bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct inelastic scattering of N2 from Ag(111). I. Rotational populations and alignment
Authors: Sitz Greg O. , Kummel Andrew C. , Zare Richard N.
Published: July 2002

The rotational state populations and the quadrupole and hexadecapole alignment moments of N2 scattered off clean Ag(111) are determined by resonance enhanced multiphoton ionization (REMPI). The scattered N2 is found to be highly aligned with its rotational angular momentum vector J parallel to the surface. The degree of alignment is found to increase with increasing rotational excitation. We see less than perfect alignment at intermediate J values indicating that the surface is not completely flat. The alignment is relatively insensitive to incident energy, incident angle, or surface temperature Ts. However, the rotational state population distributions show pronounced rainbows for higher incident energy and/or more grazing exit angle. The rotational state distributions are found to depend strongly on the final scattering angle at low Ts; this effect is markedly reduced at higher Ts. Time-of-flight measurements are used to determine the average velocity of the scattered N2 as a function of rotational level. It is found that higher rotational excitation correlates with lower average velocity and that the incident molecules lose 20%–30% of their translational energy to the solid. No correlation is found between velocity and alignment. A comparison is made with published results for the NO/Ag(111) system and a variety of theoretical models found in the literature.
Direct inelastic scattering of N2 from Ag(111). II. Orientation
Authors: Sitz Greg O. , Kummel Andrew C. , Zare Richard N. , Tully John C.
Published: July 2002

The orientation of the angular momentum of N2 scattered from clean Ag(111) is determined by resonance-enhanced multiphoton ionization. The orientation is the net helicity or handedness of the sense of rotation, i.e., clockwise vs counterclockwise. The orientation of the scattered N2 is measured along a direction perpendicular to the scattering plane. The degree and sign of the orientation is found to depend strongly on the final rotational quantum number J and on the final scattering angle. The results require that there are forces acting in the plane of the surface during the scattering. The observed behavior can be reproduced qualitatively by a conventional hard-cube, hard-ellipsoid model to which a tangential friction has been introduced to account for the in-plane forces. This produces a splitting of the rotational rainbow peak which leads to changes of sign of the orientation as a function of rotational quantum number. Thus, orientation measurements provide a unique probe of in-plane gas–surface forces.
The B̃ 1E″ state of ammonia: Sub-Doppler spectroscopy at vacuum ultraviolet energies
Authors: Ashfold M. N. R. , Dixon R. N. , Little N. , Stickland R. J. , Western C. M.
Published: July 2002

Spectra of the 2n0 progression in the B̃ 1E″–X̃ 1A′1 transition in NH3 and ND3 have been recorded at an effective resolution down to 0.008 cm−1 at the VUV equivalent energy. Line shape analysis reveals a predissociation lifetime of 0.25(2) ns for ND3 and 6.1(7) ps for NH3 independent of rotational state and v2. Measurement of the magnetic sensitivity of low J lines gave an electronic g value. Rotational analyses of the 2n0 bands with n up to 6 for ND3 and 8 in NH3 are presented and indicate some small vibrational perturbations. The observed difference between the Coriolis coupling constant ζ derived from the rotational structure and the g value indicates a mild Jahn–Teller effect.
Thermodynamic stability of single-phase fluids and fluid mixtures under the influence of gravity
Authors: Debenedetti Pablo G.
Published: July 2002

The equilibrium state attained by a single-phase fluid or fluid mixture when under the influence of gravity and subject to given external constraints differs, as is well known, from its gravity-free counterpart. The thermodynamic stability of single-phase fluids and their mixtures, however, is shown to be uninfluenced by the presence of gravity. Single-phase fluids and their mixtures are thermodynamically stable when under the influence of gravity if, and only if, the gravity-free stability criteria are satisfied everywhere throughout the fluid volume. Gravitational fields, therefore, introduce no additional mechanism whereby spontaneous fluctuations can destabilize an equilibrium state.
Isotope effect and nature of bonding in the cluster ions H+3(Ar)n and D+3(Ar)n
Authors: Hiraoka Kenzo , Mori Toshiharu
Published: July 2002

Thermochemical data, ΔH○n−1,n and ΔS○n−1,n (n=1–7), of clustering reactions, H+3(Ar)n−1+Ar=H+3(Ar)n and D+3(Ar)n−1+Ar=D+3(Ar)n, were measured with a pulsed electron-beam high-pressure mass spectrometer. The shell formation with n=3 and 6 was observed for both H+3(Ar)n and D+3(Ar)n clusters. The binding energies of D+3(Ar)n are found to be about 0.2 kcal/mol greater than those of H+3(Ar)n with n=1–3. With n≥4, the binding energies for both clusters become about the same. The Ar ligands in the cluster D+3(Ar)n are found to have slightly more restricted freedoms of motion than those in H+3(Ar)n, probably due to the smaller size of the core ion D+3 than H+3. The binding energy of H+3---Ne was also measured. The obtained binding energy (∼0.4 kcal/mol) is more than one order of magnitude smaller than that of H+3---Ar (6.69 kcal/mol). This is mainly due to the much smaller polarizability of Ne than Ar. A careful remeasurement of thermochemical data for the clustering reaction H+3+H2=H+3(H2) was also made. The obtained −ΔH°(7.01 kcal/mol) and −ΔS°(18.1 cal/mol K) values are in excellent agreement with our previous measurement.
Atoms in molecules in external fields
Authors: Bader Richard F. W.
Published: July 2002

The theory of atoms in molecules is extended to the case where the molecule is in the presence of an electromagnetic field. This theory is based upon a generalization of quantum mechanics to an open system, as obtained through a corresponding extension of Schwinger’s principle of stationary action. The extension of this principle is possible only if the open system satisfies a particular boundary condition, one which is expressed as a constraint on the variation of the action integral. This is the condition that it be bounded by a surface of zero flux in the gradient vector field of the charge density, the definition of an atom in a molecule. It is shown that this boundary constraint again suffices to define an atom as a quantum subsystem when the molecule is in the presence of an electromagnetic field. The mechanics of an open system and its properties are determined by the fluxes in corresponding vector current densities through its surface. As in the fieldfree case, the obtainment of these currents from the variation of the action integral is a direct result of the variation of the atomic surface and of the imposition of the variational constraint on its boundary. The currents in this case consist of a paramagnetic and a diamagnetic contribution, currents whose presence are a necessary requirement for the description of the properties of a system in the presence of external fields. The variational statement of the Heisenberg equation of motion obtained from the principle of stationary action is used to derive the Ehrenfest force and virial theorems for an atom in a molecule in the presence of external electric and magnetic fields. In this case, there are forces acting on the interior of the atom which arise from the magnetic pressures acting on its surface. It is shown that the molecular electric polarizability and magnetic susceptibility, like other properties, are rigorously expressible as a sum of atomic contributions.
Microscopic theory of the dynamics of polymeric liquids: General formulation of a mode–mode-coupling approach
Authors: Schweizer Kenneth S.
Published: July 2002

A formally exact, nonlinear generalized Langevin equation (GLE) for a flexible probe polymer in a dense melt has been derived using molecular phase space kinetic theory and Mori–Zwanzig projection operator techniques. An approximate, linearized dynamic memory function is developed, and the resulting GLE is specialized to the problem of an overdamped liquid of uncrossable Rouse polymers. An analytically tractable, perturbative/short time evaluation of the projected force time correlation function matrix is proposed which accounts for uncorrelated intermolecular pair interaction effects in the polymer melt. The detailed predictions for transport coefficients and various time correlation functions are determined for linear chains, and compared with recent lattice Monte Carlo simulations. Significant slowing down of all dynamical processes relative to the Rouse behavior is found, but the molecular weight scaling is not correctly described. A nonperturbative approach based on a polymeric generalization of molecular-scale mode–mode coupling theory is formulated which does properly capture the strong caging and viscoelastic effects in dense melts.

The phenomenological concepts of topological entanglements, a static tube, and primitive path are not employed, and simplified assumptions about liquid structure and mode of motion are not introduced a priori. The microscopic theory is based on an explicit nonlinear coupling of the collective fluid density fluctuations with the segmental density fields of a probe polymer. Equilibrium structural information is naturally incorporated, and the associated renormalized intermolecular potential, or vertex, is found to be spatially long range due to chain connectivity and correlation hole effects. The projected dynamics describing the time evolution of the mode-coupling part of the memory function matrix is evaluated using the short time/pair interaction theory. The polymeric mode-coupling theory can be employed as a rigorous and unified framework for qualitatively and quantitatively studying transport coefficients, material response functions, crossover phenomena, collective density fluctuation dynamical effects, nonlinear molecular architectures (e.g., ring polymers), tracer diffusion, semidilute solutions and blends.
Slow spinodal decomposition in binary liquid mixtures of polymers. IV. Scaled structure factor for later stage unmixing
Authors: Takenaka Mikihito , Izumitani Tatsuo , Hasimoto Takeji
Published: July 2002

The later-stage spinodal decomposition (SD) of polymer mixtures at deep quenches were investigated by time-resolved light scattering. The scaled structure factor F(x)≡qm (t,T)3I(x) with x≡q/qm (t,T) was analyzed as a function of time t or reduced time τ during the SD at various temperatures T’s, where qm (t,T) is the magnitude of the scattering vector q at which the first-order peak of the scattering intensity profile I(q;t,T) appears. Spectacular features in F(x) were discovered; (i) F(x) at large x is not universal with t but rather increasing with t, implying that the local structure is not scaled with a single length parameter 1/qm (t,T) used for the scaling of the global structure at x≂1, (ii) F(x) shows the second- and the third-order peaks or shoulders at x=2 and 3, and (iii) F(x) exhibits a dynamical crossover from x−n (n≂6) to x−4 with t in the asymptotic behavior at x≳2, as well as a spatial crossover from x−n to x−4 with x at τ in the sufficiently late stage.
Identification of two distinct structural and dynamical domains in an amorphous water cluster
Authors: Buch V.
Published: July 2002

This investigation is focused on dynamics and structure of an amorphous water cluster (H2O)116. The cluster was obtained in a previous study by simulation of the condensation of amorphous ice. The time evolution of the cluster is studied on a ∼400 ps time scale, using the classical trajectory technique at different temperatures. Two distinct structural and dynamical domains are identified. At 36 K the cluster displays glass-like behavior. There is also a broad distribution of molecular states extending from 1- to 5-coordinated water molecules. Upon heating to 100 K the cluster undergoes a transition to a more compact structure. The molecular energy distribution narrows down and is dominated by 3- and 4-coordinated molecules. After the transition the cluster displays liquid-like characteristics. All these changes are not reflected by the normal frequency distribution of structures along the trajectory.
Vapor–liquid equilibria of the two-dimensional Lennard-Jones fluid(s)
Authors: Smit B. , Frenkel D.
Published: July 2002

Monte Carlo simulations in the Gibbs ensemble are presented for the two-dimensional Lennard-Jones fluids. We have considered the full Lennard-Jones potential and the truncated (at 2.5σ) and shifted potential. It is shown that the critical temperature depends largely on the details of the truncation of the potential. These differences are by no means small.
‘‘Sequence space soup’’ of proteins and copolymers
Authors: Chan Hue Sun , Dill Ken A.
Published: July 2002

To study the protein folding problem, we use exhaustive computer enumeration to explore ‘‘sequence space soup,’’ an imaginary solution containing the ‘‘native’’ conformations (i.e., of lowest free energy) under folding conditions, of every possible copolymer sequence. The model is of short self-avoiding chains of hydrophobic (H) and polar (P) monomers configured on the two-dimensional square lattice. By exhaustive enumeration, we identify all native structures for every possible sequence. We find that random sequences of H/P copolymers will bear striking resemblance to known proteins: Most sequences under folding conditions will be approximately as compact as known proteins, will have considerable amounts of secondary structure, and it is most probable that an arbitrary sequence will fold to a number of lowest free energy conformations that is of order one. In these respects, this simple model shows that proteinlike behavior should arise simply in copolymers in which one monomer type is highly solvent averse. It suggests that the structures and uniquenesses of native proteins are not consequences of having 20 different monomer types, or of unique properties of amino acid monomers with regard to special packing or interactions, and thus that simple copolymers might be designable to collapse to proteinlike structures and properties. A good strategy for designing a sequence to have a minimum possible number of native states is to strategically insert many P monomers. Thus known proteins may be marginally stable due to a balance: More H residues stabilize the desired native state, but more P residues prevent simultaneous stabilization of undesired native states.
Quasidiabatic states from a
b i
n
i
t
i
o calculations by block diagonalization of the electronic Hamiltonian: Use of frozen orbitals
Authors: Pacher T. , Köppel H. , Cederbaum L. S.
Published: July 2002

The construction of quasidiabatic states is of theoretical and practical relevance in treating systems that exhibit strong nonadiabatic interactions. In the present work the implementation of the recently proposed method of block diagonalization of the electronic Hamiltonian in CI-type calculations, both theoretically and in practical calculations is investigated in detail. It is shown that the use of shifted or frozen molecular orbitals in the calculation of the adiabatic potential energy surfaces provides the most appropriate N-electron wave functions for applying the block diagonalization procedure. Freezing means to conserve the composition of the molecular orbitals at some chosen geometry, i.e., to have their coefficients unchanged with respect to the atomic orbitals with varying nuclear geometry. The use of the frozen orbitals and block diagonalization of the resulting CI matrix yields a method that treats orbital coupling and configurational interaction efficiently at the same time, leading to quasidiabatic states. Due to the mathematical simplicity of the method and to the fact, that no derivative couplings are needed during the calculations, the additional computational cost is negligible. Application to the ethylene dication illustrates the successful performance of the proposed method.
A quantum mechanical study of recombinative desorption of atomic hydrogen on a metal surface
Authors: Jackson Bret , Persson Mats
Published: July 2002

The Eley–Rideal mechanism for the recombinative desorption of hydrogen is examined theoretically. A two-dimensional collinear model is used to describe the interaction between a gas phase atom and an atom bound to a metal surface, which combine to form a desorbing molecule. The model is solved exactly by a fully quantum pseudospectral technique. Reaction probabilities and final molecular vibrational distributions are computed as a function of the kinetic energy of the incident atom, for model Cu and W surfaces. Large amounts of vibrational excitation are found, due to the large exothermicity of the reaction. Isotope effects are examined by replacing the incoming and/or bound H atoms with D.
Kinetic oscillations in catalytic CO oxidation on a cylindrical Pt single crystal surface
Authors: Sander M. , Imbihl R. , Ertl G.
Published: July 2002

A Pt single crystal of cylindrical shape (axis II [001]) whose surface exhibits all orientations of the [001] zone has been used to study coupling effects between orientations which exhibit kinetic oscillations in catalytic CO oxidation. On the clean surface, one can distinguish between an orientational range in between (110) and (320) which exhibits a 1×2 reconstruction, a nonreconstructed surface range around (210) and a hex reconstructed surface range around (100). Structural transitions proceed continuously via atomic steps between (110) and (210), while the orientational range extending from (210) to (100) is faceted. With a rotatable Kelvin probe, the orientational dependence of the work function could be followed. The results revealed that the variation of the oxygen sticking coefficient sO2 displays a mirror-like behavior with respect to the work function variation of the clean surface such that the orientation with the lowest work function (210) exhibits the highest sO2. Kinetic oscillations were studied in the 10−5 and 10−4 Torr range. By means of two Kelvin probes, the Δφ oscillations could be followed simultaneously at two different orientations, while the integral behavior of the cylinder surface was monitored via the CO2 production rate. A strong broadening of the oscillatory region in parameter space, as compared to a flat surface, was detected for the oscillatory range around (100). In the vicinity of this orientation, spatial coupling is provided via reaction fronts which propagate from (210) toward (110). Gas-phase coupling is only observed with the (110) orientations. Prolonged oscillation experiments cause faceting resulting in an increase of catalytic activity around (100), while the orientations around (210) lose catalytic activity by faceting.
Systematic ionic screening theory of macroions
Authors: Coalson Rob D. , Duncan Anthony
Published: July 2002

Techniques of lattice field theory are utilized to compute the free energy of a system of fixed charged macroions surrounded by small (atomic size) mobile ions. The grand partition function for the simple ions is written down as a functional integral over a three-dimensional auxiliary field. This functional integral is discretized on a lattice, and then subjected to saddle point analysis. The lowest order or ‘‘mean field’’ result of the analysis isolates a field which satisfies the Poisson–Boltzmann equation, and from which the Helmholtz free energy can be extracted. The formalism also provides a minimum principle for the Poisson–Boltzmann field that can be realized numerically by elementary annealing techniques. Most importantly, the mean field approximation can be systematically corrected by evaluating fluctuations around the saddle point to successive orders in an appropriate interaction strength. It is shown by numerical tests on a two-macroion system that the hierarchy of corrections converges rapidly in experimentally interesting regions of parameter space.
Hindered diffusion of a single sphere very near a wall in a nonuniform force field
Authors: Frej Nasser A. , Prieve Dennis C.
Published: July 2002

Total internal reflection microscopy is used to monitor Brownian fluctuations in the elevation of a single 10 μm sphere, levitated 0.05 to 0.35 μm above a glass plate by double-layer repulsion in a dilute aqueous salt solution. An analysis reveals that the initial slope of the autocorrelation function of the scattering intensity yields the diffusion coefficient evaluated at the weighted-average elevation in which the product of the Boltzmann factor and the square of the scattering intensity for that elevation are used as the weighting factor. When the sphere is very close to the wall, its mobility is expected to be significantly reduced owing to hydrodynamic interactions with the wall. Diffusion coefficients as small as 1.5% of the Stokes–Einstein value in the bulk were measured. Using Brenner’s relation between mobility and elevation [Chem. Eng. Sci. 16, 242 (1961)], the average elevation was inferred from the ratio of the average diffusion coefficient to its bulk value. The result is compared to the elevation obtained from the brightness of the sphere. Good agreement is obtained between the elevations inferred from the two independent measurements.
Femtosecond laser studies of the cis-stilbene photoisomerization reactions
Authors: Sension Roseanne J. , Repinec Stephen T. , Szarka Arpad Z. , Hochstrasser Robin M.
Published: July 2002

Femtosecond laser studies have been performed on the photoisomerization reactions of cis-stilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment. These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface. Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted on the ∼150 fs time scale. The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules. Specific experiments presented in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for cis* showing motion along the reaction coordinate; the detection of the trans-stilbene product using transient fluorescence and transient absorption, confirming that the reaction generates hot product states and that the Franck–Condon modes are largely spectators in the reaction; the anisotropy (alignment) of trans product molecules illustrating the effect of friction coupling overall motion to the reaction coordinate; and a theoretical treatment of three-pulse anisotropy experiments.
Atomic force microscopy and surface-enhanced Raman spectroscopy. I. Ag island films and Ag film over polymer nanosphere surfaces supported on glass
Authors: Van Duyne R. P. , Hulteen J. C. , Treichel D. A.
Published: July 2002

The surface roughness and nanometer scale structure of Ag films used for surface-enhanced Raman scattering (SERS) are characterized using atomic force microscopy (AFM). Two important types of thin film based SERS-active surface have been examined in this study: (1) Ag island films (AgIF’s) on smooth, insulating substrates and (2) thick Ag films evaporated over both preroughened and smooth substrates. AFM is demonstrated to be capable of quantitatively defining the three-dimensional (3D) structure of these roughened surfaces. The effects of mass thickness, dm, and thermal annealing on the nanostructure of AgIF’s are studied in detail. Particle size histograms are calculated from the AFM images for both ‘‘as-deposited’’ and annealed IF’s with dm=1.8 and 3.5 nm. Quantitative measurements of the SERS enhancement factor (EF) are coupled with the AFM data and interpreted within the framework of the electromagnetic theory of SERS. AFM images for thick evaporated Ag films over a monolayer of polymer nanospheres (AgFON) shows the clear presence of ‘‘random substructure roughness’’ reducing their utility as controlled roughness surfaces. Similar roughness structures are observed for thick evaporated Ag films on smooth, insulating substrates. Nevertheless, AgFON surfaces are demonstrated to be among the most strongly enhancing thin film based surfaces ever studied with EF’s comparable to those found for electrochemically roughened surfaces. Applications of FON surfaces to ultrahigh sensitivity SERS, anti-Stokes detected SERS, and surface-enhanced hyper-Raman spectroscopy (SEHRS) are reported.
Molecular dynamics algorithms and hydrodynamic screening
Authors: Dünweg Burkhard
Published: July 2002

In this paper molecular dynamics simulations of a system of Brownian particles in an explicit bath of solvent particles are considered. Generalized algorithms (Langevin simulations), in which both the Brownian particles and the solvent particles are artificially coupled to a heat bath, are analyzed for their dynamical properties on long length scales. Although such a dynamic is clearly unphysical, its analysis is useful for two reasons: The Langevin algorithm is frequently applied in an ad hoc fashion, and the deviation of its dynamical properties from the physical Hamiltonian case can be made arbitrarily small by choosing a sufficiently weak coupling to the heat bath. By a direct application of the Mori–Zwanzig projection operator formalism it is shown that the violation of global momentum conservation results in an artificial screening of the hydrodynamic interactions, with a screening length proportional to the inverse square root of the friction constant of the algorithm. The result is formally similar to expressions given in phenomenological theories of hydrodynamic screening in semidilute polymer solutions.
Ring-down cavity absorption spectroscopy of the very weak HCN overtone bands with six, seven, and eight stretching quanta
Authors: Romanini D. , Lehmann K. K.
Published: July 2002

A nonstandard, high sensitivity, absorption detection technique has been applied to the investigation of the very weak fifth, sixth, and seventh overtones of HCN at 100 Torr and 296 K. The frequency range covered is from 17 500 to 23 000 cm−1. We report high resolution, absolute absorption spectra with a noise equivalent sensitivity as low as ∼2×10−9/cm (recently improved to 7×10−10/cm). Band origins, rotational constants, and band intensities are reported and compared with calculated values. The HCN overtone spectra in the present study are not affected by any kind of perturbation, despite the high excitation energy involved.
Molecular dynamics with Langevin equation using local harmonics and Chandrasekhar’s convolution
Authors: Askar Attila , Owens Robert G. , Rabitz Herschel A.
Published: July 2002

A numerical method for studying molecular systems subject to a random force field leading to a Gaussian velocity distribution and described by the Langevin equation is presented. Two basic elements constitute the formulation: local harmonic modes and Chandrasekhar’s formula for the distribution function for a convolution involving a random function. First, by linearizing the governing Langevin equations locally and employing an orthogonal change of coordinates, an explicit solution for the displacement and velocity is constructed. Second, Chandrasekhar’s formula is employed in deriving the probability distribution function of the displacements and the velocities coming from the random forces. The local mode analysis is essential for the use of the Chandrasekhar’s formula, since we need the formal solution as a convolution of the random forces and the local Green’s function. For an illustration of the method in a significant case representative of real problems, we study a one dimensional idealization of a long chain molecule possessing internal energy barriers and subjected to an applied tension. The results are compared with the predictions of a conventional approximate method where a finite number of random realizations are generated in each time step. This truncation constitutes an approximation to obtain the desired Gaussian probability distribution function for the velocities which is reached in the limit of an infinity of random realizations. The calculations show that the conventional approximations may be acceptable only for short times, small temperatures, and average values over very long times. In particular, these approximations fail to give accurate results for transient phenomena, show slow convergence with the increase in the number of random realizations, and predict large values for the variance even in the steady regime.

The new proposed method on the other hand, (i) incorporates the mathematically and conceptually correct limit for the distribution function, (ii) is quite stable with respect to increases in the value of the time increment as well as in terms of fluctuations characterized by the variance, (iii) leads to considerable savings in computer time over the approximate method, and (iv) has the proper description during the transient regime, which is usually the most interesting phase of dynamical processes.
Toward a molecular theory of vapor-phase nucleation. III. Thermodynamic properties of argon clusters from Monte Carlo simulations and a modified liquid drop theory
Authors: Weakliem Cheryl Li , Reiss Howard
Published: July 2002

We report on the properties of the physically consistent cluster used in a theory of homogeneous vapor-phase nucleation and defined by Ellerby, Weakliem, and Reiss (paper I of this series) and by Ellerby and Reiss (paper II of this series). Specifically, the free energy of formation of argon clusters and their radial density distributions are obtained from Monte Carlo simulations. Additionally a modified liquid drop theory is presented, and the results from this macroscopic theory are compared to the simulation results. Finally, we outline the development of a rate theory which utilizes the modified liquid drop theory.
A statistical mechanical approach to heterogeneous nucleation
Authors: Lazaridis Mihalis , Ford Ian J.
Published: July 2002

The aim of the present work is to construct a model of heterogeneous nucleation based on a statistical mechanical derivation as an alternative to the classical model based on continuum thermodynamics. The result is similar to the classical approach, but the relation to the underlying microscopic physics is clearer. In our approach, we make use of the capillarity approximation using a cluster potential energy that is independent of the position of the molecules in the cluster. The model has a qualitative agreement with the experimental results of Mahata and Alofs for the heterogeneous nucleation of water on different substrates.
Differential flow-induced chemical instability in a model enzyme mechanism
Authors: Aguda Baltazar D.
Published: July 2002

General expressions are derived for the critical wave numbers for two- and three-species reaction mechanisms when their homogeneous steady states are destabilized by a relative flow of at least one species. Analysis of a model of the oscillating peroxidase–oxidase reaction indicates that the reaction exhibits this kind of instability. Computer simulations of the reaction-flow-diffusion equations generate traveling concentration waves under appropriate conditions.
The partition functions and thermodynamic properties of small clusters of rare gas atoms
Authors: Elyutin Pavel V. , Baranov Vladimir I. , Belega Elena D. , Trubnikov Dmitrij N.
Published: July 2002

The partition functions Z(T) for the clusters Arn, Krn, and Xen (n=2, 3, and 4) were calculated with the smoothed density of energy levels ρ(E). The latter was determined in the semiclassical approximation by Monte Carlo integration over the phase space and corrected by the rotational asymptotic for the lowest levels and by the trajectory separation method for the bound states above the dissociation threshold. In the temperature range of 5<T<150 K that is of crucial interest for the cluster formation studies in the supersonic jets, the results have an estimated accuracy of about 5%. The structure of the phase space of tetramer (n=4) clusters and their conformational transition dynamics were studied. The possibility of a link between such transitions and clusters melting is discussed.
Ultraviolet photodissociation study of CH3SCH3 and CH3SSCH3
Authors: Lee Y. R. , Chiu C. L. , Lin S. M.
Published: July 2002

The photodissociation processes of CH3SSCH3 at 248 and 193 nm and CH3SCH3 at 193 nm have been studied by translational spectroscopy. When excited at 248 nm, CH3SSCH3 undergoes a simple S–S bond scission to produce two CH3S fragments with an average translational energy of 33 kcal/mol. The angular distribution of the product with respect to the polarized laser is measured and fitted with an anisotropy parameter β=1.2. It indicates that the dissociation is a fast, direct process. At 193 nm, there is only a simple dissociation channel for CH3SCH3, while CH3SSCH3 undergoes a predominant C–S bond scission with the S–S bond scission as a minor channel. No angular dependence for the primary products from both CH3SCH3 and CH3SSCH3 has been observed. The observation of S+2 time-of-flight spectra shows that a major fraction of CH3S2, which is internally excited when produced, undergoes spontaneous dissociation to form slow S2 and CH3 radicals. Results obtained for the dissociation processes of both molecules at 193 nm are rationalized and comparisons with previous investigations [J. Chem. Phys. 92, 6587 (1990); 95, 5014 (1991)] are also included.
Dynamics and relaxation of an intermediate size water cluster (H2O)108
Authors: Saito Shinji , Ohmine Iwao
Published: July 2002

The potential surface, melting, surface structure, and hydrogen bond network of an intermediate size water cluster (H2O)108 are investigated. The orientation relaxations of single molecule and of collective molecules are analyzed and compared with those of liquid water. The collective orientation relaxation (COR) (i.e., dielectric relaxation) of the water cluster is found to be much faster than that of liquid water due to different boundary conditions. In both liquid and cluster, the cross correlation between individual molecular dipoles plays an important role in static and dynamic quantities. COR of the cluster yields a so-called 1/f fluctuation in contrast to the well-known Debye relaxation in liquid water. In order to understand these differences of COR between the water cluster and liquid water, the wave vector dependence of the transverse and longitudinal components of COR is examined. A surface effect on hydrogen bond network and the correlation between structural change and coordination number are analyzed.
An ab initio molecular orbital study on adsorption at the MgO surface. II. Site dependence of hydrogen chemisorption on the (MgO)4,6,8 clusters
Authors: Sawabe Kyoichi , Koga Nobuaki , Morokuma Keiji , Iwasawa Yasuhiro
Published: July 2002

Using (MgO)4, (MgO)6, and (MgO)8 clusters as models of the MgO surface, the structures and energies of H2 chemisorption on various pair sites are studied with ab initio molecular orbital calculations. The effect of Madelung potential on the chemisorption energetics is also studied. The chemisorption energy is not very sensitive to the cluster size used in the calculation. It however depends strongly on the coordination numbers of Mg and O at the chemisorption site, and decreases in the following order: the valley site ≫Mg3c–O3c≫Mg4c–O3c≳Mg3c–O4c site. The valley and the Mg3c–O3c pair sites are strongly bonding, whereas the Mg4c–O3c and the Mg3c–O4c sites are only marginally bonding. All other higher coordination sites are nonbonding. The coordination number of the nearest neighbor atoms only secondarily affects the stability of hydrogen chemisorption. The contribution of electron correlation to the chemisorption energy is essentially determined only by the localized electrons near the chemisorption site.
Impulsive excitation of coherent vibrational motion ground surface dynamics induced by intense short pulses
Authors: Banin Uri , Bartana Allon , Ruhman Sanford , Kosloff Ronnie
Published: July 2002

A framework for understanding impulsively photoinduced vibrational coherent motion on the ground electronic surface is presented. In particular strong resonant excitation to a directly dissociative electronic surface is considered. Three distinct approaches are employed. A two surface Fourier wavepacket method explicitly including the field explores this process in isolated molecules. A coordinate dependent two-level system is employed to develop a novel analytical approximation to the photoinduced quantum dynamics. The negligible computational requirements make it a powerful interactive tool for reconstructing the impulsive photoexcitation stage. Its analytical nature provides closed form expressions for the photoinduced changes in the material. Finally the full simulation of the process including the solvent effects is carried out by a numerical propagation of the density operator. In all three techniques the excitation field is treated to all orders, allowing an analysis of current experiments using strong fields, resulting in substantial photoconversion. The emerging picture is that the impulsive excitation carves a coherent dynamical ‘‘hole’’ out of the ground surface density. A rigorous definition of the dynamical ‘‘hole’’ is constructed and used to define a measure of its coherence. In particular all photoinduced time dependence in the system can be directly related to the dynamical ‘‘hole.’’ All three techniques are used to simulate the pump probe experiment on the symmetric stretch mode of I3−, including electronic and vibrational dephasing.
Corrected electrostatic model for dipoles adsorbed on a metal surface
Authors: Maschhoff Brian L. , Cowin James P.
Published: July 2002

We present a dipole–dipole interaction model for polar molecules vertically adsorbed on a idealized metal surface in an approximate analytic form suitable for estimating the coverage dependence of the work function, binding energies, and thermal desorption activation energies. In contrast to previous treatments, we have included all contributions to the interaction energy within the dipole model, such as the internal polarization energy and the coverage dependence of the self-image interaction with the metal. We show that these can contribute significantly to the total interaction energy. We present formulae for both point and extended dipole cases.
Free energy of cavity formation in solvent: Computational, methodological, and physical aspects
Authors: Beutler Thomas C. , Béguelin Daniel R. , van Gunsteren Wilfred F.
Published: July 2002

With molecular dynamics simulation techniques, the free energy of cavity formation in water is investigated and determined at 300 K and 1 atm. Different methods are compared: Widom particle insertion, thermodynamic integration, and perturbation. It is shown that the most efficient way to get accurate results is to apply Widom particle insertion for cavities up to a thermal radius of 0.25 nm followed by a thermodynamic integration with sampling at fixed thermal radii. In order to converge the result to 2.5% at one thermal radius a simulation period in the order of 100 ps is needed. The convergence behavior for thermal radii slightly above the van der Waals radius of a water molecule (0.31 nm) is poor and thus indicates instabilities in the water shell surrounding the cavity and that the free energy is a nonsmoothly increasing function of the thermal radius.
Extended continuum configurational bias Monte Carlo methods for simulation of flexible molecules
Authors: Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

The continuum configurational bias (CCB) Monte Carlo method has been extended to perform elementary moves that involve the rearrangement of inner segments of flexible chains. When regrowing inner sites, the continuity with the rest of the chain is ensured by disregarding those configurations that would imply an unrealistic elongation of the bonds once the chain is reconstructed. The formalism presented here also allows the simulation of branched chains and crosslinked-network structures. The Monte Carlo elementary moves proposed in this work are used in conjunction with an alternative method of preferential sampling in which the segments to be rearranged are chosen from a preselected region of space. The performance and capabilities of the new moves are compared to those of standard CCB and crank-shaft algorithms for simulation of melts and solutions of hard-sphere chains at high densities. Our results indicate that the methods presented here provide a fast relaxation of the bond orientation and the end-to-end orientation autocorrelation functions. Our isobaric simulations for homopolymer chains of up to 51 sites and for concentrated solutions of chain molecules in the monomer are consistent with previously reported data obtained by approximate molecular dynamics methods and by conventional Monte Carlo methods. However, small disagreements with existing data are identified at high densities. These PV results are also compared to the predictions of two recent equations of state. This comparison shows the presence of some small but systematic deviations.
Line mixing and nonlinear density effects in the ν3 and 3ν3 infrared bands of CO2 perturbed by He up to 1000 bar
Authors: Ozanne L. , Nguyen-Van-Thanh  , Brodbeck C. , Bouanich J. P. , Hartmann J. M. , Boulet C.
Published: July 2002

We present high density experimental and theoretical results on CO2–He absorption in the ν3 and 3ν3 infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. Computations are based on an impact line-mixing approach in which the relaxation operator is modeled with the energy corrected sudden (ECS) approximation. Comparisons between experimental and calculated results demonstrate the accuracy of the ECS approach when applied to band wings and band centers at moderate densities. On the other hand, small but significant discrepancies appear at very high pressures. They are attributed to a number of reasons which include nonlinear density dependence due to the finite volume of the molecules, neglected contributions of vibration to the relaxation matrix, and incorrect modeling of interbranch mixing.
Millimeter-wave spectroscopy of vibrationally excited ground state alkaline-earth hydroxide radicals (X 2Σ+)
Authors: Fletcher D. A. , Anderson M. A. , Barclay W. L. , Ziurys L. M.
Published: July 2002

Pure rotational spectra of the alkaline-earth monohydroxides have been recorded for vibrationally excited states (0 1 0), (0 2 0), (0 3 0), and (1 0 0) of the ground electronic state (X 2Σ+) using millimeter-wave absorption spectroscopy. The radicals MgOH, CaOH, SrOH, and BaOH were studied. The data for CaOH, SrOH, and BaOH were analyzed with a linear 2Σ+ model, but with the addition of two terms to account for contamination of the v2=1 2Π and v2=2 2Δ vibronic levels with 2Π and 2Δ electronic states. The data for MgOH, however, did not fit well to this linear model and is additional evidence that this species is quasilinear.
Millimeter-wave rotational spectroscopy of MgOD and CaOD (X 2Σ+)
Authors: Nuccio B. P. , Apponi A. J. , Ziurys L. M.
Published: July 2002

Pure rotational spectra of CaOD and MgOD have been recorded in the range 200–390 GHz using millimeter/sub-mm direct absorption spectroscopy. Transitions arising from the (000), (010), (020), and (100) modes have been measured for the 2Σ+ ground electronic states of these free radicals. The data were analyzed successfully using a linear 2Σ+ model for CaOD; for MgOD, only the (000) and (010) states could be fit with this Hamiltonian. Moreover, the (010) data required the addition of a substantial pΠ term to account for contamination of excited 2Π electronic states. For both species, the α2 vibration–rotation term was found to be negative, in contrast to MgOH and CaOH, suggesting a less anharmonic contribution to the bending potential in CaOD and MgOD. These measurements also indicate a shorter O–H bond in MgOH than the other alkaline earth hydroxide radicals, which likely results because this species is quasilinear.
Resonant ion-dip infrared spectroscopy of benzene–H2O and benzene–HOD
Authors: Pribble R. Nathaniel , Garrett Aaron W. , Haber Kenneth , Zwier Timothy S.
Published: July 2002

Resonant ion-dip infrared spectra of C6H6–H2O and C6H6–HOD have been recorded in the OH stretch fundamental region. The spectra provide further evidence for the unique, large-amplitude motions present in these π hydrogen-bonded complexes. In C6H6–H2O, transitions out of the lowest ortho (Π) and para (Σ) ground state levels are observed. A transition at 3634 cm−1 is assigned as an unresolved pair of parallel transitions (Σ→Σ and Π→Π) involving the symmetric stretch fundamental (at 3657 cm−1 in free H2O). In the antisymmetric stretch region, transitions at 3713, 3748, and 3774 cm−1 are assigned as Π→Σ, Σ→Π, and Π→Δ transitions, respectively. The spacing of the transitions is consistent with nearly free internal rotation of H2O about benzene’s sixfold axis in both ground and vibrationally excited states. The intensities of combination bands depends critically on the mixing of some local mode character into the symmetric and antisymmetric stretches at asymmetric positions of H2O on benzene. Surprisingly, in C6H6–HOD, five transitions are observed in the OH stretch region, all arising from the ground state zero point level. Even more unusual, the higher-energy combination bands are many times stronger than the OH stretch fundamental. The local mode OH stretch has components both parallel and perpendicular to benzene’s sixfold axis, leading to strong parallel and perpendicular transitions in the spectrum. A two-dimensional model involving free internal rotation and torsion of HOD in its plane is used to account for the qualitative appearance of the spectrum. The form of the OH(v=0) and OH(v=1) torsional potentials which reproduce the qualitative features of the spectrum are slightly asymmetric, double-minimum potentials with large-amplitude excursions for HOD over nearly 180°.
Electron-energy-loss spectroscopy of 6,6′-dimethylfulvene: First detection of the triplet state
Authors: Swiderek P. , Michaud M. , Sanche L.
Published: July 2002

Low-energy electron-energy-loss spectra of 6,6′-dimethylfulvene deposited on a thin film of solid argon are measured at a temperature of 16 K. The spectra make it possible to locate the lowest triplet state with an onset of the transition band at 1.9 eV and a vertical transition energy of approximately 2.3 eV. This is the first observation of a triplet state in a compound with a pentafulvene π-electron system. Semiempirical calculations, as well as ab initio multiconfiguration self-consistent-field calculations for fulvene using the 4-31G basis set, lead to an assignment of the observed triplet state as 1 3B2. Both 1 3B2 and the lowest excited singlet state 1 1B2 mainly arise from the excitation from the highest occupied molecular orbital to the lowest unoccupied molecular orbital. A singlet-triplet splitting of 1.05 eV is derived for these states. This is about 2.5 times smaller than the corresponding value in an open-chain conjugated 6π-system represented by 1,3,5-trans-hexatriene. The difference expresses the smaller exchange interaction between the highest occupied and lowest unoccupied molecular orbital in fulvene and related compounds. Properties of the lowest triplet state of the parent compound fulvene are calculated by use of the multiconfiguration self-consistent-field method. According to these results the fulvene π-electron system is expected to be planar in this electronic state. The stabilization with respect to a structure twisted by 90° around the exocyclic carbon–carbon bond amounts to 0.62 eV.