bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of an inclusion compound of alkyl chains in an organic matrix
Authors: Bée M. , Renault A. , Lajzérowicz-Bonneteau J. , Le Bars-Combe M.
Published: July 2002

The nitroxide C9H16NO2 forms channel inclusion compounds with linear hydrocarbons. In particular, alkane molecules CnH2n+2 (if n≥6) can be enclosed end to end in parallel channels (c.a. 5 Å diameter). At room temperature, disorder of the chains is revealed by diffuse planes on x-ray photographs. Lock-in transitions occur at low temperature: one if n≤14 and two for greater length. Inclusion compounds with shorter chains, heptane and octane, were extensively analyzed. Besides other techniques, neutron experiments were carried out with selectively deuterated samples, in order to access the individual dynamics of the alkane or of the matrix. Advantage was taken of the characteristics of different spectrometers: IN6 and IN5 at Institut Laue-Langevin in Grenoble, MIBEMOL in Saclay, and IRIS at the Rutherford Appleton Laboratory. We propose a general model for the dynamics of this system. The motion of the alkyl chains combines kink defects responsible for jumps of the CH2 around the channel axis, together with a local fast motion of the H atoms, originating from the torsions of the chains and with a component parallel to the channel axis. Moreover, the reorientations of the alkyl chains are clearly related to an inversion of the cycles of the matrix molecules around the channels. Their displacements are evident.
The weakest bond: Experimental observation of helium dimer
Authors: Luo Fei , McBane George C. , Kim Geunsik , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

Helium dimer ion was observed after electron impact ionization of a supersonic expansion of helium with translational temperature near 1 mK. The dependence of the ion signal on source pressure, distance from the source, and electron kinetic energy was measured. The signal was determined to arise from ionization of neutral helium dimer.
Femtosecond wave packet spectroscopy: Coherences, the potential, and structural determination
Authors: Gruebele M. , Zewail A. H.
Published: July 2002

Recently, we presented a formalism for extracting highly resolved spectral information and the potential of bound isolated systems from coherent ultrafast laser experiments, using I2 as a model system [Gruebele et al., Chem. Phys. Lett. 166, 459 (1990)]. The key to this approach is the formation of coherent wave packets on the potential energy curve (or surface) of interest, and the measurement of their scalar and vector properties. Here we give a full account of the method by analyzing the coherences of the wave packet in the temporal transients of molecules excited by ultrashort laser pulses, either at room temperature, or in a molecular beam. From this, some general considerations for properly treating temporal data can be derived. We also present a direct inversion to the potential and quantum and classical calculations for comparison with the experiments.
Steric effects in the state specific reaction Li+HF (v=1, j=1, m=0)→LiF+H
Authors: Loesch H. J. , Stienkemeier F.
Published: July 2002

Results of a crossed beam study on the state specific reaction Li+HF (v=1, j=1, m=0)→LiF+H at a translational collision energy of Etr=0.42 eV are reported. Angular distributions of LiF have been measured for three different distributions of the internuclear axis of HF, namely an isotropic one, one where the axis is aligned with the relative velocity of reagents V and one where the axis is aligned perpendicular to both V and the scattering plane. We find a marked influence of these collision geometries (steric effects) on (i) the angular distributions; (ii) the partition of available energy; and (iii) the integral reaction cross sections. The ratio of the latter for preferred side-on and end-on collisions with HF amounts to 1.76. From the angular distributions of products, double-differential cross sections in the center-of-mass frame are determined which exhibit in all three cases preferred backward scattering of LiF. They provide three out of four accessible moments of the orientation-dependent double-differential reaction cross section. The results are compared to quasiclassical trajectory calculations based on the potential energy surface of Chen and Schaefer [J. Chem. Phys. 72, 4376 (1980)] and to predictions of a modified direct interaction with product repulsion (DIPR) model. The latter suggests that the stringent correlation between the electric dipole moment d (the synonym for molecular axis) and the direction in which the products are ejected is relaxed and both a reorientation of the molecular axis during the approach of reagents and an interaction between the products during separation play an important role. These conclusions are supported by trajectory calculations.
Theoretical study of Ga4As4, Al4P4, and Mg4S4 clusters
Authors: Al-Laham Mohammad A. , Raghavachari Krishnan
Published: July 2002

Ab initio molecular orbital investigations of the electronic structures, bonding, and stabilities of Ga4As4, Al4P4, and Mg4S4 are reported. The effects of polarization functions and electron correlation are included in these calculations. Our results indicate that the electronegativity difference between the constituents of a mixed cluster plays a very important role in determining its ground state structure. In A4B4 mixed clusters, a distorted cubic structure with alternating atomic charges leads to a particularly stable ionic form. This Td structure consisting of two interpenetrating tetrahedra of the two constituents is the ground state structure for both Al4P4 and Mg4S4. For Ga4As4, there also exists a more covalent Si8-like Ci structure that is 3 kcal/mole more stable than the Td form. The structures and relative stabilities of these mixed clusters are compared with the results for valence-isoelectronic Si8 and Na4Cl4 clusters.
A study of O3, S3, CH2, and Be2 using Kohn–Sham theory with accurate quadrature and large basis sets
Authors: Murray Christopher W. , Handy Nicholas C. , Amos Roger D.
Published: July 2002

Density functional calculations with accurate quadrature and large basis sets are reported on ozone, thiozone, methylene, and the beryllium dimer. The Kohn–Sham self-consistent procedure is used throughout and nonlocal gradient corrected functionals (B-LYP, B-P) are used. The systems and properties chosen are notoriously difficult problems for accurate computation, nevertheless, the density functional methods perform well. It is found, in line with calculations on more straightforward problems, that the functionals involving gradient corrections are more reliable than local density functionals. The B-LYP prediction is within 5 kcal/mol of the most accurate ab initio data on the C2v–D3h energy gap in ozone and thiozone. The B-LYP functional performs reasonably well on the vibrational frequencies of ozone whilst the B-P functional gives very accurate results for the structural parameters of ozone. B-LYP is used to estimate the singlet–triplet splitting in methylene with the triplet treated in both a restricted and unrestricted formalism. In either case the correct energy gap is reproduced within 2 kcal/mol. The bond length and low frequency of the beryllium dimer are reproduced accurately by the gradient corrected functionals but the binding energy is overestimated. However, B-LYP only overestimates the binding energy by 4 kcal/mol, which is an encouraging result given the difficulty of the problem. This paper can therefore be seen as further evidence that density functionals methods, and in particular methods based on nonlocal functionals, can predict molecular properties reasonably accurately for systems that have multireference character.
Reply to Comment on: The relation between intensity and dipole moment for bending modes in linear molecules
Authors: So/rensen Georg Ole , Jo/rgensen Uffe Gråe
Published: July 2002

Our paper [J. Chem. Phys. 97, 4616 (1992) (SJ92)] and Gwaltney and Bartlett’s comments to it [J. Chem. Phys. 99, ▪▪▪ (1993) (GB93)] show that discrepancies in the literature between ab initio and experimental intensities are mainly due to inaccuracies in the computed dipole moment. SJ92 and GB93 find that the most accurate transformation between intensity and dipole moment is the one outlined by SJ92, however GB93’s intensities are more accurate than those presented in Table II in SJ92. This high accuracy is not common in the literature, particularly not for overtones and combination bands.
Evidence for the deep potential well of Li+HF from backward glory scattering
Authors: Loesch H. J. , Stienkemeier F.
Published: July 2002

Angular distributions of Li scattered from HF have been measured for various collision energies Etr ranging from 64 to 372 meV. At Etr=88 and 64 meV a clear rainbow structure at small angles is observed. At Etr=296 meV a faint peak emerges at large angles which becomes a prominent feature for all lower energies. The position of the peak always corresponds to backward scattering in the center-of-mass frame. This fact and the rather sudden rise of the backward intensity with decreasing Etr clearly identify this phenomenon as a backward glory. Based on the infinite order sudden approximation both the rainbow and the backward glory as well as the Etr dependence of these features could be rationalized using a simple empirical potential energy surface. The surface consists of two only moderately anisotropic extended regions which differ drastically in the well depths. The shallow well region has a maximal depth of ≊33 meV and is essentially determined by the rainbow scattering. The deep well region features a maximal depth of ≊300 meV and is associated with the backward glory. The well depths are in excellent to fair agreement with recent ab initio results.
A spectroscopic investigation of the nonrigid–rigid transition of (benzene)13 during free jet expansion
Authors: Easter David C. , Baronavski A. P. , Hawley Michael
Published: July 2002

We have traced the evolution of the benzene-h6 transition in (C6H6)(C6D6)12 as a function of time (distance from the nozzle) in a supersonic expansion. Taking advantage of the special properties of the isotopically substituted (benzene)13 resonant two-photon ionization spectrum, we present the first experimental evidence for a nonrigid–rigid transition in a single-size molecular cluster. Initially, under the conditions of our free-jet expansion, (benzene)13 condenses in a nonrigid form, then solidifies into a well-defined structural configuration.
On the role of conical intersections of two potential energy surfaces of the same symmetry in photodissociation. I. CH3SH→CH3S+H and CH3+SH
Authors: Yarkony David R.
Published: July 2002

Recent experimental/theoretical studies have demonstrated efficient photodissociation of CH3SH to CH3S+H and to CH3+SH. Avoided crossings have been implicated in the photodissociation process. This work demonstrates the existence of a surface of conical intersections of two states of the same symmetry relevant to the photodissociation offering new insights into the mechanism of the decomposition process. Using wave functions comprised of greater than 1.3 million configuration state functions convincing evidence is provided that the surface of conical intersections is a property of the system rather the particular expansion employed in this work. The conical intersections are determined directly, that is without prior determination of the potential energy surfaces in question.
Conductivity in quasi-one-dimensional organic metals. A new approach
Authors: Misurkin I. A. , Tchougreeff A. L.
Published: July 2002

A new model for the electric conductivity in quasi-one-dimensional organic metals is proposed. The conductivity is attributed to electron hopping between the separate segments of the one-dimensional donor stacks. The metalliclike behavior of the conductivity at the room temperature as well the low temperature conductivity maximum are explained. The relation of the model with the Anderson’s confinement hypothesis is briefly discussed.
The analysis of the chemisorption bond from uncorrelated and correlated cluster model wave functions
Authors: Ricart J. M. , Clotet A. , Illas F. , Rubio J.
Published: July 2002

The influence of the electronic correlation on the physical picture of the chemisorption bond arising from the ab initio molecular orbital cluster model approach is studied through some selected examples. Dipole moment curves and the effects induced by a uniform external electric field on the equilibrium distance are the techniques used to analyze the chemisorption bond. The comparison between uncorrelated self-consistent field (SCF) and correlated configuration interaction (CI) results for these two techniques of analysis permits us to conclude that even when the SCF interaction energy is a poor approximation of the more accurate CI value, both wave functions lead to the same qualitative picture of the chemisorption bond.
Reactions of laser ablated Be atoms with O2: Infrared spectra of beryllium oxides in solid argon
Authors: Thompson Craig A. , Andrews Lester
Published: July 2002

Pulsed-laser ablated beryllium atoms codeposited with O2 in excess argon at 10 K yielded new beryllium–oxygen molecules. The initial reaction to make BeO is endothermic, but activation energy is provided by hyperthermal Be atoms. A strong band at 1572.9 cm−1 and a weak band at 398.9 cm−1 gave oxygen isotopic shifts consistent with a BeO diatomic molecule and are assigned to the O2–BeO complex analogous to the Ar–BeO complex. A sharp band at 1413.2 cm−1 exhibited an oxygen isotopic triplet with 16,18O2 and shifts appropriate for linear OBeO. A nearly coincident band at 1412.4 cm−1 gave an isotopic doublet and shifted in agreement with linear BeOBe. Quantum chemical calculations at the second-order many-body perturbation theory [MBPT(2)] level predict 3Σ ground states and ν3 fundamentals at 1422.5 and 1418.2 cm−1 for OBeO and BeOBe, respectively, in excellent agreement with the observed values. Three bands at 1131.2, 866.3, and 522.4 cm−1 increased together on photolysis, yielded isotopic triplets, and agreed with MBPT(2) calculations for rhombic Be2O2. A sharp photosensitive band at 988.6 cm−1 gave oxygen isotopic shifts consistent with another linear OBeO species and is tentatively assigned to the molecular anion. An intense band at 1465.1 cm−1 that appeared on annealing gave oxygen isotopic shifts in agreement with calculations for linear BeOBeO. Two bands at 1288.9 and 1264.1 cm−1 appeared on photolysis and gave isotopic triplets and shifts that are in excellent agreement with calculations for branched BeBeO2. Annealing produced bands at 871.8 and 436.1 cm−1 that decreased on photolysis and are assigned to BeO3.
Calculation of transport properties from molecular dynamics simulation
Authors: Baranyai András
Published: July 2002

Using the self-diffusion as an example, we present a novel molecular dynamics method which fixes a transport property of the system to a prescribed value. We discuss the technical realization of the algorithm for this history dependent property. Owing to the monotonic temperature vs self-diffusion relationship for a given model substance at fixed density the system automatically chooses that temperature which corresponds to the input value of the self-diffusion coefficient. In principle, this approach can be applied to other transport coefficients too. In practice, however, the numerical calculation of transport properties expressed by the collective dynamics of particles is very time consuming from equilibrium molecular dynamics simulation. Our method being an equivalent of the Green–Kubo integral also performs poorly compared to nonequilibrium molecular dynamics techniques. We present details and results of model calculations for these two transport processes.
Rotational–translational correlations in photochemically desorbed molecules
Authors: Zimmermann Frank M. , Ho W.
Published: July 2002

A simple dynamical model is presented that quantitatively explains the positive correlation between rotational and translational energies characteristic of photodesorbed molecules. The model unifies two previously presented models for rotational and translational excitation mechanisms, both of which exploit the fact that electronic excitation lifetimes are extremely short in most photodesorption processes from metal and semiconductor surfaces. Observed positive rotational–translational correlations are explained to be a consequence of the distribution of excited state residence times.
Thermodynamics of thin, diblock copolymer films
Authors: Wong Ka Yiu , Trache Maria , McMullen William E.
Published: July 2002

We analyze the weak-segregation, thermodynamic predictions of a simple mean-field model of diblock copolymer melts sandwiched between surfaces that interact weakly with melts. Under many conditions, these films do not exhibit a phase transition although the segregation that develops within them resembles ordering in the bulk. When transitions occur, they result from a competition between surface and bulk effects and have no bulk analog. Thin films confined by identical surfaces and films bounded by surfaces having equal in magnitude, but opposite affinities for the monomers may exhibit first- as well as second-order intrafilm transitions when the melt–surface interactions are small. The model predicts that second-order transitions do not occur in films with large surface affinities and that films bounded by different surfaces undergo only first-order intrafilm transitions. The instabilities that lead to the second-order transitions in films with symmetry-breaking affinities disappear with increases in the film thickness.
The influence of structure on diffusion in screened Coulombic suspensions
Authors: Nilsen Stephen J. , Gast Alice P.
Published: July 2002

The static structure factor and hydrodynamics, characterized by the functions S(q) and H(q), respectively, will modify the short-time q-dependent diffusion coefficient from its free particle value, D0, as D(q)=D0[H(q)/S(q)]. We probe these dynamics with diffusing wave spectroscopy on low volume fraction suspensions of highly charged, monodisperse colloidal particles, where we can neglect the hydrodynamic contributions and probe the effect of structure alone. We vary the range of the screened Coulombic interactions via suspension ionic strength to obtain a range of interparticle structures from liquid-like to ordered crystalline arrays. From the initial decay of the autocorrelation function of the multiply scattered laser light, we obtain a q-weighted average of D(q). Modeling the interactions with the hard-core Yukawa pair potential and the structure with the rescaled mean spherical approximation, we attribute slowed dynamics in the suspension to peaks in S(q). We vary the particle size, laser wavelength, and suspension volume fraction to probe the effect of various features in S(q) on the dynamics.
Pitchfork and winged-cusp singularities in iodate–arsenous acid reaction
Authors: Li Ru-Sheng
Published: July 2002

The iodate–arsenous acid reaction was reported to be able to exhibit hysteresis bistability, including mushrooms and isolas, in a continuous flow stirred tank reactor (CSTR) when an additional flow of solvent is introduced [N. Ganapathisubramanian and K. Showalter, J. Chem. Phys. 80, 4177 (1984)]. Based on their kinetic data and the empirical rate law, it is shown that the reaction may also exhibit pitchfork-type and winged cusp-type singularities if additional flows of the reactants are introduced to the CSTR.
Resonances in the CH+N2→HCN+N(4S) reaction: The dynamics of a spin-forbidden process
Authors: Seideman Tamar
Published: July 2002

The dynamics of the CH+N2(X 1Σ+g)→HCN+N(4S) reaction is studied theoretically for the first time. A simple two-dimensional model is developed, treating the reaction dynamics on the doublet and on the quartet Born–Oppenheimer surfaces of CHN2 by exact quantum mechanics and the coupling between the two electronic states within first-order perturbation theory. Summation over total angular momentum states is carried out within the J-shifting approximation and the Boltzmann rate constant is computed over the temperature range of interest for combustion T≲1700 K. The reaction probability exhibits a rich resonance pattern, manifesting the existence of long-lived quasibound intermediate states on both the doublet and the quartet surfaces. These resonances affect the dynamics profoundly, being the driving force behind the spin-changing reaction. The thermal rate constant increases with temperature in an Arrhenius type fashion and in qualitative agreement with high-temperature experiments.
Electron-impact dissociation of HCl: Translational energy and angular distributions of excited hydrogen atoms
Authors: Ogawa Teiichiro , Tsuboi Toshiyuki , Nakashima Keiji
Published: July 2002

Electron-impact dissociation of HCl for the formation of excited hydrogen atoms (n=4) has been investigated by measuring Doppler profiles of the Balmer lines and their angular dependence at a high optical resolution using a Fabry–Perot interferometer. The translational energy distribution (TED) and the angular difference Doppler profile were obtained. There are five major dissociation processes for the formation of H* (n=4). The threshold energy and the TED peak of the five components are (1) 19 and 2.5; (2) 25 and 7.2; (3) 29 and 1.7; (4) 36 and 5.1; and (5) ≳40 and 8–12 eV, respectively. Formation of components 1 and 4 is anisotropic and parallel with respect to the electron beam. Component 1 should be produced by predissociation through the Rydberg states with the Σ symmetry converging to either the 4Σ or 2Σ− state and then those converging to the A 2Σ+ state. The asymmetry parameter (β) of component 1 was determined to be about 0.62, and the intermediate excited state for the formation of component 1 has a lifetime equal to the rotational period. Component 2 would be produced through the Rydberg states converging to the 2Π state. Component 3 would be produced through high-lying doubly excited Rydberg states converging to either the (4)2Π or (4)2Σ+ state. Component 4 should be produced through doubly excited repulsive states with the Σ symmetry.
A second harmonic generation analog of optical rotatory dispersion for the study of chiral monolayers
Authors: Byers J. D. , Yee H. I. , Hicks J. M.
Published: July 2002

Surface second harmonic generation (SHG) is shown to display optical rotatory effects dependent on the handedness of chiral molecules (2,2′-dihydroxy-1,1′binaphthyl) at various interfaces. The polarization of the surface SHG radiation is rotated with respect to that of the input fundamental radiation in a direction indicative of the chirality of the surface molecules. The origin of SHG-ORD lies in the electric dipole-allowed χ(2) tensor elements, one of which is associated with the handedness of surface species. The SHG-ORD rotation is independent of the density of surface species for an optically pure system, but is dependent on the chiral excess for mixtures of enantiomers. It is wavelength dependent but resonance is not required to observe the rotation. The experiments show that while SHG-ORD has origins quite distinct from ordinary ORD, it can be utilized to measure chiral structures of surfaces.
Algebraic approach to vibrational spectra of tetrahedral molecules: Application to methane
Authors: Lemus R. , Frank A.
Published: July 2002

We present an algebraic approach to describe the vibrational excitations of polyatomic molecules. The model is based on the coupling of anharmonic oscillators and is characterized by combining the strengths of the Lie-algebraic methods with those of point symmetry techniques. We illustrate the algebraic method for tetrahedral molecules and apply it to the construction of the complete vibrational spectra of methane up to four quanta.
New phase diagram for N-arylacrylamide/acrylamide/sodium acrylate copolymer gel
Authors: Katayama Seiji , Shimizu Masanori , Akahori Yukio
Published: July 2002

Examination was made of changes in the volume of N-arylacrylamide(NAA)/acrylamide/sodium acrylate copolymer gel immersed in acetone–water and dioxane–water mixtures. The gel underwent reversible volume change from a swollen to collapsed state via discontinuous volume change with increase in solvent composition. Volume change at the transition point decreased with increase in NAA content, and the transition point shifted first toward lower solvent content with increase in NAA content and then toward higher solvent content. This is a volume phase transition characterized by two conventional patterns. Plots of transition solvent composition as a function of gel polymer composition indicated a nobel phase diagram for the polymer gel, which is essentially equivalent to miscibility diagram usually observed for polymer solutions.
A lumped model for H2/O2 oxidation in the oscillatory regime
Authors: Li Genyuan , Rabitz Herschel
Published: July 2002

A lumped model for H2/O2 oxidation in the oscillatory regime is constructed by using the approach of approximate constrained nonlinear lumping based on an algebraic method within nonlinear perturbation theory. The fast variables approach zero rapidly such that the lumped model is constructed within a slow manifold by setting the identified fast variables to zero. The model dimension is reduced from 7 to 4. The variables representing the concentrations of H2, O2, H2O, and H are explicitly kept unlumped. The lumped model accurately reproduces the main features of the original system, such as oscillatory periods and concentration profiles.
Instabilities of cubic autocatalytic waves on two and three dimensional domains
Authors: Milton Robert A. , Scott Stephen K.
Published: July 2002

Planar wave fronts in autocatalytic chemical systems propagate with a constant wave form and velocity provided that the reactant and autocatalytic species have similar diffusion coefficients. Such waves are also stable to spatial perturbations. Circular or spherical fronts show a constant wave form and a velocity that increases towards the planar wave velocity as the radius increases with time. These are again stable to spatial perturbation if the reactant and autocatalyst have similar diffusivities. However, if the ratio of the diffusion coefficients δ exceeds some critical value δ*≊2.3 a different situation arises. For cylindrical or spherical geometries, unperturbed waves decelerate as they expand if δ≳δ*. For all geometries, the smooth waves may become unstable to spatial perturbation if δ≳δ* although there are some additional requirements. In Cartesian systems, the width of the reaction zone transverse to the direction of propagation must exceed some minimum value W*≊6 (in dimensionless units) and the wave number of the imposed perturbation must be less than kcr,max≊0.15. For circular or spherical waves, the conditions for the growth of perturbations also involves the radius of the wave at the moment the perturbation is applied. A set of expansions based on small curvature and small departures of δ from unity have been derived allowing the instantaneous wave velocity to be written in the form v(θ,φ,τ)=c+v1κ+v2Δtrκ, where the coefficients c, v1, and v2 depend on δ and κ is the instantaneous curvature.
Quantum-state-specific dynamics of the dissociative adsorption and associative desorption of H2 at a Cu(111) surface
Authors: Rettner C. T. , Michelsen H. A. , Auerbach D. J.
Published: July 2002

We have determined the dependence of the dissociative adsorption probability in the zero coverage limit, S0, for H2 on Cu(111) as a function of translational energy, Ei, and incidence angle, θi, vibrational state, v, and rotational state, J. We have also obtained information on the effect of surface temperature, Ts, on this probability. These results have been obtained by combining the findings of two separate experiments. We have obtained the form of the dependence of S0 on Ei at Ts=925 K for a range of quantum states from desorption experiments via the principle of detailed balance. We have obtained absolute S0 values from direct molecular beam adsorption experiments, which reveal that S0 scales with the so-called ‘‘normal energy,’’ En=Ei cos2 θi. The desorption experiments provide detailed information for J=0 to 10 of H2(v=0) and for J=0 to 7 of H2(v=1). The beam experiments additionally provide information on the adsorption of H2(v=2), averaged over J. All measurements are consistent with adsorption functions with an s-shaped form, which can be described by S0=A(1+erf(x))/2, where x=(En−E0)/W. Values of W are ∼0.16 and 0.13 eV for v=0 and v=1, respectively, at Ts=925 K, falling by about 0.05 eV for Ts=120 K, and with only a slight dependence on J. Values of A are insensitive to v and J, with a value of ∼0.25. S(En,v,J) curves are thus similar for different v and J, but shifted in En.

In contrast, we find that the values of E0, which determine the mid-point of the curves, have a strong dependence on v and J. Specifically, E0 for H2(v=0) molecules is about 0.6 eV, falling to 0.3 and 0.1 eV for H2(v=1) and H2(v=2), respectively. Translational energy is thus about twice as effective as vibrational energy in promoting dissociation. E0 rises with increasing J at low J, before falling at high J, indicating that rotational motion hinders adsorption for low rotational states (J<4), and enhances adsorption for high rotational states (J≳4). Results are compared with similar studies on the D2/Cu(111) system and with recent calculations. Finally, these results are used to predict the dependence of the rate of dissociation on temperature for a ‘‘bulb’’ experiment with ambient hydrogen gas in contact with a Cu(111) surface. This simulation yields an activation energy of 0.47 eV for temperatures close to 800 K, compared to a literature value of 0.4 eV from experiment. Analysis of the temperature dependence reveals that the dominant reason for the increase in rate at high temperature is the increase in population of the high energy tail of the translational energy distribution.
Quantum limitations on dynamics and control
Authors: Shapiro Moshe , Brumer Paul
Published: July 2002

Quantum mechanics is shown to impose a rigid limitation on the extent to which dynamics can be controlled. Specifically, consider a system in an initial subspace ℋ0 of dimensionality M0, which evolves to populate subspaces ℋ1, ℋ2 of dimensionality M1, M2. Then, if M2≥M0, it is possible to prevent transitions from ℋ0 into ℋ2 only under extremely stringent conditions. Classical mechanics is shown to impose no such limitation.
Alternative ansätze in single reference coupled-cluster theory. III. A critical analysis of different methods
Authors: Szalay Péter G. , Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

There are several important formal requirements that should be satisfied if a quantum chemical method is to offer a widely applicable approach. This paper considers a variety of potential exponential ansätze, and assesses their attributes pertaining to different desiderata, in particular satisfaction of a symmetric generalized Hellmann–Feynman theorem, exactness of the method in the untruncated limit and finiteness of the equations. The ansätze are the standard coupled-cluster (CC); the expectation value (XCC); symmetric expectation value (SXCC); unitary (UCC); and the extended (ECC) coupled-cluster approaches. ECC satisfies most specified desiderata, but it does not satisfy a symmetric GHF theorem. SXCC is shown to be a symmetrized approximation to ECC, which does satisfy a symmetric GHF theorem, but SXCC is not exact in the untruncated limit. A new alternative ansatz, strongly connected SC-XCC is proposed which satisfies all desiderata except that the equations are nonterminating. Considering the necessity of approximation for all high level alternative CC methods, the nonfiniteness of the energy functional and stationary equations might not be a major limitation.
Molecular dynamics of linear and branched alkanes
Authors: Mondello Maurizio , Grest Gary S.
Published: July 2002

We have extended a recently introduced united atom model of n-alkanes to investigate the liquid-state dynamics of squalane, a molecule with six, symmetrically placed, methyl side groups. We compare our results with experimental measurements of diffusion in the same system and with experimental and simulation results on n-decane and n-tetracosane. The model reproduces the significantly different temperature dependence of diffusion in squalane and n-alkanes of similar mass. The results of a detailed comparative study of the intrachain local dynamics in the different molecules makes apparent its correlation with global single-chain relaxation processes. For linear alkanes, we also make a comparison between united atom and asymmetric united atom models.
Monte Carlo simulation of exciton trapping in quasi-one-dimensional antiferromagnetic CsMnCl3⋅2H2O
Authors: Karachevtsev V. A. , Levitsky I. A. , Slavin V. V.
Published: July 2002

Computer simulation of incoherent exciton migration in a quasi-one-dimensional (q-1-D) lattice with randomly distributed traps has been performed. The model assumes both efficient and inefficient exciton trapping. Simulative curves are fitted to the experimental exciton decay curves in q-1-D antiferromagnetic crystals CsMnCl3⋅2H2O (CMC) doped with copper ions (1% and 0.1%). The hopping rate along a chain (W) and trapping rate (U) have been defined. These rates are compared with those obtained from fitting the theoretical curves (1-D model of exciton migration) to the experimental ones.
Response to ‘‘Comment on ‘Density functional calculation of nuclear chemical shifts’ ’’ [J. Chem. Phys. 104, 1163 (1996)]
Authors: van Wüllen Christoph
Published: July 2002

The comment presents numerical results indicating that the gauge dependence of the SOS method is not a practical problem. This gauge dependence was analyzed in the paper commented on, and the theoretical analysis therein remains valid.
Quantum calculations for rotational energy transfer in nitrogen molecule collisions
Authors: Huo Winifred M. , Green Sheldon
Published: July 2002

Rotational energy transfer in collisions of nitrogen molecules has been studied theoretically, using the N2–N2 rigid-rotor potential of van der Avoird et al. [J. Chem. Phys. 84, 1629 (1986)]. For benchmarking purposes, converged close coupling (CC) calculations have been carried out to a total energy of about 200 cm−1. Coupled states (CS) approximation calculations have been carried out to a total energy of 680 cm−1, and infinite order sudden (IOS) approximation calculations have also been carried out. The CC and CS cross sections have been obtained both with and without identical molecule exchange symmetry, whereas exchange was neglected in the IOS calculations. The CS results track the CC cross sections rather well: between 113–219 cm−1 the average deviation is 14%, with accuracy improving at higher energy. Comparison between the CS and IOS cross sections at the high energy end of the CS calculations, 500–680 cm−1, shows that IOS is sensitive to the amount of inelasticity and the results for large ΔJ transitions are subject to larger errors. State-to-state cross sections with even and odd exchange symmetry agree to better than 2% and are well represented as a sum of direct and exchange cross sections for distinguishable molecules, an indication of the applicability of a classical treatment for this system. This result, however, does not apply to partial cross sections for given total J, but arises from a near cancellation of the interference terms between even and odd exchange symmetries on summing over partial waves. In order to compare with experimental data for rotational excitation rates of N2 in the n=1 excited vibrational level colliding with ground vibrational level (n=0) bath N2 molecules, it is assumed that exchange scattering between molecules in different vibrational levels is negligible and direct scattering is independent of n so that distinguishable molecule rigid rotor rates may be used. With these assumptions good agreement is obtained. Although the IOS approximation itself is found to provide only moderately accurate values for rate constants, IOS/ECS scaling methods, especially if based on fundamental rates obtained from coupled channel results, are found to provide generally good accuracy.
Through bond and through space interactions in oligo–alkoxythiophenes: A spectroscopic study
Authors: Villa E. , Agosti E. , Castiglioni C. , Gallazzi M. C. , Zerbi G.
Published: July 2002

The aim of this work is to understand the molecular structural properties which may justify the great observed stability of doped alkoxy polythiophenes. Infrared frequency and intensity spectroscopy and Raman spectroscopy are used as probes. Suitably synthesized oligomers with increasing chain length and preassigned chemical structure have been studied together with several model molecules. The existence of through-space and through-bond S- - -O interactions has been revealed. The extent of the topologically dependent perturbation induced by the charge injection by the oxygen atoms is detected and measured. The inter- and intraring delocalizations have been monitored by the observation of the softening of the strongly Raman active ‘‘amplitude mode’’ and by the red shift of the electronic spectra. The conformation of the side chains is trans planar in the solid state and collapses in a liquidlike premelting state before the melting of the crystal.
Hückel-type semiempirical implementation of a variational method for determining electronic band gaps
Authors: Cedillo Andrés , Parr Robert G.
Published: July 2002

A previously derived [J. Chem. Phys. 103, 7645 (1995)] general variational principle, for determining the hardness (or band gap) of electronic systems, is applied to π-electron systems by a straightforward simple parameterization of the hardness kernel. For conjugated polyenes and annulenes, the hardness (band gaps) are essentially the same as (although no identical with) those predicted by the Hückel method. In contrast with the Hückel method, one need not assume that the total electronic energy is a sum of one-electron energies. Comparisons are made with Nalewajski’s Charge Sensitivity Analysis.
Long-time behavior of the angular velocity autocorrelation function
Authors: Masters A. J.
Published: July 2002

In this Communication we present a hydrodynamic analysis to show that the asymptotic long-time tail of the angular velocity autocorrelation function of a Brownian particle is independent of the particle’s size or shape—a result in agreement with simulation and in accord with previous predictions from microscopic theories. The reason that earlier hydrodynamic treatments had predicted a shape dependent tail is because they implicitly assumed the particle to have a fixed orientation. Reorientation of the particle must be included to obtain the true long-time behavior.
Monte Carlo simulations of the adsorption of amphiphilic oligomers at hydrophobic interfaces
Authors: Wijmans Christopher M. , Linse Per
Published: July 2002

The adsorption of amphiphilic oligomers from solution to different interfaces has been studied using a lattice model. The oligomers consist of one hydrophobic and one hydrophilic block and can form micellar aggregates in solution at sufficiently high concentrations. We have investigated the relationships among the amphiphile bulk concentration, its chemical potential, and adsorbed amount. Below the critical micellization concentration (cmc) the amphiphile chemical potential is a logarithmic function of the concentration, above the cmc it becomes constant. When the amphiphile solution is in contact with a hydrophobic surface, the hydrophobic blocks adsorb to the surface and the hydrophilic blocks protrude into the solution. As the adsorbed amount increases, the adsorbed molecules become laterally more confined and stretch away from the interface. The adsorption isotherm shows a plateau at high bulk concentration, which is due to the constant chemical potential above the cmc. The influence on the adsorption of both the interaction between the amphiphile and the solvent, and between the amphiphile and the interface are investigated. Finally, the adsorption at a hard hydrophobic surface is compared with the adsorption at a fluid interface.
Adsorbate order-disorder effects on recombinative thermal desorption: Equivalence between dynamic Monte Carlo simulations and self-consistent cluster approximations
Authors: Weinketz Sieghard , Cabrera G. G.
Published: July 2002

The thermally activated desorption of dissociated diatomic species from a metallic surface is described as a lattice-gas problem on a square lattice with nearest- and next-nearest neighbor interactions between the adsorbates and investigated within dynamic Monte Carlo simulations. In the limit of fast diffusion with respect to desorption, it can be shown that the desorption rate depends directly on the local order induced by the interactions within the adsorbate layer. Therefore, by employing an appropriate quasi-equilibrium cluster approximation for the local order (beyond the quasi-chemical approximation), a differential equation can be derived that depends on self-consistently calculated structure forms, reproducing quantitatively the temperature-programmed desorption spectra simulated with the Monte Carlo procedure. In this way it can be shown that the time evolution obtained from the dynamic Monte Carlo algorithm is indeed “correct,’’ and on the other hand, that it can be successfully substituted by a “cheaper’’ cluster approximation.
A molecular dynamics study of photothermal compression of colloidal crystals
Authors: Zahorchak James C. , Kurnikova Maria G. , Coalson Rob D.
Published: July 2002

When irradiated with a laser, the lattice constant of a colloid crystal changes in response to the temperature gradient created in the solvent. Using constant pressure/constant temperature molecular dynamics simulations, the expansion and compression of charge stabilized colloid crystals is investigated. Two systems are considered, both of which correspond approximately to samples which have been experimentally characterized. It is shown that these colloid crystals do not melt over a temperature range of 298 K to 368 K. One system only expands with increasing temperature, while the other initially expands and then contracts. Colloid number density is calculated as a function of temperature at the center of the heated region. An analytical model of the dependence of equilibrium lattice parameter on external pressure for a crystal characterized by zero colloid temperature is presented. Because these crystals remain relatively rigid even in the physically relevant temperature range, it is argued that a zero colloid temperature model should be qualitatively reasonable. Indeed, the model calculations support the basic conclusions drawn from full molecular dynamics simulations, and by comparison highlight effects due to finite temperature motion of the colloid particles.
Fullerene symmetry reduction and rotational level fine structure: The Buckyball isotopomer 12C59 13C
Authors: Reimer Tyle C. , Harter William G.
Published: July 2002

Rotational level structure is investigated for a number of lower symmetry fullerene structures including the Buckyball isotopomer 13C 12C59, and it is compared to that of 12C60 using quantum mechanical and semiclassical theory. Rotational spectra of 13C 12C59 differs markedly from that of icosahedrally symmetric Buckyball because the presence of a single additional neutron completely breaks its rotational symmetry from Ih for C60 down to a single Cs reflection plane for 13C 12C59. Nevertheless, most rotational energy levels remain surprisingly clustered and well ordered. Predictions are made for types of spectroscopic structure resulting from reduction of icosahedral symmetry to Cs, C2v, C3v, and C5v such as might be encountered in intrahedrally doped XC59. Semiclassical techniques help to label the spectra of molecules undergoing such extreme symmetry breaking and to explain why high J levels still maintain so much order and degeneracy under these conditions. These techniques may also be useful toward understanding the dynamics of hindered rotors in solution or solid fullerite, as well as in the interpretation of high resolution gas phase spectra of fullerene molecules or ions.
State-resolved photodesorption of CO from Ag(111): Nonthermal desorption at 1064 nm
Authors: Fleck L. , Beuhler R. J. , White M. G.
Published: July 2002

State-resolved measurements of CO molecules desorbed from Ag(111) by 1064 nm radiation have been performed using (VUV+UV) resonant multiphoton ionization. The CO translational and rotational energies are found to be essentially constant over a wide range of 1064 nm laser fluences indicating a non-thermal desorption mechanism.
Spin-dependent effects in charge carrier photogeneration in amorphous molecular semiconductor with charge transfer-complex centers
Authors: Davidenko N. A. , Kuvshinsky N. G.
Published: July 2002

The paper is devoted to the investigation of multiplicity and evolution of multiplicity in the course of life of electron-hole pairs in the films of poly-N-epoxypropylcarbazole, containing the CT-complexes as the electron-hole pairs photogeneration centers. The main results were obtained from the analysis of the effects of external magnetic and electric fields influence on photoconductivity.
Dielectric relaxation in the lead–iron oxide glass system
Authors: Mandal S. , Ghosh A.
Published: July 2002

The dielectric properties of the PbO–Fe2O3 glassy system are reported in the frequency range 102–106 Hz and in the temperature range 300–500 K. The experimental data have been analyzed in the light of the dipolar relaxation model. It is observed that this model is consistent with the frequency and temperature dependence of the complex dielectric constant. The network former PbO influences strongly the dielectric properties, yielding large values of static and high-frequency dielectric constants and a narrow distribution of relaxation times.
Lattice vibrations of semirigid molecules: Low-frequency vibrations of 4,4-difluorobenzophenone and 4,4-dichlorobenzophenone
Authors: Kirin D. , Volovšek V.
Published: July 2002

The low-frequency Raman and far infrared spectra of 4,4-difluorobenzophenone and 4,4 - dichlorobenzophenone have been recorded in the high-temperature phase. The assignment of the lattice vibrations based on the semirigid-molecule model is presented. The strong influence of intermolecular forces on a number of molecular normal modes is predicted by the lattice dynamics calculations. Particularly, the torsional molecular modes, involving librations of phenyl rings, are sensitive to the external influences. The calculations also predict that several molecular modes are almost unaffected by the crystal field and an explanation of such behavior is proposed. Comparison with experimental and calculated results for other flexible molecules is discussed.