bluuebunny/crossref_metadata_2025 huggingface dataset:
Short-time charge motion in Hen+ clusters
Authors: Seong Jeonghee , Janda Kenneth C. , Halberstadt Nadine , Spiegelmann Fernand
Published: July 2002

The mechanism of short-time charge delocalization in Hen+ clusters, n=3, 14, 40, and 112, is studied using time-dependent wave packets for the electronic states for fixed nuclear positions chosen randomly from quantum Monte Carlo distributions for the neutral clusters. A DIM (diatomics-in-molecules) potential is used to calculate electronic eigenvalues and eigenvectors for the positively charged clusters, and a coherent superposition of states is created to initially localize the charge on a single atom. The charge distribution as a function of time is then calculated by propagating the initial wave function for 200 fs, during which little nuclear motion will occur. The charge motion is separated into three categories. For initial ionization of relatively isolated atoms in the cluster, the charge is only weakly coupled to other atoms, and there is little charge motion in 200 fs. For initial positions at which two or three atoms are coupled, but isolated from the rest of the cluster, the charge resonates among these atoms. For initial ionization positions at which more than three atoms are coupled, the charge can effectively delocalize over that part of the cluster. For the first two types of positions, which tend to occur near the surface of the cluster, He2+ is likely to form and the cluster will fragment before significant charge migration can occur. Charge delocalization tends to occur for initial ionization at the third type of location in the core of the cluster. For the initial positions that lead to charge delocalization, the distance and direction of the delocalization was characterized.
Interaction of cyclobutane with the Ru(001) surface: Low-temperature molecular adsorption and dissociative chemisorption at elevated surface temperature
Authors: Hagedorn Christopher J. , Weiss Michael J. , Chung C.-H. , Mikesell Peter J. , Little R. Daniel , Weinberg W. Henry
Published: July 2002

We have studied the interaction of cyclobutane with the hexagonally close-packed Ru(001) surface. High-resolution electron energy loss spectroscopy (HREELS) has been used to identify the vibrational modes of both c-C4H8 and c-C4D8 adsorbed at 90 K as a function of cyclobutane exposure. We have observed a vibrational mode not observed in the gas phase at 2600 cm−1 (2140 cm−1) which is attributed to the strong interaction of the cyclobutane C–H (C–D) bonds with the ruthenium surface. Two different adsorption geometries for cyclobutane on Ru(001) have been proposed based on the dipolar activity of this softened C–H mode. We have also measured the trapping-mediated dissociative chemisorption of both c-C4H8 and c-C4D8 at surface temperatures between 190 and 1200 K. The measured activation energies with respect to the bottom of the physically adsorbed well for c-C4H8 and c-C4D8 are 10 090±180 and 10 180±190 cal/mol, respectively. The trapping-mediated chemisorption of cyclobutane is believed to occur via C–C bond cleavage, as judged by the absence of a kinetic isotope effect. The measured ratios of the preexponential factors for desorption relative to reaction of 21±2 and 47±4 for c-C4H8 and c-C4D8 respectively, are in the expected range considering the greater entropy gain associated with the transition state for desorption relative to the transition state for C–C bond cleavage.
Calculation of nuclear magnetic shieldings. XIII. Gauge-origin independent relativistic effects
Authors: Baba T. , Fukui H.
Published: July 2002

The gauge-origin independent expression for the relativistic nuclear magnetic shieldings was derived from the Douglas-Kroll transformation of the no-pair equation and the use of the gauge including atomic orbitals (GIAOs) proposed by London. Using our expression the relativistic spin free effect on the nuclear magnetic shieldings was evaluated for the four hydrogen halide molecules, HF, HCl, HBr, and HI, at the coupled Hartree-Fock (CHF) level with uncontracted Cartesian Gaussian-type basis sets. It was found that the GIAO-CHF results are very similar to the shielding values calculated with the fixed gauge origins at the halogen nuclei. The calculated results showed that the spin independent relativistic effect produces high-field shifts at both the halogen nuclei and protons in the hydrogen halides. However, the computed spin free effect was too small to interpret the very large upshield proton shifts observed in HBr and HI molecules.
Jahn–Teller effects in the coronene anions and cations
Authors: Kato Takashi , Yoshizawa Kazunari , Yamabe Tokio
Published: July 2002

The vibronic (vibrational–electronic) interactions and Jahn–Teller distortions in the mono- and trianions and the mono- and trications of coronene are discussed. E2g modes of vibration remove the orbital degeneracies in the highly symmetric D6h molecule to lead to D2h distortions. We calculate and analyze the vibrational modes and the linear vibronic coupling constants of the anions and cations of coronene as well as its deutero-form using the (Becke and Lee, Yang, and Parr) density-functional method. Although there are twelve E2g modes of vibration in coronene, some of them appear to play a significant role in the Jahn–Teller distortions. The lowest frequency mode of 370 cm−1, the C–C stretching mode of 1668 cm−1, and the C–H stretching modes of 3177 and 3193 cm−1 give large coupling constants in the anions. In particular, the 1668 cm−1 mode plays the most important role in the Jahn–Teller distortions both in the anions and cations. There is electron–hole symmetry in the coupling constants in the anions and cations. We also demonstrate an important H/D isotope effect. The highest two C–H stretching modes are important for the Jahn–Teller distortions in coronene; however, the corresponding C–D stretching modes are less important for the Jahn–Teller distortions in deutero-coronene.
Molecular diffusion on solid surfaces: A lattice-model study
Authors: Raut Janhavi S. , Fichthorn Kristen A.
Published: July 2002

Molecular diffusion on surfaces does not adhere to the basic assumptions of the adsorbate hopping model. Large molecules such as n-alkanes can bind at more than one site on surfaces. Their diffusion involves multiple hops to various nearest and non-nearest neighbor sites. In a recent study [J. S. Raut and K. A. Fichthorn, J. Chem. Phys. 108, 1626 (1998)], we proposed a simple heterogeneous lattice model to describe the behavior of these molecules on surfaces. In this work, we have carried out kinetic Monte Carlo simulations to verify the model and study the tracer and chemical diffusion of these molecules at different coverages and temperatures. Interestingly the tracer diffusion of a single molecule can be described by a solution of the lattice model obtained using the simplifying assumption of uncorrelated hopping out of different sites. The coverage dependence of tracer diffusion can also be described by a simple lattice model. We compare results from the kinetic Monte Carlo simulations to molecular-dynamics simulations and demonstrate that a lattice-based hopping model does account for all the relevant features of short chain diffusion on surfaces. The chemical-diffusion coefficient increases with increasing coverage, due to a reduction in configurational entropy.
A hierarchical model for surface effects on chain conformation and rheology of polymer solutions. I. General formulation
Authors: Mavrantzas Vlasis G. , Beris Antony N.
Published: July 2002

The flow behavior of polymer solutions near a solid surface (either neutral or adsorbing) is modeled through a new, hierarchical (macroscopic and microscopic) approach which enables the thermodynamically consistent extension of equilibrium (static) considerations to nonequilibrium (flow) conditions. The approach involves two steps: First, the set of primary, independent, variables defining the state of the system at the macroscopic level is chosen, and a complete set of transport and constitutive equations is constructed for them through a two fluid, Hamiltonian model. In the present work, the macroscopic variables include the polymer chain concentration, the macroscopic fluid velocity, and the conformation tensor (defined as the tensor of the second moment of the chain end-to-end vector). The governing equations involve the (extended) free energy or Hamiltonian of the system, H, and are valid both in the bulk of the fluid and in the interfacial region. Thus, to solve them one needs to specify H. This is done in a second step, by invoking a microscopic model, which consistently takes into account the simultaneous effect on chain conformations of both the solid boundary and the imposed flow field. Solid boundary effects are taken into account in the solution of a diffusion equation for the chain propagator G(r,n;r0) which represents the weighted probability that an n-segment long chain which starts at r0 will end at position r. Flow field effects are taken into account through the definition of a generalized propagator G′(r,n;r0,α), which further depends on the apparent strain tensor α, representing chain deformation effects due to flow. The present part of the paper describes the general formulation of the approach and its relevance with previous works. Results from applying the methodology to the case of a polymer solution flowing past a purely repulsive surface (a wall) are presented in the second part of this work.
The Hartree approximation in dynamics of polymeric manifolds in the melt
Authors: Rostiashvili V. G. , Rehkopf M. , Vilgis T. A.
Published: July 2002

The Martin–Siggia–Rose functional integral technique is applied to the dynamics of a D-dimensional manifold in a melt of similar manifolds. The integration over the collective variables of the melt can be simply implemented in the framework of the dynamical random phase approximation. The resulting effective action functional of the test manifold is treated by making use of the self-consistent Hartree approximation. As an outcome the generalized Rouse equation of the test manifold is derived and its static and dynamic properties are studied. It was found that the static upper critical dimension, duc=2D/(2−D), discriminates between Gaussian (or screened) and non-Gaussian regimes, whereas its dynamical counterpart, d̃uc=2duc, distinguishes between the simple Rouse and the renormalized Rouse behavior. We have argued that the Rouse mode correlation function has a stretched exponential form. The subdiffusional exponents for this regime are calculated explicitly. The special case of linear chains, D=1, shows good agreement with Monte-Carlo simulations.
The NO2 vibronic levels near the X 2A1–A 2B2 conical intersection: Jet cooled laser induced fluorescence between 11 680 and 13 900 cm−1
Authors: Delon Antoine , Jost Remy
Published: July 2002

Following our previous LIF [J. Chem. Phys. 95, 5701, (1991) and 103, 1732 (1995)] and ICLAS [Chem. Phys. 190, 207 (1995)] studies on NO2, we observed by LIF the NO2 vibronic levels between 11 680 and 13 900 cm−1. These observations allow us to characterize the vibronic levels resulting from the conical intersection between the X 2A1 and A 2B2 electronic PESs. Globally, we observed by LIF 78 vibronic levels of B2 vibronic symmetry, among which 61 had previously been observed by ICLAS. Conversely, one vibronic level observed at 13 088 cm−1 by ICLAS has not been observed by LIF. The 17 new levels have been observed thanks to a better sensitivity (about a factor 10) and resolution (0.3 GHz). The LIF intensities range over four orders of magnitude. The 79 (78+1) observed vibronic levels represent 83% of the 95 levels of B2 vibronic symmetry calculated in that range; 85 X 2A1 levels of b2 vibrational symmetry and 10 A 2B2 levels of a1 vibrational symmetry. The missing levels are expected to have a weak A 2B2 electronic character and then a very weak intensity. Consequently, these missing levels are expected not to be significantly shifted by X 2A1–A 2B2 vibronic interaction. In addition, 85 hot bands have been observed by LIF in the same range (among which only 15 have been observed by ICLAS). For most of the vibronic levels, the rotational constants (A,B) and spin splittings have been determined from their few lowest rotational levels. The zero order vibronic levels can be classified according to a polyad number, N=N(bend)+2N(stretch). The present results cover the polyads four to seven. This experimental data, combined with laser induced dispersed fluorescence spectra (LIDFS), [J. Chem. Phys. 108, 6638 (1998)] which give access to polyads one to five, contributes to a quantitative interpretation of the strong interaction between the X 2A1 and A 2B2 PESs. The integrated density of states of vibronic levels of B2 symmetry is carefully discussed up to 19 360 cm−1 and the next neighbor spacing distribution (NNDS) is presented for a (almost?) complete set of 65 levels located between 12 500 and 13 860 cm−1. These data contributes to the characterization of the transition from regularity, below 10 000 cm−1, to quantum chaos, which is fully developed above ∼16 500 cm−1 within the NO2 vibronic degrees of freedom.
Saturation and lifetime effects on degenerate four-wave mixing spectroscopy
Authors: Delve Paul A. , Whitaker Benjamin J.
Published: July 2002

The effect of rotational predissociation on the degenerate four-wave mixing (D4WM) spectrum of the A(2A″)←X(2A′) band of HCO is investigated. We find that D4WM signal intensities depend on the excited state lifetime to the fourth power in the limit that the rotational predissociation rate is much faster than the collision depopulation rate. The effect can significantly reduce the apparent rotational line strength even when the predissociation is not readily observable through its influence on the line profile as a result of the concomitant homogeneous line broadening. The effects of rotational predissociation are generally more pronounced for the higher rotational states, and, consequently, failure to take account of its influence on the D4WM rotational line strength can lead to a significant distortion of the band contour. Since D4WM has been widely applied as a temperature probe, in, for example, combustion, the conclusion is that this effect must be taken into account when interpreting D4WM spectra from species which exhibit rotational predissociation.
Nonlinear chemical reactions in dispersed media: The effect of slow mass exchange on the steady-state of the Schlögl models
Authors: Vanag V. K. , Nicolis G.
Published: July 2002

The Schlögl-1 and bistable Schlögl-2 models in a spatially extended system are studied with the aid of a probabilistic cellular automaton (PCA) emulating chemical dynamics in a dispersed medium. Among the unusual effects observed are the significant deviation of the steady-state behavior from the deterministic value for Schlögl-1 model and two new bifurcations for Schlögl-2 model found under the conditions of intensive stirring and low rate constant kex of mass exchange between the adjacent cells of the PCA. One of these bifurcations results from the suppression of bi- and trimolecular reactions under the condition that the average number of particles 〈nX〉 in a cell is smaller than 1 and mass exchange rate is slow. The other bifurcation results from the competition between the mass exchange rate and the rate of autocatalysis in separate cells.
A new nucleation method by electron cyclotron resonance enhanced microwave plasma chemical vapor deposition for deposition of (001)-oriented diamond films
Authors: Zhang W. J. , Sun C. , Bello I. , Lee C. S. , Lee S. T.
Published: July 2002

A new nucleation method, which is different from bias-enhanced nucleation, was employed for the preparation of (001)-oriented diamond films on untreated, mirror-polished silicon substrates. The nucleation was realized in an electron cyclotron resonance enhanced microwave plasma at a pressure of about 10−3 Torr which was 4 orders of magnitude lower than that normally used for bias-enhanced nucleation (∼tens Torr). Scanning electron microscopy and Raman spectroscopy were used to investigate the surface morphology and phase purity of the deposited diamond films. The new findings may provide us a route to further understand the nucleation mechanism of diamond films by chemical vapor deposition.
Nanosecond transient grating studies of jet-cooled NO2
Authors: Tang Y. , Schmidt J. P. , Reid S. A.
Published: July 2002

We report the application of nanosecond transient grating spectroscopy to probe the excited state level structure of NO2 near 487.9 nm. Intensity and polarization grating transients were obtained in one-color (DFWM) and two-color (TC-RFWM) configurations. Most grating transients exhibit pronounced beats, arising from coherences among excited state hyperfine levels, which are enhanced in polarization grating experiments. Analysis of these beats yields detailed information on the excited state hyperfine level structure at a resolution 100 times smaller than the bandwidth of the applied laser fields. The grating transients are compared to predictions of the grating decomposition method (GDM) using Fourier transform analysis.
Constant density activation energies and the role of activated dynamics in liquids above their melting points
Authors: Ferrer Maria Luisa , Kivelson Daniel
Published: July 2002

For a liquid above its melting point (Tm), the activation energy determined at constant density, E∞(ρ), is associated with “true” activated dynamics, while the ratio E∞(ρ)/E∞(P), where E∞(P) is the activation energy determined at constant pressure, is a measure of the relative importance of activation dynamics. If E∞(ρ)≫Tm, the liquid is called “strong,” and for strong liquids it appears that E∞(ρ)/E∞(P)>0.5; these inequalities taken together indicate relaxations dominated by activated dynamics. If E∞(ρ)≈Tm, the liquid is called “weak,” and for weak liquids E∞(ρ)/E∞(P)<0.5; these inequalities taken together indicate relaxations dominated by nonactivated processes. Although E∞(ρ) can be big or small, E∞(P) seems to be appreciably larger than Tm. Many glass-formers are quite “strong,” even some such as orthoterphenyl, which is normally classified as “fragile,” and, consequently, the relaxations of these are likely to be dominated by activated dynamics over the entire temperature range from well above melting down to the glass transition; the relaxation of others, such as toluene, seem not to be activated at temperatures above melting.
Potential energy surface and vibrational eigenstates of the H2–CN(X 2Σ+) van der Waals complex
Authors: Kaledin Alexey L. , Heaven Michael C. , Bowman Joel M.
Published: July 2002

A four-dimensional potential energy surface has been calculated for the long-range interaction between rigid CN(X) and rigid H2(X). Electronic structure calculations were performed using single-reference wave function with singles and doubles configuration interaction. Davidson and counterpoise corrections were applied. The atomic orbital basis set was of avtz quality with f-type basis functions removed. The interaction energy at a grid of 865 points was fit by a standard expression in terms of the two in-plane angles, the dihedral angle, and the distance between the diatoms’ centers of mass. This fit facilitated examination of the surface and calculation of vibrational eigenstates. The potential exhibits two distinct minima: linear H–H…N–C and T-shaped H2…C–N, 100 and 68 cm−1 below the CN+H2 dissociation asymptote, respectively. Bound states for CN–H2 and CN–D2 were calculated for zero total angular momentum (J=0, ignoring spin). Approximate calculations for levels with |K|>0 (where K is the body-fixed projection of J) are also reported. States correlating with j=1 H2/D2 were found to be more deeply bound than those that correlate with j=0 H2/D2. The binding energies of CN–orthoH2 and CN–paraD2 are predicted to be 26 and 34 cm−1, respectively. These values are in reasonable agreement with a recent spectroscopic determination from this laboratory. Nuclear wave functions show that the average geometry changes dramatically on excitation of the intermolecular vibrations.
High resolution infrared spectroscopy of single SF6 molecules in helium droplets. I. Size effects in He4 droplets
Authors: Hartmann Matthias , Pörtner Nikolas , Sartakov Boris , Toennies J. Peter , Vilesov Andrei F.
Published: July 2002

Single SF6 molecules have been embedded in large He4 droplets (N̄4≈103–104 atoms) in a molecular beam and studied via infrared laser depletion spectroscopy. The rotational fine structure of the ν3 transition is analyzed with an effective third order gas phase Hamiltonian of SF6 yielding seven spectroscopic parameters as a function of the measured mean droplet size. From the intensities of the rotational lines the rotational temperature in droplets with N̄4>103 atoms is determined to be Trot=0.38(1) K consistent with theoretical estimates. Quantitative information on the efficiency of evaporative cooling of the droplets could be obtained by increasing the droplet temperature up to Trot=0.55 K by many successive inelastic collisions with He4 atoms from the background gas. For small droplets the absorption maximum shows an increasing redshift with respect to the gas phase and only small downward shift with droplet sizes N̄4>2×103. This could not be explained quantitatively with the excluded volume model assuming a liquid drop nor by the expected decrease in the internal pressure with increasing droplet radius. The much smaller decrease in the redshift observed is shown to provide evidence that the nearest neighbor shell of He4 next to the molecule is almost incompressible providing direct experimental evidence for the existence of a dense snowball-like shell structure of He4 atoms around the molecule as predicted theoretically.
The Gaussian-2 method with proper dissociation, improved accuracy, and less cost
Authors: Fast Patton L. , Sánchez Marı́a Luz , Corchado José C. , Truhlar Donald G.
Published: July 2002

The Gaussian-2 method (G2) is modified by deleting the empirical high-level correction and instead using empirical coefficients to extrapolate to full configuration interaction and an infinite basis set. The resulting method, called multicoefficient Gaussian-2 (MCG2) is less expensive than G2 but a factor of 1.7 more accurate for molecules composed of H and first-row atoms.
Orientation of pyrimidine in the gas phase using a strong electric field: Spectroscopy and relaxation dynamics
Authors: Franks Karen J. , Li Hongzhi , Kong Wei
Published: July 2002

Orientation of pyrimidine in a strong electric field was measured using resonantly enhanced multiphoton ionization (REMPI) and laser induced fluorescence (LIF). The ion and fluorescence yields showed preference for perpendicular excitation relative to the orientation field, implying a perpendicular relationship between the permanent dipole and the transition dipole. Calculation results using a linear variation method reproduced the observed spectral features, overall transition intensity, and polarization preference of the excitation laser. The permanent dipole of the S1 state of pyrimidine was thereby determined to be +0.6 Debye. Measurements of polarization preferences in photoexcitation, i.e., linear dichroism, provide a direct approach for determination of transition dipole moments. A general theory of deriving directions of transition dipoles relative to permanent dipoles based on this type of measurement/calculation was also developed. In addition, we report observations of complex relaxation dynamics of pyrimidine in an electric field. At 50 kV/cm, the overall fluorescence yield was quenched to a quarter of its value under field free conditions. The spectral intensity distribution exhibited dependence on the delay time of the ionization laser in the REMPI experiment. Qualitative assignments of the REMPI spectra revealed that the loss of signal strength with delay time was primarily from levels containing high rotational angular momenta. Elimination of contributions from levels with M′⩾3 in the calculation was sufficient to reproduce experimental spectra recorded with a delay time of 200 ns. These observations and interpretations agree with previous reports on photophysical properties of pyrimidine, including relaxation and quenching in a magnetic field.
Determination of potential parameters for alkanes
Authors: Lopez Rodriguez A. , Vega C. , Freire J. J.
Published: July 2002

Potential parameters for n-alkanes can be obtained either from second virial coefficient calculations or from simulations of the vapor–liquid equilibria. In this note, it is shown that although each method yields different parameters, differences are moderate. These differences are smaller when the second virial coefficient is fitted at high temperatures.
Photodissociation and recombination of solvated I2−: What causes the transient absorption peak?
Authors: Delaney N. , Faeder J. , Parson R.
Published: July 2002

Using nonadiabatic molecular dynamics simulations, we present evidence that the 2 ps peak in the pump–probe spectrum of I2− dissociated inside CO2 clusters is due to transitions from the ground state to the spin–orbit excited states, rather than to excited-state absorption as previously assigned.
Assessment of S(0,φ) from multiply scattered light
Authors: Banerjee Sukanta , Shinde Rajesh , Sevick-Muraca Eva M.
Published: July 2002

Two-wavelength measurements of isotropic scattering coefficient in concentrated, multiply scattering colloidal suspensions are used to extract S(0,φ) values, using a small wave number expansion of the interparticle structure factor, S(q,φ). Owing to the small particle size (150 nm) and near-infrared wavelengths (670–820 nm) used in this study, the approximation is reasonable and allows accurate estimation of S(0,φ). The estimated values of S(0,φ) agree well with that predicted using the Carnahan–Starling equation of state.
Dynamics on statistical samples of potential energy surfaces
Authors: Ball Keith D. , Berry R. Stephen
Published: July 2002

Prior work [K. D. Ball and R. S. Berry, J. Chem. Phys. 109, 8541 (1998); 109, 8557 (1998)] has demonstrated that master equations constructed from a complete set of minima and transition states can capture the essential features of the relaxation dynamics of small systems. The current study extends this work by examining robustness of master equations based only on statistical samples of the surface topography, to make it possible to work with larger systems for which a full topographical description is either impossible or infeasible. We ask whether such “statistical” master equations can predict relaxation on the entire potential energy surface. Our test cases are Ar11 and Ar13, for which we have extensive databases: 168 geometrically distinct minima and 1890 transition states for Ar11, and 1478 minima and 17,357 saddles for Ar13 which we assume represent complete set of stationary points. From these databases we construct statistical sample sets of transition sequences, and compare relaxation predictions based on these with those obtained from the master equations representing the full potential surfaces, and with results of molecular dynamics simulations. The slowest, rate-controlling relaxation timescale converges at moderate temperatures as the number of sequences in a sample reaches ∼1000, approaching convergence for as few as 100 sequences. The asymptotic value of the slowest nonzero relaxation rate is essentially identical to that from the full potential energy surface. Equilibrium properties from the statistical samples match those of the full surface. To achieve convergence within a factor of 2 of full-surface rates, the number of sequences required is approximately the same for Ar13 as for Ar11. Precise convergence, however, appears to scale as the number of stationary points. These results reveal how the reliability and precision of kinetic predictions from statistical master equations depends on the size of the statistical database.
Surface properties of diluted aqueous solutions of 1,2-pentanediol
Authors: Glinski Jacek , Chavepeyer Guy , Platten Jean-Karl
Published: July 2002

The surface tensions σ of aqueous solutions of 1,2-pentanediol were measured in a temperature range between 288 and 308 K using the Wilhelmy plate method. The surface entropies, enthalpies, densities, and compositions were calculated. The results were interpreted in terms of specific interactions of the hydrophobic parts of the molecules adsorbed at the interface. Increasing surface concentration of diol molecules causes their rearrangement at the surface, similar to that observed earlier for water+n-butanol system.
State-resolved collisional quenching of highly vibrationally excited pyridine by water: The role of strong electrostatic attraction in V→RT energy transfer
Authors: Elioff Michael S. , Fraelich Margaret , Sansom Rebecca L. , Mullin Amy S.
Published: July 2002

We report experiments that investigate the influence of long-range attractive forces on collisional energy loss from highly vibrationally excited molecules. State-resolved studies of energy transfer from highly vibrationally excited pyridine (μ=2.2 D) to water (μ=1.8 D) in a low-pressure environment at 298 K have been performed using high-resolution transient absorption spectroscopy of water at λ≈2.7 μm. Pyridine in its ground electronic state with 37 900 cm−1 of vibrational energy was prepared by absorption of pulsed ultraviolet light (λ=266 nm) to the S1 state, followed by rapid internal conversion to the S0 state. Collisions between vibrationally excited pyridine and water that result in rotational and translational excitation of the ground vibrationless state of H2O (000) were investigated by monitoring the populations of individual rotational states of H2O (000) at short times following pyridine excitation. The infrared probe of water was the highly allowed asymmetric stretching (000→001) transition. The nascent distribution of rotationally excited H2O (000) states is well described by a thermal distribution with a rotational temperature of Trot=770±80 K. Doppler-broadened transient linewidth measurements yield the velocity distributions of the recoiling H2O (000) molecules that correspond to center-of-mass translational temperatures of Ttrans∼515 K for all water rotational states investigated. Additionally, rate constants for energy gain in individual water states were determined, yielding an integrated rate constant of k2int=1.1×10−11 cm3 mol−1 s−1 for the appearance of H2O (000) with Erot=1000–2000 cm−1. These results are compared with previous relaxation studies of excited pyrazine (μ=0 D) with water and of excited pyridine with CO2 (μ=0 D), and the influence of electrostatic attraction on the relaxation dynamics is discussed.
Diatomics-in-ionic-systems and ab initio predictions for the stationary points on potential energy surfaces of the (HF)n clusters (n=3–6)
Authors: Grigorenko Bella L. , Moskovsky Alexander A. , Nemukhin Alexander V.
Published: July 2002

Predictions of the diatomics-in-ionic-systems model for the variety of stationary points on the potential energy surfaces of the hydrogen fluoride clusters (HF)n (3⩽n⩽6) are compared to the results of ab initio MP2/6-311+G(2d,2p) calculations as well as to the results of the polarizable mechanics model of Hodges et al. [J. Phys. Chem. A 102, 2455 (1998)]. The diatomics-in-ionic-systems scheme which relies on the balanced treatment of neutral and ionic contributions to the electronic properties of polyatomic species within the diatomics-in-molecules theory takes into account here the mixing of the FH and F−H+ electronic states. The corresponding mixing coefficient serves as a single principal adjustable parameter of the model, finally selected by the reference value of the binding energy of (HF)3. It is shown that structures and energies of the main cyclic isomers are in a good agreement with the best estimates of Quack and Suhm [Conceptual Perspectives in Quantum Chemistry (Kluwer, Dordrecht, 1997)]. Every prediction of this model for the stationary points corresponding to 16 higher energy structures of (HF)n is confirmed by the MP2 ab initio data.
Glass transition of one-dimensional molecular chains of p-nitroaniline confined in AlPO4-5 nanopores revealed by dielectric spectroscopy
Authors: Wübbenhorst M. , Klap G. J. , Jansen J. C. , van Bekkum H. , van Turnhout J.
Published: July 2002

The dynamics of molecular chains of hydrogen-bonded p-nitroaniline (pNA), confined in the one-dimensional pore system of AlPO4-5, was studied by broadband dielectric spectroscopy (10−2–106 Hz). Two relaxation processes were observed. A fast process (the α process) shows above 72 °C a Vogel–Fulcher–Tammann behavior typical of glass-forming materials, indicating cooperative dynamics of the “condensed” pNA chains. Below the glass temperature the α-relaxation peak broadens significantly and obeys an Arrhenius law. Both observations imply a change in length scale of cooperativity ξ from a temperature-controlled to a chain-length-dependent one. The second relaxation process is tentatively assigned to structural relaxations of the AlPO4-5 host lattice itself, since it is also present in calcined (empty) crystals.
Percolation for a model of statistically inhomogeneous random media
Authors: Quintanilla J. , Torquato S.
Published: July 2002

We study clustering and percolation phenomena for a model of statistically inhomogeneous two-phase random media, including functionally graded materials. This model consists of inhomogeneous fully penetrable (Poisson distributed) disks and can be constructed for any specified variation of volume fraction. We quantify the transition zone in the model, defined by the frontier of the cluster of disks which are connected to the disk-covered portion of the model, by defining the coastline function and correlation functions for the coastline. We find that the behavior of these functions becomes largely independent of the specific choice of grade in volume fraction as the separation of length scales becomes large. We also show that the correlation function behaves in a manner similar to that of fractal Brownian motion. Finally, we study fractal characteristics of the frontier itself and compare to similar properties for two-dimensional percolation on a lattice. In particular, we show that the average location of the frontier appears to be related to the percolation threshold for homogeneous fully penetrable disks.
Spin echoes of nuclear magnetization diffusing in a constant magnetic field gradient and in a restricted geometry
Authors: Sen Pabitra N. , André Axel , Axelrod Scott
Published: July 2002

We study the influence of restriction on Carr–Purcell–Meiboom–Gill spin echoes response of magnetization of spins diffusing in a bounded region in the presence of a constant magnetic field gradient. Depending on three main length scales: LS pore size, LG dephasing length and LD diffusion length during half-echo time, three main regimes of decay have been identified: free, localization and motionally averaging regime. In localization regime, the decay exponent depends on a fractional power (2/3) of the gradient, denoting a strong breakdown of the second cumulant or the Gaussian phase approximation (GPA). In the other two regimes, the exponent depends on the gradient squared, and the GPA holds. We find that the transition from the localization to the motionally averaging regime happens when the magnetic field gradients approach special values, corresponding to branch points of the eigenvalues. Transition from one regime to another as a function of echo number for a certain range of parameters is discussed. In this transition region, the signal shows large oscillations with echo number. For large n, asymptotic behavior sets in as a function of n for the decay exponent per echo. This is true for all values of the parameters LS, LG, and LD.
Unraveling the energy dependence in large ΔE (V→RT) energy transfer: Separation of ΔE and probability in the collisional relaxation of highly vibrationally excited pyrazine (Evib=36 000 to 41 000 cm−1) by CO2
Authors: Wall Mark C. , Lemoff Andrew E. , Mullin Amy S.
Published: July 2002

The energy dependence of collisional relaxation of highly vibrationally excited pyrazine through collisions with CO2 has been investigated for pyrazine vibrational energies Evib=36 000–41 000 cm−1. Highly excited pyrazine was prepared by absorption of tunable pulsed UV laser light at four wavelengths between 251 and 271 nm. Data for pyrazine excitation at 246 and 266 nm have been published previously and are included with the data presented here. We have used high resolution transient absorption spectroscopy to measure nascent distributions of rotational and translational energy gain in CO2 molecules in their ground vibrationless (0000) state that are populated through collisions with excited pyrazine molecules. Our results reveal that substantial amounts of rotational and translational energy are imparted to CO2(0000) molecules for all pyrazine energies studied and that the magnitude of this energy changes very little for a 13% change in the pyrazine vibrational energy. In contrast, state-resolved rates for appearance of individual CO2(0000) rotational states show a strong dependence on the precise value of the donor energy and increase by nearly an order-of-magnitude for a 5000 cm−1 increase in pyrazine energy. The high energy part of P(E,E′) is obtained from our energy gain measurements for each pyrazine excitation energy and these distributions are compared with experimental measurements of 〈ΔE〉 and its energy dependence.
Nonlinear optical properties of push–pull stilbenes based on a strong carbocation acceptor moiety
Authors: Paci Barbara , Schmidt Claudia , Fiorini Céline , Nunzi Jean-Michel , Arbez-Gindre Cécile , Screttas Constantinos G.
Published: July 2002

Nonlinear optical properties of push–pull stilbenes based on an alkylamino donor and a strong carbocation acceptor moieties are analyzed in the present study. The highly electron-acceptor character of the carbocation group makes this kind of molecule promising material for second-order nonlinear optical applications. Photoinduced intramolecular charge-transfer is studied by second harmonic generation in solution using time-resolved nondegenerate six-wave mixing. The fully characterized nonlinear dye Disperse Red 1 is used as a reference. The second-order nonlinear properties of the carbocation molecules are compared to a more standard amino–nitro push–pull stilbene. Kerr ellipsometry measurements aiming at a better understanding of the nonlinear excited state spectroscopy of the molecules are also reported. They reveal a very short excited state lifetime of the molecules. Optimization of this lifetime should lead to an improvement of the six-wave mixing response of such compounds.
Spectroscopic and theoretical characterization of linear centrosymmetric N≡N⋅⋅H+⋅⋅N≡N
Authors: Verdes Dorinel , Linnartz Harold , Maier John P. , Botschwina Peter , Oswald Rainer , Rosmus Pavel , Knowles Peter J.
Published: July 2002

The first high resolution infrared spectrum of the ionic complex N2⋅⋅H+⋅⋅N2 and its deuterated derivative is reported. The spectra were obtained in direct absorption in a supersonic slit nozzle plasma. The observed rovibrational transitions were assigned to the ν3 antisymmetric NN stretching vibration and the spectrum is consistent with a linear centrosymmetric equilibrium structure. The band origin is found at 2352.2364(6) cm−1 and the ground state rotational constant is determined as B″=0.081 809(14) cm−1. The assignment is supported by ab initio calculations including electron correlation effects. The best estimate for the equilibrium structure is Re (NN)=1.095 Å and re (N⋅⋅H)=1.277 Å. The transition moment of the ν3 band of N2⋅⋅H+⋅⋅N2 is predicted to be 0.21 D, an order of magnitude larger than for the NN stretching vibration of HN2+. The equilibrium dissociation energy De for fragmentation into N2 and HN2+ is calculated to be ∼5900 cm−1.
Production of HCO from propenal photolyzed near 300 nm: Reaction mechanism and distribution of internal states of fragment HCO
Authors: Jen Shih-Hui , Chen I-Chia
Published: July 2002

The photodissociation dynamics of propenal in the near UV region is studied by detecting fluorescence of nascent fragment HCO in its transition B̃ 2A′−X̃ 2A′000. The yield is small and the wavelength range 300–287 nm of production is narrow. From the onset of yield of HCO versus the photolysis wavelength, the threshold of the formation of C2H3+HCO is determined to be 95.9±0.6 kcal/mol. At photolysis energy 34 032 cm−1 rotational states up to N=14 for K=0 and Ka=2 of HCO are populated. The fluorescence intensity is corrected for both the quantum yield of fluorescence and the effect of axis switching to yield the population of rotational states of HCO. The Ka=1 doublet states and the two spin states are about equally populated. The calculated distributions of HCO according to phase-space theory disagree with the experimental data indicating a nonstatistical distribution. Hence, radical products are expected to emanate from the triplet surface with a small exit barrier; this process competes with intersystem crossing back to the ground electronic state to generate molecular products C2H4+CO so causing a small yield of HCO and rapid depletion of excited propenal. This explains why the rate coefficient (>2×108 s−1) from the appearance curve of fragment HCO is greater than the calculated dissociation rate of triplet propenal.
A dynamically heterogeneous coupled dissipation scenario of glass transition
Authors: Huang Y. N. , Wang Y. N. , Riande E.
Published: July 2002

A dynamically heterogeneous coupled dissipation scenario of the glass transition is presented. The model predicts a splitting from a high-temperature single (αβ)-relaxation to low-temperature α- and β-processes at a certain temperature, TS. The model also predicts a glass transition and some anomalies dependent on the thermal history during the glass transition. Some general features of glass formers emerge from the predictions of the model proposed.
Dynamics of the O(3P)+HCl reaction on the 3A″ electronic state: A new ab initio potential energy surface, quasi-classical trajectory study, and comparison to experiment
Authors: Ramachandran B. , Schrader Ethan A. , Senekowitsch Jörg , Wyatt Robert E.
Published: July 2002

A new potential energy surface for the lowest 3A″ electronic state of the O(3P)+HCl system is presented. This surface is based on electronic energies calculated at the multireference configuration interaction level of theory with the Davidson correction (MR-CI+Q) using the Dunning cc-pVTZ one-electron basis sets. The ab initio energies thus obtained are scaled using the scaled external correlation (SEC) method of Brown and Truhlar. The SEC-scaled energies are fitted to a simple analytical expression to yield a potential energy surface which correlates the reactants O(3P)+HCl(1Σ+) to the products OH(2Π)+Cl(2P). The reaction barrier on this surface lies at an O–H–Cl angle of 131.4° at an energy of 9.78 kcal/mol above the asymptotic O+HCl minimum. This barrier is 1.3 kcal/mol higher than that on the potential energy surface obtained by Koizumi, Schatz, and Gordon (KSG) [J. Chem. Phys. 95, 6421 (1991)] and 1.1 kcal/mol lower than the S2 surface of Ramachandran, Senekowitsch, and Wyatt (RSW) [J. Mol. Struct. (Theochem) 454, 307 (1998)]. The dynamics of the reaction O(3P)+HCl(v=2; j=1,6,9)→OH(v′,j′)+Cl on this potential surface is studied using quasi-classical trajectory (QCT) propagation and the results are compared to the experimental observations of Zhang et al. [R. Zhang, W. J. van der Zande, M. J. Bronikowski, and R. N. Zare, J. Chem. Phys. 94, 2704 (1991)]. The broad distribution of collision energies in the experiment is modeled by computing weighted averages of the quantities of interest with the weighting factor at each collision energy determined by the collision energy distribution.
Molecular-dynamics study of rotational alignment of NO+ drifting in helium—velocity and angular momentum distribution functions
Authors: Baranowski R. , Thachuk M.
Published: July 2002

Collision-induced rotational alignment of NO+ ions drifting in a helium buffer gas is studied with molecular dynamics using the ab initio potential surface of S. K. Pogrebnya et al. [Int. J. Mass Spectrom. Ion Proc. 149/150, 207 (1995)], obtained via a coupled-cluster singles–doubles approximation. We examine average translational and rotational temperatures, velocity and angular momentum distributions, and the dependence of these quantities on the applied electric field. The distributions show that angular momentum is preferentially aligned perpendicular to the electric field vector. We investigate the mechanism of this alignment through a multipolar moment expansion, and propose and demonstrate the accuracy of a bi-Maxwellian analytic form for describing the angular momentum distribution.
Periodic orbit–Quantum mechanical investigation of the inversion mechanism of Ar3
Authors: Guantes Raul , Nezis Anastasios , Farantos Stavros C.
Published: July 2002

The inversion mechanism of a T-shaped Ar3 is studied both classically and quantum mechanically. Regular states, localized in the region of the transition state for the inversion of the axial argon atom are found and are assigned by the symmetric stretch stable periodic orbits which emanate from the saddle point of the potential. These states inhibit the inversion process. States which promote the inversion are mainly irregular, but a few of them are localized and they have their nodes perpendicularly arranged along periodic orbits which originate from saddle node bifurcations. The two types of periodic orbits, inhibiting and isomerizing, are used to produce distinctly different spectra and to extract the corresponding eigenfunctions by solving the time dependent Schrödinger equation using a variable order finite difference method [J. Chem. Phys. 111, 10827 (1999), preceding paper].
A quantum mechanical-Poisson–Boltzmann equation approach for studying charge flow between ions and a dielectric continuum
Authors: Gogonea Valentin , Merz Kenneth M.
Published: July 2002

This paper presents a theoretical model for the investigation of charge transfer between ions and a solvent treated as a dielectric continuum media. The method is a combination of a semiempirical effective Hamiltonian with a modified Poisson–Boltzmann equation which includes charge transfer in the form of a surface charge density positioned at the dielectric interface. The new Poisson–Boltzmann equation together with new boundary conditions results in a new set of equations for the electrostatic potential (or polarization charge densities). Charge transfer adds a new free energy component to the solvation free energy term, which accounts for all interactions between the transferred charge at the dielectric interface, the solute wave function and the solvent polarization charges. Practical calculations on a set of 19 anions and 17 cations demonstrate that charge exchange with a dielectric is present and it is in the range of 0.06–0.4 eu. Furthermore, the pattern of the magnitudes of charge transfer can be related to the acid–base properties of the ions in many cases, but exceptions are also found. Finally, we show that the method leads to an energy decomposition scheme of the total electrostatic energy, which can be used in mechanistic studies on protein and DNA interaction with water.