory and master equations to provide a connection between that topography and the dynamics on the surface. This analysis, in particular, permits an interpretation of the glass-forming or ‘‘focusing’’ character of the surface. The method is illustrated with a model system derived, with simplifications, from the 19-atom Lennard-Jones cluster. The method also leads naturally to control problems including the determination of optimum conditions for forming glasses or selected structures, such as particular crystal structures or folded protein structures.
Finite-difference approach to solving Heisenberg’s operator equations of motion: Application to one-dimensional time dependent Hamiltonians
Authors: Sharafeddin Omar A.
Published: July 2002

We reviewed and expanded on the finite difference approach introduced by Moncrief [Phys. Rev. D 28, 2485 (1983)] and Bender [Phys. Rev. Lett. 55, 901 (1985)] for solving Heisenberg’s operator equations of motion. In this approach, finite-difference recurrence relations are used to evolve the matrix representation of the operators in time. The advantages and disadvantages of this approach for the study of quantum processes in real time are discussed. The approach performed very well as illustrated by examples of harmonic and Morse oscillators interacting with continuous wave and pulsed laser fields.
Study of low-lying electronic states of ozone by multireference Mo/ller–Plesset perturbation method
Authors: Tsuneda T. , Nakano H. , Hirao K.
Published: July 2002

The geometry and relative energy of the seven low-lying electronic states of ozone and the ground state of ozonide anion have been determined in C2v symmetry by the complete active space self-consistent field (CASSCF) and the multireference Mo/ller–Plesset perturbation (MRMP) methods. The results are compared with the photodetachment spectra of O−3 observed recently by Arnold et al. The theoretical electron affinity of ozone is 1.965 eV, which is 0.14 eV below the experimental result of 2.103 eV. The calculated adiabatic excitation energies (assignment of Arnold et al. in parentheses) of ozone are 3A2 0.90 eV (1.18 eV), 3B2, 1.19 eV (1.30 eV), 3B1, 1.18 eV (1.45 eV), 1A2, 1.15 eV (∼1.6 eV), 1B1, 1.65 eV (2.05 eV), and 1B2, 3.77 eV (3.41 eV), respectively. Overall the present theory supports the assignment of Arnold et al. However, the simple considerations of geometry and energy are insufficient to determine a specific assignment of the 3B2 and 3B1 states. The dissociation energy of the ground state of ozone is computed to be 0.834 eV at the present level of theory. The present theory also predicts that none of the excited states lies below the ground state dissociation limit of O3.
Coexistence and phase separation in clusters: From the small to the not-so-small regime
Authors: Wales David J. , Doye J. P. K.
Published: July 2002

We generalize the necessary and sufficient conditions for finite systems to exhibit van der Waals-type loops (or ‘‘S-bends’’) in thermodynamic functions. In the infinite limit, such features are forbidden by Van Hove’s theorem, and phase separation occurs instead. However, in small systems the energetic cost of the interface associated with phase separation is too great, and there is no contribution to the partition function from regions of phase space corresponding to two phases coexisting in contact. We derive a simple model that can correctly describe both limits, and investigate how the onset of phase separation will affect observable thermodynamic properties.
Chain collapse and phase separation in poor-solvent polymer solutions: A unified molecular description
Authors: Raos Guido , Allegra Giuseppe
Published: July 2002

We present a molecular theory of the phenomena of single-chain collapse and phase separation into a polymer-rich and a polymer-poor phase, which occur in polymer solutions below the Θ temperature. The theory extends the Fourier self-consistent approach of Allegra and Ganazzoli from the study of single-chain properties to that of an ensemble of interacting chains. We derive an expression for the free energy of a ‘‘Gaussian cluster’’ made up of ν chains of length N (ν=1,2,3,...; N≫1). In the limit ν→∞ this yields a mean-field expression for the solution free energy per chain as a function of the reduced temperature τ=(T−Θ)/T, the polymer volume fraction φ and the mean-square radius of gyration of the chains. From this we calculate the chain dimensions in solution and several thermodynamic properties, such as the osmotic pressure and the polymer–solvent phase diagram. We find that the contraction ratio of the chain radius of gyration is a single-valued function of (τB+K1Fφ)√N, where B and K1 specify the strength of the two- and three-body interactions and F is a polymer-dependent positive constant. We provide numerical evidence for a possible universality of the binodal line for different polymer-solvent systems; the spinodals do not share this characteristic of universality.
Reaction dynamics of Mg(3s3p 1P1) with CH4: Elucidation of reaction pathways for the MgH product by the measurement of temperature dependence and the calculation of ab initio potential energy surfaces
Authors: Liu Dean-Kuo , Ou Yaw-Ren , Lin King-Chuen
Published: July 2002

Using a pump–probe method, we have obtained the nascent bimodal rotational distribution of MgH (v″=0 and 1) products formed in the reaction of Mg(3s3p 1P1) with CH4. The low-N component of the distribution in the v″=0 state is much larger than that in the v″=1 state, whereas the high-N component in the v″=0 state is roughly equivalent to that in the v″=1 state. The MgH (v″=0) rotational distributions at three temperatures, 770, 830, and 880 K, were measured. The bimodal distribution does not change with temperature within a small experimental error. The findings suggest that the bimodal nature results from the same process, supporting a mechanism of Mg insertion into the C–H bond, irrespective of the geometry of the entrance approach. The result is consistent with that of Kleiber et al. using the far-wing scattering technique, and is supported by Chaquin et al.’s theoretical calculations. We also calculated two-dimensional potential energy surfaces for the excited and ground states of the reaction system. The calculation suggests that two possible trajectories are responsible for the production of MgH following a nonadiabatic transition. One trajectory, weakly dependent on the bending angle of H–Mg–CH3, is related to formation of the low-N component. The other trajectory evolves through a linear geometry of the intermediate complex prior to dissociation, causing a strong anisotropy in the PES. This second trajectory corresponds to the population of rotationally and vibrationally hot states. An alternative explanation of the low-N distribution is also discussed.
Low-frequency Raman scattering of aqueous solutions of L-xyloascorbic acid and D-araboascorbic acid
Authors: Wang Yan , Tominaga Yasunori
Published: July 2002

Depolarized low-frequency Raman spectra of aqueous solutions of L-xyloascorbic acid and its epimer D-araboascorbic acid have been investigated as a function of concentration at 30 °C. The influence of fluorescence in the low-frequency Raman spectral intensity of D-araboascorbic acid aqueous solution was removed by the background correction. The reduced χ″(ν̄) spectrum, which corresponds to the imaginary part of the dynamical susceptibility due to the dynamical structure of water in aqueous solutions, was analyzed with the superposition of one Cole–Cole type relaxation mode and two damped harmonic oscillator modes. The effect of L-xyloascorbic acid on the dynamical structure of water is less than that of D-araboascorbic acid.
Global geometry optimization of (Ar)n and B(Ar)n clusters using a modified genetic algorithm
Authors: Gregurick Susan K. , Alexander Millard H. , Hartke Bernd
Published: July 2002

A modified deterministic/stochastic genetic algorithm (DS-GA) method is proposed for the determination of the global minimum of atomic clusters described by pairwise analytic interaction potentials. Our modification of the standard GA method involves a coarse local minimization of each member of the population at every generation, as well as including the gradient into the fitness function. For Lennard-Jones (Ar)n clusters with n<30, the DS-GA converges far more quickly to the global minimum than either conventional GA methods or random search procedures. An application of this DS-GA is made to heterogeneous clusters of B(2P) with multiple Ar atoms. The interaction potential is given by the lowest state of a 3×3 electronic Hamiltonian. The Ar–Ar potential and the lower energy (Π state) B–Ar potential are very similar. In contrast, the higher energy (Σ state) B–Ar interaction is essentially repulsive. Consequently, the B atom is nearly always found to substitute for one of the atoms in the corresponding (Ar)n+1 cluster with the fewest number of nearest neighbors.
Neutralization and delayed ionization in fullerene surface collisions: Fragmentation and ionization rates as a route to activation energies
Authors: Weis Patrick , Rockenberger Jörg , Beck Rainer D. , Kappes Manfred M.
Published: July 2002

The interaction of C+60 and C+70 ion beams with a surface of highly oriented pyrolitic graphite was investigated by probing the ionization and fragmentation rates of scattered species within a time window of 20 μs following impact. Neutralization/reionization and fragmentation behavior was observed and followed by a pulsed deflection field applied to the surface at variable delays after the collision event. An almost complete collisional neutralization of the incident projectile was found. For an impact energy of 140–180 eV, a significant part of the scattered species was found to reionize by delayed electron emission within the experimental time window. The associated reionization and fragmentation kinetics were modeled with a system of differential equations assuming a simple unimolecular reaction diagram. Rate constants for delayed ionization and fragmentation were calculated as functions of internal energy and respective activation energies with the ‘‘finite heat bath’’ model (Klots) and the Rice–Ramsperger–Kassel–Marcus expression, respectively. The calculated and measured (deflection field delay dependent) ion intensities were compared in a fit procedure. The best fit led to an activation energy for the fragmentation of C+60 (C+60→C+58+C2) of 6.6±0.5 eV. This translates to an activation energy of 7.1±0.5 eV for the fragmentation of neutral C60 (using the experimentally determined ionization potential of C58). For C+70 we obtained an identical (within error) activation energy for fragmentation (C+70→C+68+C2) of 6.6±0.5 eV.
Role of electron pairing in collisional dissociation of Na+9 by a rare-gas atom
Authors: Nonose Shinji , Tanaka Hideki , Mizuno Tomoyuki , Hirokawa Jun , Kondow Tamotsu
Published: July 2002

A sodium cluster ion, Na9+, having a closed electronic shell was allowed to collide with a rare-gas atom (He or Ne), and collisionally fragmented ions were measured by using a tandem mass-spectrometer equipped with several octapole ion guides. The mass spectrum of the product ions show that sodium atom and dimer are released dominantly from the cluster ion, Na9+. The dependencies of the absolute cross sections for the Na and the Na2 release on the collision energy were determined for collision energies up to 3 eV in the center of mass frame. The cross section for the Na2 release increases much more rapidly than that for the Na release as the collision energy increases, while the cross sections for the Na and the Na2 release start to rise at almost the same collision energy of 0.75 ± 0.1 eV. The collision-energy dependencies are explained by orbital correlation diagrams for the Na8+ + Na and the Na7+ + Na2 systems on the basis of a model that a spherical jellium of Na9+ is deformed by collision of a rare-gas atom. The total absolute cross sections are estimated by a two-step scheme: collisional excitation and dissociation.
Simulation of ultrafast dynamics and pump–probe spectroscopy using classical trajectories
Authors: Li Zhiming , Fang Jian-Yun , Martens Craig C.
Published: July 2002

In this paper, we develop a method for accurately modeling ultrafast molecular dynamics and pump–probe spectroscopy using classical trajectory simulations. The approach is based on a semiclassical limit of the Liouville formulation of quantum mechanics. Expressions for the nonstationary classical phase space probability density created by an ultrashort laser pulse on an excited electronic state, and the observable fluorescence signal resulting from a pump–probe experiment, are derived in the weak-field limit using perturbation theory. By introducing additional approximations, these expressions are cast in a form that can be directly implemented using classical trajectory integration and ensemble averaging. The method is tested against multisurface time-dependent quantum mechanical wave packet calculations for a one-dimensional model system representing I2 photodissociation-recombination in a static Ar lattice. Nearly quantitative agreement between the exact calculations and the trajectory-based method is obtained. Although demonstrated for a one-dimensional system, the method is easily incorporated in conventional molecular dynamics programs, allowing efficient treatment of many-body ultrafast dynamics and spectroscopy.
An experimental study of HF photodissociation: Spin–orbit branching ratio and infrared alignment
Authors: Zhang J. , Riehn C. W. , Dulligan M. , Wittig C.
Published: July 2002

Single rotational levels of HF (v=3) were prepared by using overtone excitation and these molecules were then photodissociated by ultraviolet (UV) radiation at 193.3 nm. Time-of-flight spectra of the hydrogen atom fragment provided the spin–orbit state distribution of the fluorine fragment. Changing the UV photolysis laser polarization confirmed an A 1Π←X 1Σ+ electronic transition in the photodissociation step. Photodissociation of HF at 121.6 nm is also reported. Infrared (IR) induced alignment of the diatom was studied by monitoring the IR laser polarization dependence of the H-atom product angular distribution. Depolarization due to hyperfine interaction was studied by using the R(0) transition. Agreement with theory is excellent.
Two-photon ionization studies of binary aromatic van der Waals clusters: Benzene...chlorobenzene and (chlorobenzene)2
Authors: Lu Wenyun , Hu Yihua , Lin Zhenyang , Yang Shihe
Published: July 2002

Jet-cooled van der Waals dimers of benzene (B) and chlorobenzene (BCl) were studied by one-color resonant two-photon ionization through the S0→S1 transition of the chlorobenzene. The spectra of BCl...B and (BCl)2 in the 0–0 band both show two main features with different spectral shifts. These two main features are attributed to two different geometries: perpendicular T-shaped geometry and parallel displaced geometry. This assignment is supported by the relative intensity dependence on the internal temperature and our ab initio calculations at the MP2 level. The binding energies of BCl...B and (BCl)2 were measured near the two-photon ionization dissociation threshold in a reflectron time-of-flight mass spectrometer (RTOF/MS). Our estimated binding energies of BCl...B and BCl...BCl are 0.14±0.01 eV and 0.15±0.01 eV, respectively, consistent with their increasing dipole moment and polarizability in that order.
Quantum mechanical and semiclassical dynamics at a conical intersection
Authors: Ferretti A. , Granucci G. , Lami A. , Persico M. , Villani G.
Published: July 2002

We present simulations of wave-packet dynamics for a model of a conical intersection in two dimensions. The potential energy surfaces and couplings are functions of a total symmetrical coordinate and of a symmetry breaking one. The wave packet crosses the coupling region once, moving essentially in the direction of the symmetrical coordinate. The dynamics are determined by two methods, one quantum mechanical and the other semiclassical, based on trajectories and surface hopping. The semiclassical approximation is quite adequate for low coupling strengths in the diabatic representation, less so for larger couplings. Approximate analytic solutions for the two-dimensional problem and for one-dimensional analogs are provided, in order to generalize the numerical results and to analyze the reasons of the discrepancies between semiclassical and quantum mechanical results.
Investigation on the reflection and transmission properties of complex absorbing potentials
Authors: Riss U. V. , Meyer H.-D.
Published: July 2002

The reflection and transmission properties of different complex absorbing potentials (CAPs) are studied using WKB and scaling procedures which make the results transferable to any mass and kinetic energy. Explicit formulas are obtained which describe the reflection and transmission properties of monomial CAPs −iηxn with high accuracy. These properties are now well understood. The approximate results are compared to exact analytical results available for quadratic CAPs, and to numerical results obtained by wave packet propagation followed by an energy resolved analysis. The approximate, but accurate, description of the action of the CAP is finally used to determine optimal CAP parameters. CAP length, strength, and order can now be chosen in such a way that the sum of reflection and transmission is minimized. Optimal parameters are compiled for different energies and energy intervals.
Molecular dynamics with multiple time scales: The selection of efficient reference system propagators
Authors: Stuart Steven J. , Zhou Ruhong , Berne B. J.
Published: July 2002

Several heuristic rules are developed to assist in the implementation of the reversible reference system propagator algorithm (rRESPA). This is done through the use of examples, illustrating the use of properly chosen rRESPA splits of various types, as well as the dangers associated with improperly chosen ones. It is concluded that a particle-based rRESPA split should be used only when there is a great disparity in particle masses, and that a force-based split should be used only when there is no persisting opposition between forces in the system which are integrated with different time steps.
Evolution of quantum system in order domain of Chebyshev operator
Authors: Chen Rongqing , Guo Hua
Published: July 2002

A cosine transform between the order and angle of the Chebyshev operator is identified. Because the order and angle form a conjugate pair similar to energy and time, the Chebyshev state can be considered as a cosine-type evolution state in the order domain, analogous to a time-dependent wave packet. The order/angle formulation is analytically equivalent to the time/energy formulation, but the former may have some numerical advantages in certain applications. This is illustrated by examining the spectral method and the filter-diagonalization method in both formulations.
Evidence for a partial breakdown of the molecular orbital picture in the ionization spectra of large saturated hydrocarbons
Authors: Deleuze Michael S. , Cederbaum Lorenz S.
Published: July 2002

The x-ray photoionization spectra of large saturated hydrocarbons have been investigated by means of one-particle Green’s function calculations. These spectra saturate overall rather quickly to their asymptotic form with increasing system size. The results obtained indicate a severe breakdown of the molecular orbital picture of ionization above a binding energy threshold of about 23 eV, for n-alkane chains ranging from n-propane to n-nonane, or cycloalkane compounds such as cyclobutane, cyclopentane, and cyclohexane. In spite of the fast multiplication of satellite solutions, shake-up lines remain confined above that threshold, as a result of the delocalization properties of one-electron canonical states. The ring closure is shown to emphasize the dispersion of photoionization intensity into satellites. Conformational changes, on the other hand, have only marginal effects on the convoluted correlation bands.