bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodissociation of Ar–HCl: An energy-resolved study of the dynamics of total fragmentation into H+Ar+Cl
Authors: Juanes-Marcos J. C. , Garcı́a-Vela A.
Published: July 2002

UV photolysis of Ar–HCl is simulated by means of an exact wave packet treatment in three dimensions. The focus of the work is on the mechanism of indirect dissociation of the hydrogen atom, which leads to total fragmentation of Ar–HCl into H, Ar, and Cl. The results predict for this photodissociation path a probability of about 13% of the photolysis process. The remaining probability would be associated with direct photodissociation of the H fragment. Kinetic-energy distributions of the hydrogen fragments produced by indirect photodissociation are calculated for different excitation energies of Ar–HCl. The distributions reflect a pronounced structure of peaks associated with broad and overlapping resonances of the system. The resonance structure is present in the whole energy range covered by the absorption spectrum. Hydrogen atoms initially populating the resonances can dissociate from the cluster extensively cooled down, after several collisions with Ar and Cl. A mechanism is suggested for the fragmentation process due to indirect photodissociation, which involves successive jumps of the hydrogen to lower-energy resonances, induced by the collisions. A classical collisional model is proposed to rationalize qualitatively the fragmentation dynamics.
Laser-induced fluorescence of the CD2CFO radical
Authors: Inomata Satoshi , Furubayashi Masashi , Imamura Takashi , Washida Nobuaki , Yamaguchi Makoto
Published: July 2002

The laser-induced fluorescence spectrum of the B̃ 2A″→X̃ 2A″ transition of the CD2CFO radical has been observed in the region 316–335 nm. The radical was produced by 193 nm photolysis or by fluorine atom reaction with acetyl-d3 fluoride. The spectrum of CD2CFO was similar to that of CH2CFO reported previously except for small isotope shifts in the range 7–343 cm−1. The isotope shifts support the assignment of these spectra to fluorinated vinoxy radicals, and rule out the alternate assignment to FCO proposed by others. The X̃→B̃ electronic transition energy (T0) for CD2CFO was measured to be 29 867 cm−1, which is only 7 cm−1 lower than that for CH2CFO. From an analysis of the laser-induced single vibronic level fluorescence, some of the vibrational frequencies can be assigned for the ground electronic state; ν3(CO str.)=1735; ν4(CD2 sciss.)=1043; ν5(CF str.)=1248; ν6(CD2 rock.)=774; ν7(CC str.)=863; ν8(CCF bend)=597; and ν9(CCO bend)=370 cm−1. For the B̃ 2A″ state, ν3=1772; ν4=1073; ν5=1241; ν6=783; ν7=827; ν8=530; and ν9=370 cm−1. These assignments are supported by ab initio calculations. Among these fundamental frequencies, the ν4 and ν6 modes showed the largest isotope shifts, although isotope effects were observed in all the above vibrational fundamentals. The radiative lifetimes of the excited CD2CFO and the quantum yield of formation of the CH2CFO radical from photolysis of CH3CFO at 193 nm are also reported.
A quantum-mechanical study of the dynamics of the N(2D)+H2→NH+H reaction
Authors: Honvault Pascal , Launay Jean-Michel
Published: July 2002

We have studied the low energy quantum dynamics of the N(2D)+H2(X 1Σg+)→NH(X 3Σ−)+H(2S) reaction. We use the hyperspherical method and a recently published ab initio potential energy surface. We find a forward–backward symmetry in the differential cross sections which is characteristic of a complex formation. We also present rotational and vibrational integral cross sections.
Molecular dynamics simulation of vibrational energy relaxation of highly excited molecules in fluids. III. Equilibrium simulations of vibrational energy relaxation of azulene in carbon dioxide
Authors: Heidelbach C. , Vikhrenko V. S. , Schwarzer D. , Fedchenia I. I. , Schroeder J.
Published: July 2002

The expressions for vibrational energy relaxation (VER) rates of polyatomic molecules in terms of equilibrium capacity time correlation functions (TCFs) derived in the first paper of this series [J. Chem. Phys. 110, 5273 (1999)] are used for the investigation of VER of azulene in carbon dioxide at low (3.2 MPa) and high (270 MPa) pressure. It is shown that for both cases the VER times evaluated on the basis of the same potential model via solute–solvent interaction capacity TCFs by means of equilibrium molecular dynamics (EMD) simulations satisfactorily agree with the nonequilibrium (NEMD) molecular dynamics [J. Chem. Phys. 110, 5286 (1999)] and experimental [J. Chem. Phys. 105, 3121 (1996)] results as well. Thus it follows that these methods can complement each other in characterizing VER from different points of view. Although more computational power and refined methods of dealing with simulated data are required for EMD simulations, they allow the use of powerful tools of equilibrium statistical mechanics for investigating the relaxation process. To this end, an analysis of VER mechanisms on the basis of normal mode and atomic representations is carried out. The influence of temperature and CO2 pressure on azulene normal mode spectra and solvent assisted intermode coupling in connection with the eigenvector structure is investigated in great detail. The normal mode capacity cross-correlation matrix reveals the significance of intermode coupling, which significantly contributes to intramolecular vibrational energy redistribution (IVR). As a new concept, partial normal mode relaxation rates are introduced. It is shown that these rates demonstrate similar properties as the energy exchange rates through particular normal modes in nonequilibrium simulations. Atomic spectra and friction coefficients are characterized by a complicated frequency dependence due to contributions from many normal modes. Atomic capacity TCFs and partial relaxation rates are analyzed and reveal a similar picture to that obtained from NEMD simulations. These results show that VER and IVR cannot be separated from each other and have to be considered as mutually connected processes.
Simulations of the shape of a regularly branched polymer as a model of a polymacromonomer
Authors: Shiokawa Kohzoh , Itoh Katsunori , Nemoto Norio
Published: July 2002

Computer simulations were performed to examine the effects of the main (backbone) and side (branch) chain size on the shape and dimensions of a polymacromonomer using the bond fluctuation model in which bond cutting is allowed. The polymacromonomer was treated as a self-avoiding chain having N bonds in the main chain and n bonds in the side chains which are attached to every main chain element. In the simulation N and n were varied up to 64. A power law relation, 〈S2〉m∼N2νm, was obtained for each value of n, where 〈S2〉m is the mean-square radius of gyration of the main chain. The exponent 2νm increased monotonically from 1.24±0.02 at n=1 to 1.95±0.05 at n=64, indicating that the shape of the main chain gradually varied with increasing n, from a self-avoiding coil-like structure to an extended rod-like form. The mean-square radius of gyration of the side chain moiety, 〈S2〉s was independent both of N and also of the position of the branching point along the main chain for every value of n. A power law relation was also found between 〈S2〉s and n, with exponent 2νs=1.20±0.01. This exponent is the same as that for a linear chain, while 〈S2〉s is about 1.2 times the value 〈S2〉 of a linear chain with the same degree of polymerization. This strongly suggests that all side chains maintain a three-dimensional self-avoiding coil-like form, irrespective of the main chain, as it extends from a coil-like form to a rod-like form with increasing n.
Finite-size effects and the stabilized spin-polarized jellium model for metal clusters
Authors: Payami M.
Published: July 2002

In the framework of spherical geometry for jellium and local spin density approximation, we have obtained the equilibrium rs values, r̄s(N,ζ), of neutral and singly ionized “generic” N-electron clusters for their various spin polarizations, ζ. Our results reveal that r̄s(N,ζ) as a function of ζ behaves differently depending on whether N corresponds to a closed-shell or an open-shell cluster. That is, for a closed-shell one, r̄s(N,ζ) is an increasing function of ζ over the whole range 0⩽ζ⩽1, and for an open-shell one, it has a decreasing part corresponding to the range 0<ζ⩽ζ0, where ζ0 is a polarization that the cluster assumes in a configuration consistent with Hund’s first rule. In the context of the stabilized spin-polarized jellium model, our calculations based on these equilibrium rs values, r̄s(N,ζ), show that instead of the maximum spin compensation (MSC) rule, Hund’s first rule governs the minimum-energy configuration. We therefore conclude that the increasing behavior of the equilibrium rs values over the whole range of ζ is a necessary condition for obtaining the MSC rule for the minimum-energy configuration; and the only way to end up with an increasing behavior over the whole range of ζ is to break the spherical geometry of the jellium background. This is the reason why the results based on simple jellium with spheroidal or ellipsoidal geometries show up MSC rule.
Energy-dependent cross sections and nonadiabatic reaction dynamics in F(2P3/2,2P1/2)+n–H2→HF(v,J)+H
Authors: Nizkorodov Sergey A. , Harper Warren W. , Chapman William B. , Blackmon Bradley W. , Nesbitt David J.
Published: July 2002

High-sensitivity direct IR laser absorption methods are exploited to investigate quantum state-resolved reactive scattering dynamics of F+n-H2(j=0,1)→HF(v,J)+H in low-density crossed supersonic jets under single collision conditions. Nascent rotational state distributions and relative cross sections for reactive scattering into the energetically highest HF (v=3,J) vibrational manifold are obtained as a function of center-of-mass collision energies from Ecom=2.4 kcal/mole down to 0.3 kcal/mole. This energy range extends substantially below the theoretically predicted transition state barrier [Ebarrier≈1.9 kcal/mole; K. Stark and H. Werner, J. Chem. Phys. 104, 6515 (1996)] for the lowest adiabatic F(2P3/2)+H2 potential energy surface, therefore preferentially enhancing nonadiabatic channels due to spin–orbit excited F*(2P1/2) (ΔEspin–orbit=1.15 kcal/mole) in the discharge source. The HF (v=3,J) cross sections decrease gradually from 2.4 kcal/mole down to the lowest energies investigated (Ecom≈0.3 kcal/mole), in contrast with exact adiabatic quantum calculations that predict a rapid decrease below Ecom≈1.9 kcal/mole and vanishing reaction probability by Ecom≈0.7 kcal/mol. Further evidence for a nonadiabatic F*(2P1/2) reaction channel is provided by nascent rotational state distributions in HF (v=3,J), which are >2–3-fold hotter than predicted by purely adiabatic calculations. Most dramatically, the nascent product distributions reveal multiple HF (v=3,J) rovibrational states that would be energetically inaccessible from ground state F(2P3/2) atom reactions. These quantum state resolved reactive scattering studies provide the first evidence for finite nonadiabatic dynamics involving multiple potential energy surfaces in this well-studied “benchmark” F+H2 reaction system.
Relationship between bipolar moments and molecule-frame polarization parameters in Doppler photofragment spectroscopy
Authors: Rakitzis T. Peter , Hall Gregory E. , Costen Matthew L. , Zare Richard N.
Published: July 2002

In terms of the molecular-frame polarization parameters aq(k)(p), an equation is derived that describes the shape of a photofragment Doppler profile as a function of the three angles Γ, Δ, and Φ that specify the photolysis and probe laser polarizations about the detection axis. This expression is specialized to linearly polarized photolysis and probe laser beams. For the particular value of the angle between the probe laser polarization and the detection axis, Δ=π/2, this equation can be reduced to the form of well-known laboratory-frame expressions that use the bipolar moment formalism introduced by Dixon. Comparison of these forms shows the equivalence of the two formalisms and gives the relationships between the bipolar moments βQK(k1k2) and the molecule-frame aq(k)(p) parameters. We show that linear combinations of the bipolar moments completely describe photofragment polarization in the molecular frame and possess distinct quantum mechanical significance. In particular, it is shown that the coherent contribution to the photofragment alignment is proportional to the linear combination (1/5)β02(02)−(1/7)β02(22)−(12/35)β02(42).
Sequential vibrational relaxation of polyatomic molecules at surfaces: C2HD and C2H2 scattered from LiF(001)
Authors: Wight A. C. , Penno M. , Miller R. E.
Published: July 2002

Reported here is a detailed study of the vibrational relaxation of C2H2 and C2HD upon scattering from LiF(001). While direct (specular) scattering shows no vibrational quenching for either molecule, the residence times associated with trapping–desorption are long enough to give significant quenching. By studying this quenching as a function of surface temperature, thereby varying the residence times, we show that vibrational relaxation proceeds via multiple steps. The differences between C2H2 and C2HD quenching can be understood in terms of the presence of a Fermi resonance in the former, which enhances the relaxation rates associated with the early steps. A simple kinetic model is used to fit the data, confirming that vibrational quenching proceeds via a cascading mechanism, which is likely to be quite general for polyatomic molecules.
Folding of a 16-residue helical peptide using molecular dynamics simulation with Tsallis effective potential
Authors: Pak Youngshang , Wang Shaomeng
Published: July 2002

We have demonstrated that a molecular dynamics simulation method in conjunction with a Tsallis effective potential enables a 16-residue model peptide to fold into a complete α-helix in a reasonably short time. The current study also indicates that one can practically observe reversible foldings of the peptide with the method, mainly due to its superior capability of overcoming potential energy barriers. Therefore it is anticipated that the new method may provide a quite efficient conformational searching tool for systems with many degrees of freedom such as proteins and DNAs.
The influence of buoyant convection on the operation of the upward thermal diffusion cloud nucleation chamber
Authors: Ferguson Frank T. , Nuth Joseph A.
Published: July 2002

Recently, the stable operation of the upward thermal diffusion cloud chamber with respect to buoyancy-induced convection has become a concern in obtaining reliable nucleation data. During chamber operation, evidence of strong convective currents are clearly visible due to the curved trajectories of entrained droplets. A potential problem exists when these flows are much smaller in magnitude; there is no visible evidence of convection, yet these minute flows may result in systematic errors in the nucleation data calculated via 1D diffusive models of the transport mechanisms within the chamber. To examine whether such flows are possible and the characteristics of these flows we have developed an extension to recent 2D modeling of the nucleation chamber which includes buoyancy-induced, convective motion. We have examined both wet and dry chamber operation with an example case of 1-propanol in helium at a pressure of 1.18 bar. In addition, for the dry wall case we examined the effect of overheating the chamber wall and varying the chamber diameter. Results indicate that, for the representative cases investigated, very subtle convective flows can exist and that these minute flows can affect the maximum attainable supersaturation along the chamber centerline. Finally, a list of general recommendations are given for minimizing the possibility of such flows within the cloud chamber.
Surface diffusion potential energy surfaces from first principles: CO chemisorbed on Pt{110}
Authors: Ge Q. , King D. A.
Published: July 2002

Lateral potential energy curves for the chemisorption of CO on Pt{110} (1×1) and (1×2) along different azimuthal directions have been calculated using density functional theory slab calculations. In contrast to the simple models almost universally used, the results along 〈11̄0〉 show that there is a barrier of ∼0.15 eV between bridge and atop sites. Both bridge and atop sites are local minima. Diffusion along 〈100〉 on the (1×1) surface is strongly inhibited by a barrier ⩾1.2 eV. Quasielastic helium atom scattering data require reanalysis in the light of these results. The free energy, determining the most stable site at finite temperatures, includes a significant vibrational entropy term in the atop site.
Histogram filtering: A technique to optimize wave functions for use in Monte Carlo simulations
Authors: Snajdr Martin , Dwyer Jason R. , Rothstein Stuart M.
Published: July 2002

Wave functions are optimized using a histogram-based technique that deals with the statistical error plaguing traditional Monte Carlo optimizations. Following a sensitivity study on H2+, we variance- and energy-optimize explicitly correlated wave functions for He (up to 18 variational parameters), H2 (up to 10 parameters), and LiH (up to 32 parameters). To gauge the convergence of the variational energy as the quality of the wave functions improves, we adopt some simple ones from the literature in addition to more sophisticated ones unique to this paper. One for LiH has the lowest variational energy for a compact, explicitly correlated wave function to date. For the molecules we determine the optimal bond distance at the same time as we optimize either the variational energy or the variance of the local energy, but agreement with experiment is reasonable only for the energy optimizations. The energy of variance-optimized molecular wave functions appears to converge slowly to the energy optimization results as the wave function quality improves. Variance optimizations done keeping the bond distance fixed equal to the exact value improves the energy somewhat.
Mobility of oxygen atoms generated from photolysis of O3 isolated in argon matrices
Authors: Ning Xi-Jing , Zhang Lu-Ning , Chen Mo-Hua , Qin Qi-Zong
Published: July 2002

The mobility of oxygen atoms generated from ultraviolet (UV) photolysis of O3 isolated in argon matrices is investigated both theoretically and experimentally. The simulations via molecular dynamics show that O(1D) atoms generated in single, double, and triple substitutional sites can migrate through the matrices by a distance about 1 nm within several picoseconds, but the photogenerated O(3P) atoms are confined to the original trapping sites. In order to examine the mobility of the O(1D) atoms experimentally, the probing molecules of CF3Br (or CF3I) are so highly diluted in the matrix of O3/Ar that the separations between the probing molecules and the precursor ozone molecules are sufficiently large, and the reaction products CF3OBr (or CF3OI) are observed after irradiation with a 266 nm laser. Based on an analysis of the reaction rates, it is shown that the products are only generated from the probing molecules reacting with the O(1D) but not O(3P) atoms, suggesting that matrices can serve as a “sieve” for selecting quantum states in chemical reactions.
van der Waals isomers and ionic reactivity of the cluster system para-chlorofluorobenzene/methanol
Authors: Riehn C. , Buchhold K. , Reimann B. , Djafari S. , Barth H.-D. , Brutschy B. , Tarakeshwar P. , Kim K. S.
Published: July 2002

This combined experimental and computational study is aimed at elucidating the structure and reactivity of heterogeneous molecular clusters. We report results for the system para-chlorofluorobenzene/methanol (pClFB/MeOH). Particularly, three different van der Waals (vdW) isomers of the neutral (1:1) aggregate (π,σF,σCl) have been assigned by comparison of experimental infrared frequencies in the O–H and C–H stretch regions measured by IR/R2PI depletion spectroscopy with calculated frequencies at MP2/6-31+G(d) and B3LYP/6-31+G(d) level. The isomers are weakly hydrogen-bonded complexes with methanol’s OH group coordinated toward the aromatic π-electron cloud, the fluorine or the chlorine substituent, respectively. This assignment is corroborated by the UV and IR spectra of the corresponding monosubstituted benzene/methanol complexes. After resonant photoionization of pClFB⋅(MeOH)n, for n=1 besides vdW fragmentation, no chemical reactivity was observed for any isomer. The investigation of aggregates with n⩾5 indicated a substitution reaction taking place to produce chloroanisole+, emphasizing the importance of neutral precursor cluster size over cluster structure for intracluster ion–molecule reactions.
Three-dimensional picture of dynamical structure in liquid water
Authors: Svishchev Igor M. , Zassetsky Alexander Yu.
Published: July 2002

This paper presents a methodology with which to study the local density distributions in molecular liquids and their fluctuations in any spatial direction. The distinct part of the van Hove density–density correlation function for liquid water is calculated in molecular dynamics simulations. Because of the pronounced nonspherical intermolecular interactions this pair-density function is direction dependent in the local molecular frame. We explicitly resolve the distinct van Hove function in the local frame of water molecules. The dynamics of the tetrahedrally coordinated (hydrogen bonded) and the interstitial molecules in liquid water are examined. The spectrum of the pair-density fluctuations for the tetrahedrally coordinated molecules in supercooled and ambient water exhibits a well-known translational mode at 200 cm−1 and a collective relaxation mode at lower frequencies, at approximately 10 cm−1 at 263 K. The correlation time of this relaxation process decreases with temperature, from 2.2 ps at 238 K to 1.4 ps at 298 K. The spectrum for the interstitial coordination also features a 10 cm−1 mode. It represents a secondary relaxation process in water different from a much slower Debye process. As temperature increases this relaxation mode tends to disappear from the pair-density fluctuations.
Observation of metastable atomic nitrogen adsorbed on Ru(0001)
Authors: Diekhöner L. , Baurichter A. , Mortensen H. , Luntz A. C.
Published: July 2002

Exposing a Ru(0001) surface to an atomic beam of N produces a series of different states of atomic N adsorbed on the surface. For low atom doses, well-known low coverage states are produced, but for higher atom doses, several previously unknown higher coverage states are sequentially filled. These states exhibit well defined temperature programmed desorption (TPD) peaks which shift to considerably lower temperatures with N coverage. The highest N coverage obtainable is almost 1 ML N/Ru. Recent density functional calculations demonstrate that the N–Ru energy decreases significantly with N coverage, and in fact predict that N adsorbate states are not thermodynamically stable relative to associative desorption at high coverage. The observed high coverage states must, therefore, be metastable with lifetimes determined by the height of the barrier between gas phase N2 and the adsorbed atomic states. Simple analysis of the TPD features in combination with the theoretical adsorption energies allows us to estimate these coverage dependent barrier heights. We find that the barrier heights increase significantly with coverage, and this is important in the metastability of the adsorbed states. A comparison of nitrogen adsorption on Ru(0001) with oxygen adsorption on Ru(0001) surface is stressed throughout.
Intramonomer correlation contributions to first-order exchange nonadditivity in trimers
Authors: Wormer Paul E. S. , Moszynski Robert , van der Avoird Ad
Published: July 2002

An expression for the nonadditive first-order exchange contribution in terms of the one- and two-particle density matrices of the isolated monomers is given. This formula is used to derive the intramonomer correlation contribution to the first-order exchange nonadditivity with the density matrices of the isolated monomers computed at the second-order Møller–Plesset level. As an illustrative example of the theory the trimer Ar2–HF is considered. It is shown that the correlation contribution is far from negligible, not only in relation to the corresponding first-order uncorrelated exchange, but also in comparison to the other three-body terms. Moreover, it is fairly anisotropic, so that it cannot be neglected in accurate calculations of nonadditive effects.
Intramolecular energy-band dispersion in oriented thin films of n-CF3(CF2)22CF3 observed by angle-resolved photoemission with synchrotron radiation
Authors: Miyamae Takayuki , Hasegawa Shinji , Yoshimura Daisuke , Ishii Hisao , Ueno Nobuo , Seki Kazuhiko
Published: July 2002

Angle-resolved ultraviolet photoelectron spectra were measured for oriented films of perfluorotetracosane, n-CF3(CF2)22CF3, as a model compound of poly(tetrafluoroethylene) using synchrotron radiation. The main spectral features showed continuous changes in both peak positions and intensities with photon energy, incident angle, and photoelectron take-off angle. The intramolecular energy-band dispersion of poly(tetrafluoroethylene) was mapped from the photo-energy dependence of the normal emission spectra.
Critical points and reaction paths characterization on a potential energy hypersurface
Authors: Ramquet Marie-Noëlle , Dive Georges , Dehareng Dominique
Published: July 2002

Most of the time, the definitions of minima, saddle points or more generally order p (p=0,…,n) critical points, do not mention the possibility of having zero Hessian eigenvalues. This feature reflects some flatness of the potential energy hypersurface in a special eigendirection which is not often taken into account. Thus, the definitions of critical points are revisited in a more general framework within this context. The concepts of bifurcation points, branching points, and valley ridge inflection points are investigated. New definitions based on the mathematical formulation of the reaction path are given and some of their properties are outlined.
Photodissociation spectroscopy of the Ca+–Ne complex
Authors: Reddic J. E. , Pullins S. H. , Duncan M. A.
Published: July 2002

The weakly bound complex Ca+–Ne is produced by laser ablation in a pulsed nozzle cluster source and low-lying electronic states are studied with resonance enhanced photodissociation spectroscopy (REPD). The lowest band system correlates to the forbidden D←22S atomic transition. Sharp structure to the blue of this transition is assigned to the C 2Σ+←X 2Σ+ system. A vibrational progression (ωe′=21.5 cm−1) and its convergence limit determine the excited-state dissociation energy of D0′=64±5 cm−1. Two electronic systems in the complex are derived from the P←22S atomic transition. A doublet progression to the red of the atomic transition is assigned to the D 2Πr←X 2Σ+ system. Extrapolation of the vibrational progression (ωe′=134.5 cm−1) determines an excited-state dissociation energy of D0′=875±100 cm−1. A broad continuum to the blue of the atomic transition is assigned to the E 2Σ+←X 2Σ+ transition. Using the C 2Σ+ excited-state convergence limit, the X 2Σ+ ground-state dissociation energy is determined to be D0″=103±5 cm−1. Rotational analyses from bands in both systems produce a ground-state bond length of r0″=3.77±0.05 Å.
Accurate ab initio determination of magnetic interactions and hopping integrals in La2−xSrxCuO4 systems
Authors: Calzado Carmen J. , Sanz Javier F. , Malrieu Jean Paul
Published: July 2002

The nature of magnetic interactions and electron transfer processes in La2−xSrxCuO4 systems are studied, by means of an ab initio embedded cluster model approach, using a difference dedicated configuration interaction (DDCI) procedure. For the undoped system, the crucial role played by the ligand to metal charge transfer (LMCT) configurations in the magnetic process makes necessary the use of an enlarged DDCI space, which explicitly takes account of the relaxation of these LMCT configurations. This procedure allows us to approach the experimental magnetic coupling constant value. In hole-doped systems, the value obtained for the electron transfer integral, t, is of 0.54–0.57 eV. The extra hole, characterized from the nature and occupation of differential natural orbitals, has a strong p character (approximately 50%) and is essentially localized in CuO2 planes. These results are in agreement with the experimental evidence about these kinds of compounds. Neither the value of t nor the nature of the extra hole are seriously affected by the optimization of the orbitals used in the CI expansion. This suggests that a t–J effective Hamiltonian is an adequate model to study the electronic properties of these systems.
Nanometer-scale resolution and depth discrimination in near-field optical microscopy studies of electric-field-induced molecular reorientation dynamics
Authors: Mei Erwen , Higgins Daniel A.
Published: July 2002

Electric-field-induced molecular reorientation dynamics in polymer-dispersed liquid crystal (PDLC) films are characterized in detail using near-field scanning optical microscopy (NSOM) methods developed previously [Mei and Higgins, J. Phys. Chem. A 102, 7558 (1998)]. In these experiments, a modulated electric field is applied between the aluminum-coated NSOM probe and an indium-tin-oxide (ITO) substrate. The field causes reorientation of the liquid crystal within the ITO-supported PDLC film. The reorientation process is observed by near-field optical means. In this paper, it is conclusively shown that under appropriate conditions the dynamics observed occur in extremely small volumes, and are substantially confined within the near-field optical regime. The volume in which the dynamics are probed may be controlled by varying the experimental parameters (i.e., field strength and modulation frequency) employed. Conclusive evidence for confinement is obtained from both theoretical arguments and experimental results. Calculations of the electric fields in a model dielectric medium show that the largest fields occur very near the NSOM probe. Experimental observation of spatial variations in the threshold (i.e., the “Frederiks transition”) for liquid crystal reorientation provide further evidence. The most direct evidence is provided by the observation of sub-diffraction-limited resolution in dynamics images of ≈ 1 μm thick samples. Spatial variations in the observed dynamics are interpreted to reflect the energetics of local liquid crystal organization, the details of the reorientation process, and also polymer/liquid-crystal interfacial interactions. Finally, important information on the local rotational viscosity and elastic force constants within individual liquid-crystal droplets is obtained.
A new mechanism for the enhancement of activated bimolecular reactions by rotational excitation
Authors: Miklavc Adolf , Perdih Marko , Smith Ian W. M.
Published: July 2002

The kinematic mass model (KMM), which has been developed to examine the dynamics of activated bimolecular reactions, has here been adapted to examine how orientational effects associated with reagent rotation influence the rotational state dependence of reaction cross-section. It is shown that, for reactions where the critical dividing surface (CDS) and the equipotential contours near to the CDS are “prolate,” i.e., elongated in the direction of the longitudinal molecular axis, rotation favors impact on the CDS near collinear geometries where the barrier to reaction is least, with the result that reaction cross-sections are enhanced with increasing reagent rotation. In the case of the rotational velocity being comparable with, or greater than, the relative translational velocity, this enhancement can greatly exceed that due to part of the rotational energy being available for barrier crossing. The KMM model, allowing for this orientational effect, has been applied to the reactions O+HCl (DCl) and O+H2 on well-known model potential energy surfaces (PESs) where both the CDSs and the equipotential contours near the CDS are prolate. The results agree well with those from trajectory calculations. The role of the above effects of reagent rotation in the case of surfaces of nonprolate shapes is discussed qualitatively.
Femtosecond stimulated emission pumping: Characterization of the I2− ground state
Authors: Zanni Martin T. , Davis Alison V. , Frischkorn Christian , Elhanine Mohammed , Neumark Daniel M.
Published: July 2002

Femtosecond stimulated emission pumping in combination with femtosecond photoelectron spectroscopy is used to characterize the potential energy function of the I2−(X̃ 2Σu+) ground state up to vibrational energies within 2% of the dissociation limit. The frequency and anharmonicity of this state are measured at a series of vibrational energies up to 0.993 eV by coherently populating a superposition of ground state vibrational levels using femtosecond stimulated emission pumping, and monitoring the resulting wave packet oscillations with femtosecond photoelectron spectroscopy. The dissociative I2−(Ã′ 2Πg,1/2) state is used for intermediate population transfer, allowing efficient population transfer to all ground state levels. Using the measured frequencies and anharmonicities, the X̃ 2Σu+ state has been fit to a modified Morse potential with the β-parameter expanded in a Taylor series, and the bond length, well depth, and υ=0–1 fundamental frequency set equal to our previously determined Morse potential [J. Chem. Phys. 107, 7613 (1997)]. At high vibrational energies, the modified potential deviates significantly from the previously determined potential.
A five-site model for liquid water and the reproduction of the density anomaly by rigid, nonpolarizable potential functions
Authors: Mahoney Michael W. , Jorgensen William L.
Published: July 2002

The ability of simple potential functions to reproduce accurately the density of liquid water from −37 to 100 °C at 1 to 10 000 atm has been further explored. The result is the five-site TIP5P model, which yields significantly improved results; the average error in the density over the 100° temperature range from −37.5 to 62.5 °C at 1 atm is only 0.006 g cm−3. Classical Monte Carlo statistical mechanics calculations have been performed to optimize the parameters, especially the position of the negative charges along the lone-pair directions. Initial calculations with 216 molecules in the NPT ensemble at 1 atm focused on finding a model that reproduced the shape of the liquid density curve as a function of temperature. Calculations performed for 512 molecules with the final TIP5P model demonstrate that the density maximum near 4 °C at 1 atm is reproduced, while high-quality structural and thermodynamic results are maintained. Attainment of high precision for the low-temperature runs required sampling for more than 1 billion Monte Carlo configurations. In addition, the dielectric constant was computed from the response to an applied electric field; the result is 81.5±1.5 at 25 °C and the experimental curve is mirrored from 0–100 °C at 1 atm. The TIP5P model is also found to perform well as a function of pressure; the density of liquid water at 25 °C is reproduced with an average error of ∼2% over the range from 1 to 10 000 atm, and the shift of the temperature of maximum density to lower temperature with increasing pressure is also obtained.
Time-of-flight measurement of CH4 molecules rainbow scattered from a LiF(001) surface
Authors: Tomii Takashi , Kondo Takahiro , Hiraoka Tomomi , Ikeuchi Toshiyuki , Yagyu Shinjiro , Yamamoto Shigehiko
Published: July 2002

Time-of-flight spectra of CH4 molecules scattered from a LiF(001) surface have been obtained at various scattering angles by making use of a cross-correlation chopper blade for the [100] and [110] azimuthal directions. The incident translational energies of CH4 molecules are varied in the 190–500 meV range, while the target surface temperature is maintained at 300 K. The experimental results have been examined in relation to the washboard model [J. C. Tully, J. Chem. Phys. 92, 680 (1990)] which is modified here to take into account the speed distribution of incoming molecules. A qualitative agreement on their angular dependence has been obtained both in the mean speed and the energy spread of the scattered CH4 molecules, which reflects the effect of the strong corrugation of the sample surface. Their quantitative differences are partly explained by the translation-rotational excitation of CH4 molecules during collision.
HCl photodissociation on argon clusters: Effects of sequential solvation and librational preexcitation
Authors: Žďánská Petra , Slavı́ček Petr , Jungwirth Pavel
Published: July 2002

Photolysis of the HCl molecule surface solvated on clusters with 2 to 12 argon atoms is investigated by means of quantum molecular dynamics simulations. Two basic questions are addressed: (i) How does the cage effect change upon increasing the size of the cluster, and (ii) how can caging be influenced by an infrared (IR) excitation of HCl hindered rotation (libration) prior to ultraviolet (UV) photolysis. The efficiency of caging is discussed in terms of measurable quantities. In the time domain, temporary populations of the trapped hydrogen atom are monitored, while in the energy domain short-lived vibrational resonances are observed as a fine structure in the hydrogen kinetic energy distribution. While caging is negligible for the smallest clusters, it becomes more efficient upon increasing the cluster size, and for 12 solvent atoms the cage effect is already very strong. Finally, it is shown that while in the ground state the hydrogen atom points essentially toward the rare gas cluster, in excited librational states hydrogen is directed mostly away from argon atoms. As a consequence, caging of the photodissociating hydrogen atom in the case of a surface solvated HCl molecule can be efficiently “turned off” by librational preexcitation.
Auxiliary basis sets for grid-free density functional theory
Authors: Glaesemann Kurt R. , Gordon Mark S.
Published: July 2002

Density functional theory (DFT) has gained popularity because it can frequently give accurate energies and geometries. The evaluation of DFT integrals in a fully analytical manner is generally impossible; thus, most implementations use numerical quadrature over grid points. The grid-free approaches were developed as a viable alternative based upon the resolution of the identity (RI). Of particular concern is the convergence of the RI with respect to basis set in the grid-free approach. Conventional atomic basis sets are inadequate for fitting the RI, particularly for gradient corrected functionals [J. Chem. Phys. 108, 9959 (1998)]. The focus of this work is on implementation of and selection of auxiliary basis sets. Auxiliary basis sets of varying sizes are studied and those with sufficient flexibility are found to adequately represent the RI.
Theoretical investigation of the autoionization process in molecular collision complexes: He*(2 3S)+Li(2 2S)→He+Li++e−
Authors: Movre M. , Thiel L. , Meyer W.
Published: July 2002

A complete ab initio treatment is applied to the autoionization process in the He*(2s 3S)+Li(2s 2S) collisional complex. Feshbach projection based on orbital occupancy, implemented in a multireference configuration interaction (MRCI) code, defines the resonance state and provides the entrance channel potential curve as well as all pertinent information on the resonance–continuum coupling. The l-dependent coupling elements in local approximation are obtained by projecting a compact one-electron function, named Penning molecular orbital (PMO), onto the wave function of the ejected electron with proper energy. The continuum wave function is obtained by coupled channel calculations in the static-exchange approximation. A converged set of seven complex coupling matrix elements, used in the nuclear dynamics calculation based on a complex Numerov algorithm, fully describes the electron angular momentum transfer. The calculated angle-dependent spectra, as well as the total, angle-, and energy-integrated ionization cross sections agree well with available experimental data.
Mode-specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer
Authors: Hogiu S. , Werncke W. , Pfeiffer M. , Dreyer J. , Elsaesser T.
Published: July 2002

Vibrational relaxation in the electronic ground state initiated by intramolecular back-electron transfer (b-ET) of betaine-30 (B-30) is studied by picosecond time-resolved anti-Stokes Raman spectroscopy. Measurements were carried out with B-30 dissolved in slowly as well as in rapidly relaxing solvents. We observed a risetime of the Raman band with the highest frequency near 1600 cm−1 which is close to the b-ET time τb-ET of B-30. For B-30 dissolved in propylene carbonate (τb-ET∼1 ps), the population of this mode exhibits a rise time of 1 ps whereas vibrational populations between 400 and 1400 cm−1 increase substantially slower. In contrast, in glycerol triacetin (τb-ET∼3.5 ps) and in ethanol (τb-ET∼6 ps) rise times of all modes are close to the respective b-ET times. Within the first few picoseconds, direct vibrational excitation through b-ET is favored for modes with the highest frequencies and high Franck–Condon factors. Later on, indirect channels of population due to vibrational energy redistribution (IVR) become effective. Thermal equilibrium populations of the Raman active modes are established within 10 to 15 ps after optical excitation.
Erratum: “Three-body collision contributions to recombination and collision-induced dissociation. II. Kinetics” [J. Chem. Phys. 109, 6714 (1998)]
Authors: Pack Russell T , Walker Robert B. , Kendrick Brian K.
Published: July 2002

A programming error, which affects many of the numbers, tables, and figures of this paper, is corrected. It does not affect any of the conclusions of the work.
Infrared study of water–benzene mixtures at high temperatures and pressures in the two- and one-phase regions
Authors: Furutaka Seiya , Ikawa Shun-ichi
Published: July 2002

Infrared spectra of water–benzene mixtures have been observed at temperatures and pressures in the 473–648 K and 100–350 bar ranges. The OH stretching band intensities of HDO in the benzene-rich phase in the two-phase region and in the one-phase region were obtained as a function of temperature and pressure. The band intensity, as a measure of water concentration, increases by about three times as the temperature rises from 473 to 523 K, while it is almost independent of pressure in the 100–350 bar range at these temperatures. At higher temperatures, on the contrary, the absorption intensities exhibit remarkable pressure dependence. They increase by an order of magnitude as the pressure increases from 100 to 350 bar. These temperature-pressure dependent changes of water concentration can be properly understood with a phase diagram of the water–benzene mixture. The absolute concentration of water in the benzene-rich phase and in the homogeneous one phase has been estimated from the absorption intensities by using a simple empirical relationship. The resulting values are consistent with literature data, which were obtained by composition analyses of the mixtures. The effect of temperature and pressure on the water solubility in the benzene-rich phase has been argued in terms of energies of solvation and cavity creation for a solute molecule. Coalescence of the hydrogen-bonded and hydrogen-bond-free OH stretching bands has been observed at higher temperatures and discussed in terms of rotational motion of water molecules in a hydrogen-bonded cluster.
Comparison of static polarizabilities of Cun, Nan, and Lin (n⩽9) clusters
Authors: Calaminici Patrizia , Köster Andreas M. , Vela Alberto , Jug Karl
Published: July 2002

This paper presents the first study of static polarizabilities and polarizability anisotropies of copper clusters up to nine atoms calculated in the framework of density functional theory. The calculations were of all-electron type and have been performed by using a finite field approach implemented in the density functional program ALLCHEM. A newly developed first-order field induced copper basis set for density functional calculation was employed. A gradient-corrected exchange-correlation functional has been used. All cluster structures were fully optimized. The calculated polarizabilities of copper clusters are compared with experimental polarizabilities of sodium and lithium clusters. This comparison shows that the size dependency of the static polarizabilities per atom of copper clusters posseses the same trend as that observed in sodium clusters. However, the absolute polarizabilities of the copper clusters are considerably smaller as those of the sodium clusters.
Electronic structure determination of chromium boride cation, CrB+
Authors: Kalemos Apostolos , Mavridis Aristides
Published: July 2002

The CrB+ cation molecular system has been investigated with the help of semi-quantitative basis sets [(7s6p4d3f )Cr/(4s3p2d1f )B] and highly correlated (valence) multi-reference wave functions. Out of a possible manifold of 70 states correlating to the Cr+(6S, 6D, 4D, 4G)+B(2P) atomic states, we have explored a total of 35 states spanning an energy range of about 3.4 eV. The ground state is of X 7Σ+ symmetry with a binding energy of 28.8 kcal/mol at an internuclear distance of 2.242 Å. The next three excited states 1 5Σ+, 2 5Π, and 37Π with energy splittings 7.1, 12.9, and 24.3 kcal/mol from the X state, have binding energies of 21.8, 16.5, and 5.1 kcal/mol, respectively. For practically all states we report potential energy curves, total energies, the most common spectroscopic parameters, while we discuss the binding modes using simple chemical diagrams based on valence-bond concepts.
Exact solution for the spatially inhomogeneous nonlinear Kac model of the Boltzmann equation
Authors: Cornille Henri
Published: July 2002

We study the spatially inhomogeneous Kac model of the nonlinear Boltzmann equation in 1+1+1 dimensions (velocity v, time t, position x). We obtain an exact solution which is the product of a Maxwellian with a time-dependent width by a second-order polynomial in the velocity variable. The solution satisfies a specular reflection condition at the boundary x=x0. The position x0−x appears linearly and only in the odd part of the velocity distribution, the range of x0−x being arbitrarily large but finite in order to maintain the positivity of the distribution. The local density is spatially homogeneous. Further, a particular linear relation between the moments of the cross section must be satisfied. The most general Maxwellian width has two relaxation times and their ratio is a function of the moments of the cross sections. Depending on whether this ratio is larger than or smaller than 1 we find contraction or expansion. The solution relaxes towards a Maxwellian equilibrium solution. Studying the Tjon overpopulation effect of high velocity particles, we find that it depends weakly on the initial condition, and strongly on both the microscopic model of cross section and on the ratio of the two relaxations times. We give a simple criterion (linked to the distinction between contraction and expansion) for the existence of the effect. Theoretically and numerically we test its validity and its failure.
Separability of the Dirac equation in a class of perfect fluid space-times with local rotational symmetry
Authors: Iyer B. R. , Vishveshwara C. V.
Published: July 2002

Chandrasekhar’s technique for separation of the Dirac equation in the Kerr background is applied to perfect fluid space-times with local rotational symmetry. These space-times fall into three distinct types. It is found that in case (1) the Dirac equation separates if the space-time is at least ‘‘locally static’’ while in case (3) it separates if the space-time is at least ‘‘locally diagonal,’’ in contrast to the massless case where Dhurandhar, Vishveshwara, and Cohen showed that the Hertz potential is separable in all cases. In case (2), however, the Dirac equation is separable in all those cases where the Hertz potential for neutrinos is separable.