bluuebunny/crossref_metadata_2025 huggingface dataset:
Collision-induced desorption of physisorbed CH4 from Ni(111): Experiments and simulations
Authors: Beckerle J. D. , Johnson A. D. , Ceyer S. T.
Published: July 2002

The desorption of CH4 physisorbed on Ni(111) is observed to be induced by collision with Ar atoms incident with energies less than 2 eV. The absolute cross section for collision-induced desorption of CH4 in the low coverage limit of an isolated CH4 molecule and from a saturated CH4 monolayer is measured as a function of the kinetic energy and incident angle of the Ar beam. The dominant mechanism for collision-induced desorption is determined to involve the direct collision of the incident Ar with the physisorbed CH4. Indirect, surface mediated desorption processes and multiple desorptions are found to be unimportant. Three-dimensional, classical molecular dynamics simulations based upon a hard sphere/hard cube model of the direct collision mechanism show that the complicated dependence of the desorption cross section at low CH4 coverage on the Ar energy and incident angle is the result of two competing dynamical effects: the increase in the geometrical collision cross section and the decrease in the Ar kinetic energy that can be transferred to CH4 motion normal to the surface as the Ar incident angle increases. Multiple Ar–CH4 collisions and mirror collisions are found to make relatively minor contributions to the cross section for collision induced desorption. Normal energy accommodation during the CH4-surface collision plays a significant role in determining the threshold energy for desorption. At high CH4 coverage, the obstruction of small impact parameter, head-on Ar–CH4 collisions by neighboring CH4 molecules at large angles of incidence is the origin of the difference in the cross section observed for low and high CH4 coverage.
Microscopic dynamics of flow in molecularly narrow pores
Authors: Bitsanis Ioannis , Somers Susan A. , Davis H. Ted , Tirrell Matthew
Published: July 2002

Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong density variations across the pore render the usual dependence of the local viscosity on local density inappropriate. At separations greater than four molecular diameters flow can be described by a simple redefinition of local viscosity. In narrower pores a dramatic increase of effective viscosities is observed and is due to the inability of fluid layers to undergo the gliding motion of planar flow. This effect is partially responsible for the strong viscosity increases observed experimentally in thin films that still maintain their fluidity.
Theoretical study of the dissociative recombination of NO+ with slow electrons
Authors: Sun Hosung , Nakamura Hiroki
Published: July 2002

The multichannel quantum defect theory (MQDT) has been applied to the dissociative recombination of NO+ by collisions with slow electrons. The Rydberg states (superexcited states of the second kind) are fully incorporated by using the quantum defect functions. Five valence-type dissociative superexcited states of the first kind (A′ 2Σ+, I 2Σ+, B 2Π, L 2Π, B′ 2Δ) are all considered as the dissociation channels. Since the recently obtained quite reliable information on the superexcited states is employed in the calculation, the present results are considered to be the most accurate ones at this moment. Energy dependence of the cross sections shows intriguing resonance structures and varies from state to state, reflecting the relative dominance of the three basic important quantities: quantum defect function, electronic coupling, and Franck–Condon factor. The results are compared with other calculations and the available experiments.
Dynamics on potential energy surfaces with a conical intersection: Adiabatic, intermediate, and diabatic behavior
Authors: Manthe U. , Köppel H.
Published: July 2002

The nuclear dynamics on potential energy surfaces which are strongly vibronically coupled through a conical intersection is investigated by exact (numerical) integration of the time-dependent Schrödinger equation. Results for realistic model systems including three nuclear degrees of freedom are presented: C2H+4, pyrazine and NO2. It was found previously for C2H+4 that the wave packet moves after an initial decay mainly on the lower adiabatic surface. This observation could be confirmed also for pyrazine and NO2. By varying the coupling strength λ also other types of behavior could be identified. A transition from adiabatic via intermediate to a diabatic nature of the system dynamics is found upon decreasing λ. Population of diabatic and adiabatic states in the long time limit are calculated by classical phase-space statistics. Analytic relations between the population of the diabatic states and the dynamical observables, coordinates and momenta, are derived. They allow for a deeper understanding of their long time limits.
Transitions induced in a double minimum system by interaction with a quantum mechanical heat bath
Authors: Meyer R. , Ernst R. R.
Published: July 2002

A quantum mechanical treatment of a double minimum system interacting with a heat bath is presented for the purpose of interpreting experimental data on transfer kinetics in condensed hydrogen-bonded systems. The model describes the transfer motion in one or two dimensions. The heat bath is represented by a set of harmonic oscillators and the interaction by a term linear in the system coordinates and in the bath coordinates. Extending an earlier random field approach, the present treatment consistently accounts for the quantum nature of the total system. With crystalline benzoic acid dimer used as an example, the master equation for the populations of the energy levels of the hydrogen transfer motion is derived. Transition probabilities consistent with the principle of detailed balance are obtained, based on a representation with explicit off-diagonal tunnel interactions for pairs of states localized on different sides of the barrier and with diagonal terms describing the rearrangement of the heat bath as a consequence of the tunneling motion. The activation of the double minimum transfer process with increasing temperature is related to the excitation of the local vibrations in the two potential wells.
Formulation of molecular integrals over Gaussian functions treatable by both the Laplace and Fourier transforms of spatial operators by using derivative of Fourier-kernel multiplied Gaussians
Authors: Honda Masaru , Sato Kikue , Obara Shigeru
Published: July 2002

General recurrence formulas for molecular integrals over Gaussian functions are derived by introducing the derivative of Fourier-kernel multiplied Gaussians (DFGs). The DFG allows us to formulate on the same ground molecular integrals over the Cartesian Gaussians, modified Hermite Gaussians, and the Gaussians multiplied by phase factors exp[ik⋅(r−R)] with spatial operators including any number of both the Laplace and Fourier transforms for one- and two-electron spatial operators. Thus the present formulation has a wider applicability than that given by Obara and Saika [J. Chem. Phys. 89, 1540(1988)], where the basis functions are the Cartesian Gaussians and the spatial operators are those in the Laplace transform with at most one kernel of the Fourier transform. Furthermore the present formulation inherits the characteristic features of the above one, such as being capable of dealing with (1) molecular integrals with both nonrelativistic and relativistic spatial operators, (2) any order of the derivative of these integrals with respect to the function centers, and (3) leading us to the efficient scheme of computing the integrals by virtue of the recursive nature of the formulation.
Internuclear distance and repulsive potential between noble-gas atoms: Combining rules based on repulsive force
Authors: Carlton Terry S. , Gittins Christopher M.
Published: July 2002

Hartree–Fock calculations show that the internuclear distance R or repulsive potential U for a mixed pair AB of noble-gas atoms He, Ne, or Ar is very nearly the average of the same property for AA and BB, provided values for AA, BB, and AB are at the same repulsive force. For RAB greater than half the equilibrium internuclear distance the maximum errors in these combining rules are 2% for RAB and 3% for UAB. The maximum error in the predicted function UAB(R) is 8%. Force-dependent dipole moment values for HeAr are predicted from moments for HeNe and NeAr with an accuracy of 6% or better. Consideration of force per unit area suggests why errors in RAB predictions are positive and are largest for HeAr. Nonoverlap of the interacting atoms is shown to be a nonessential feature of the distortion model on which the combining rules are based.
Chemical etching of vicinal Si(111): Dependence of the surface structure and the hydrogen termination on the pH of the etching solutions
Authors: Jakob P. , Chabal Y. J.
Published: July 2002

Infrared spectroscopy is used to study the etching process of stepped Si(111)9° surfaces as a function of the pH of the etching HF solutions. This process results in complete H termination of the silicon surface, including terraces, steps, and defects; the surface structure can therefore be well studied using infrared (IR) spectroscopy. Polarized IR absorption spectra of the Si–H stretching vibrations (i.e., in the region 2060–2150 cm−1) vary dramatically as the pH of the etching solutions increases from 2.0 to 7.8. In general, higher pH solutions yield sharper bands and more easily assigned spectra, making it possible to identify the step and terrace species and thus to infer the surface structure and step morphology (i.e., to investigate the etching process). The data are explained by a model involving different etching rates for each individual surface species: The highest rate of removal is for isolated adatom defects located on (111) planes and the lowest is for the ideally H-terminated (111) planes themselves. For proper conditions of pH and etching time, atomically straight steps are formed due to faster removal of kinks than etching of the straight step themselves. The influence of steric hindrance is invoked to account for such preferential etching.
Resonant two-photon ionization spectroscopy of jet-cooled Au3
Authors: Bishea Gregory A. , Morse Michael D.
Published: July 2002

A band system of jet-cooled Au3 has been located in the near infrared region of the spectrum using resonant two-photon ionization spectroscopy. The origin band is located at 13 354.15 cm−1 and the system extends more than 700 cm−1 further to the blue. The excited state displays a radiative lifetime of approximately 28 μs, corresponding to an absorption oscillator strength of f≊0.0003. Accordingly, it is thought that the transition corresponds to a spin-forbidden doublet (S=1/2) to quartet (S=3/2) transition, which is made allowed by spin–orbit contamination, presumably in the upper state. A progression in a totally symmetric stretching vibration (ω=179.7 cm−1 ) is obvious in the spectrum, along with a much weaker progression in another mode, which displays an interesting pattern of splittings. Although no assignment is absolutely unambiguous, various candidates are presented. The most likely of these assigns the system as an Ã 4E′←X̃ 2E′ transition in the D3h point group, with both the ground X̃ 2E′ and excited Ã 4E′ states undergoing Jahn–Teller distortion. The vibronic levels of the Ã 4E′ state have been fitted assuming a linear Jahn–Teller effect in a system with both spin–orbit splitting and a significant anharmonicity in the Jahn–Teller active e′ vibrational mode. The combined effects of anharmonicity in the Jahn–Teller active mode and spin–orbit coupling appear not to have been previously investigated; they are therefore examined in some detail.
High resolution laser spectroscopy of the B 1Π–X 1Σ+ transition of 23Na85Rb
Authors: Wang You-Chang , Kajitani Masaru , Kasahara Shunji , Baba Masaaki , Ishikawa Kiyoshi , Katô Hajime
Published: July 2002

High resolution spectrum of the B 1Π–X 1Σ+ transition of the NaRb molecule was measured with the technique of the Doppler-free laser polarization spectroscopy. Molecular constants of the B 1Π(v=0–12) and X 1Σ+(v=0–6) states of 23Na85Rb were determined. The energy levels of the B 1Π state were found to present many irregularities due to perturbations. The resonance fluorescence spectrum following an excitation to a strongly perturbed level was measured. The fluorescence to the (1)3Σ+ state, which consists of discrete lines followed by a continuum band, was observed in addition to the fluorescence lines to rovibrational levels of the X 1Σ+ state. The perturbing state to the B 1Π(v=8,J=15–21) levels is identified as the (1)3Π state by comparing the observed fluorescence spectra with the selection rules for perturbations and radiative transitions. The dissociation limit of the (1)3Σ+ state, which separates into the Na(3s2S1/2)+Rb(5s2S1/2) atoms, was deduced from the spectrum. The dissociation energies of the X 1Σ+, (1)3Σ+, and B 1Π states were determined to be 5030±2, 182±2, and 1319±2 cm−1, respectively.
Stoichiometric network analysis of the oxalate–persulfate–silver oscillator
Authors: Clarke Bruce L.
Published: July 2002

This paper illustrates an approach that can refine mechanisms and obtain information about rate constants from dynamical phase diagrams which show the regions of oscillation of a mechanism as a function of the experimental parameters. Possible mechanisms for the experimentally oscillating oxalate–persulfate–silver system are examined. Starting with a proposed mechanism by Ouyang and de Kepper, which they could not make oscillate, we show that some variations of the mechanism are stable for all nonnegative values of the rate constants. Other variations are unstable. For these variations, feedback cycles that lead to instability are compared with a conceptual picture of feedback in the experimental system. One unstable mechanism fits the picture well. Its unimportant reactions are omitted and an analytical solution for the unstable region using 13 adjustable parameters is obtained. The rate constants are adjusted to match this solution to the experimentally measured phase diagram. A good fit can only be obtained if [O2] is too low and k1 is much smaller than the known value. Both discrepancies are resolved if Ag2+ oxidizes water. The analysis predicts the width of the unstable region can increase when more O2 enters the reactor.
Reactions of highly excited oxygen atoms O(2p33s,5S) with simple gas molecules
Authors: Mori T. , Kanou K. , Mizuta K. , Kuramasu T. , Ishikawa Y. , Arai S.
Published: July 2002

The electron pulse irradiation of 10 atm He containing one of CO, CO2, NO, NO2, or N2O at small amounts produced highly excited oxygen atoms O(2p33s,5S), which subsequently disappeared by their reactions with parent molecules. The rate constants have been determined from the absorption decay curves at 777.3 nm O(2p33p,5P)←O(2p33s,5S). The rate constants for the reactions of O(2p33s,5S) with Xe, H2, N2, CH4, and C2H6 could be determined in the irradiation of mixtures of He, O2, and these gases with electron pulses. The upper limit of a quenching rate constant was estimated for Kr. The observed rate constants showed good correlation with the quenching rate constants of Kr(4p55s,3P2) or Xe(5p56s,3P2) by the same gas molecules.
Many-body dispersion forces of polarizable clusters and liquids
Authors: Cao J. , Berne B. J.
Published: July 2002

A system of atoms with embedded Drude dispersion oscillators interacting through dipole–dipole forces is simulated. Using path integrals it is shown that after the coordinates of the dispersion oscillators are integrated out, the atoms interact through many-body dispersion forces to all orders of the dipole–dipole interaction. Simulations are carried out on clusters to see if the presence of many-body forces leads to ground state geometries different from those predicted from two-body potentials. In addition, the polarizability tensor of clusters is determined as a function of cluster size. Simulations are also carried out for fluids to see how many-body forces affect the pair correlation function. Lastly, the long-range interaction between van der Waals clusters is compared with the predictions of a summation over site–site two-body interactions. It is found that many-body forces have only a minor effect on the low energy geometries of van der Waals clusters, a somewhat surprising result given that many-body forces do give an important contribution to surface free energies of clusters and liquids. The vibrational frequencies of the breathing mode decrease by approximately 10%.
The potential energy curves of the X 1Σg+ ground states of Mg2 and Ca2 using the interacting correlated fragments model
Authors: Dyall K. G. , McLean A. D.
Published: July 2002

We report ground-state potential energy curves for Mg2 and Ca2 calculated in a large Slater-type basis using the interacting correlated fragments (ICF) model of Liu and McLean. Within the framework of the model, we study convergence of the potential curves with respect to the amount of intrafragment electron correlation introduced into each of the interacting atoms. The ICF model requires localized orbitals and the convergence rate can be affected dramatically by the localization choice; we illustrate and discuss this effect. Successful application of the ICF model requires careful treatment of the basis set superposition error and we discuss this. Our best calculations with four electrons correlated in Mg2 give De=458.8 cm−1 compared with an estimated lower limit for the four correlated electron calculation of 464 cm−1 and an experimental value of 429.6 cm−1. The bulk of the difference between calculation and experiment is attributed to intrafragment core–valence correlation effects which decrease the dissociation energy by nearly 30 cm−1. In parallel calculations on Ca2, our computed four correlated electron De is 1236 cm−1 compared to the experimental 1095.4 cm−1, showing that core–valence electron correlations are responsible for decreasing the dissociation energy by approximately 149 cm−1.
On the hard sphere bridge function
Authors: Taylor Mark P. , Lipson J. E. G.
Published: July 2002

The bridge function for a simple fluid can be written as a diagrammatic expansion involving either the Mayer f-function f(r) or the total correlation function h(r). At each order, the h-bond expansion includes all f-bond diagrams to that order plus an infinite subset of higher order f-bond diagrams. Thus, taken to any order, the h-bond expansion is generally considered more accurate than the f-bond expansion. However, here we show that for hard spheres, the lowest order h-bond approximation for the bridge function is much less accurate than the corresponding f-bond approximation.
Excited-state dipole moments of SO2. II
Authors: Heitz S. , Schmalzigaug K. , von Moers F. , Hese A.
Published: July 2002

Determination of electric dipole moments on SO2 molecules in the region of the Ã 1A2 and B̃ 1B1 states is reported. The Stark splittings and shifts of single rovibronic lines in several vibronic bands were measured by high-resolution laser spectroscopy in a supersonic molecular beam. Electric-field strengths of up to 12 kV/cm were applied. The dipole moment of the Ã 1A2 state was determined to be 1.79(5) D, whereas an indication of a clearly smaller dipole moment was found for the B̃ 1B1 state.
Local structure of model polymeric fluids: Hard-sphere chains and the three-dimensional fluctuating bond model
Authors: Yethiraj Arun , Dickman Ronald
Published: July 2002

Monte Carlo simulations are performed for athermal polymers in the three-dimensional fluctuating bond lattice model. Polymer molecules composed of 20 and 50 freely-jointed beads are studied at volume fractions ranging from 0.2 to 0.35. Chain dimensions, intramolecular correlation functions, and intermolecular correlation functions are compared to results of off-lattice simulations for a similar freely-jointed chain model. It is found that the intramolecular correlation functions are qualitatively similar both on and off the lattice; there are quantitative differences which arise because of the larger persistence lengths of the off-lattice chains. At low densities the intermolecular correlation functions are similar in the two models, but significant differences appear at higher densities because of the limited extent of packing in the lattice fluid. We conclude that the lattice model adequately describes the effect of configurational entropy on fluid structure, but is inadequate to address packing effects which are important in the study of polymer melts. The volumetric behavior of the two model fluids is also discussed.
Vibrational distribution of CH3 produced by the reaction of O(1D2) atom with CH4
Authors: Suzuki Toshinori , Hirota Eizi
Published: July 2002

The nascent vibrational distribution of CH3 produced by the reaction O(1D2)+CH4→OH+CH3 (ΔH0=−182 kJ mol−1) has been investigated by infrared diode laser kinetic spectroscopy. The reaction was initiated by the generation of O(1D2) atoms by excimer laser photolysis of N2O or O3 at a total pressure of 200 mTorr, and the ν2 (out-of-plane bending) bands of CH3, v2=1←0 up to 4←3, were measured as functions of time. The vibrational distribution of ν2 (v≤3) was found to be noninverted and much less excited than a prior distribution. The fraction of the available energy released to the ν2 vibration, <fv(CH3ν2)≳, is estimated to be about 0.019. A similar noninverted vibrational distribution of CH3 was observed in the reaction of CH4 with O(3P) (ΔH0=8 kJ mol−1), which is known to be a direct hydrogen abstraction reaction. The results may suggest that the CH3 moiety is gradually relaxed to a planar structure during these reactions.
Paramagnetism of closed shell diatomic hydrides with six valence electrons
Authors: Sauer Stephan P. A. , Enevoldsen Thomas , Oddershede Jens
Published: July 2002

We have investigated the potential temperature independent van Vleck paramagnetism of closed shell diatomic hydrides with six valence electrons. More specifically, we have studied the magnetizability of the first row hydrides BeH−, BH, CH+, of the second row hydrides MgH−, AlH, and SiH+, and of the third row hydride GeH+. The magnetizability was calculated using a gauge origin independent method at the uncorrelated SCF level within the random-phase approximation (RPA) as well as at different correlated levels within the second-order polarization propagator approximation (SOPPA) or various coupled cluster polarization propagator approximations (CCDPPA/CCSDPPA). We find that BH, CH+, and SiH+ are paramagnetic, MgH−, AlH, and GeH+ are diamagnetic and BeH− is a borderline case tilting towards paramagnetism. It is primarily variations in the diamagnetic contribution to the magnetizability that determine the overall sign of the magnetizability.
Crossed beam studies of four-atom reactions: The dynamics of OH+CO
Authors: Alagia Michele , Balucani Nadia , Casavecchia Piergiorgio , Stranges Domenico , Volpi Gian Gualberto
Published: July 2002

The angular and velocity distribution of CO2 product from the reaction OH+CO at 14.1 kcal/mole collision energy has been obtained in a crossed molecular beam study. The reaction is found to proceed through an osculating HOCO complex living about one rotational period. About 65% of the available energy goes into product translation.
Intramolecular bond length dependence of the anisotropic dispersion coefficients for H2–rare gas interactions
Authors: Wormer Paul E. S. , Hettema Hinne , Thakkar Ajit J.
Published: July 2002

Effective states arising from variational perturbation calculations in a full configuration interaction basis are used to calculate dynamic multipole polarizabilities for H2 at seven different bond lengths. These are combined with previously calculated dynamic polarizabilities for rare gas atoms to obtain the intramolecular bond length dependence of the anisotropic C6, C8, and C10 dispersion coefficients for H2–X (X=He, Ne, Ar, Kr, Xe) interactions. The results are generally in good agreement with previous semiempirical estimates where available.
Metal/conjugated polymer interfaces: A theoretical investigation of the interaction between aluminum and trans-polyacetylene oligomers
Authors: Fredriksson C. , Brédas J. L.
Published: July 2002

Molecular model systems are used to quantum chemically investigate the interface between aluminum and trans-polyacetylene. Modifications to the chemical and electronic structure of trans-polyacetylene oligomers upon interaction with a submonolayer of aluminum are studied at the semiempirical and ab initio Hartree–Fock levels. An aluminum atom is found to react strongly with a carbon atom of the trans-polyacetylene chain to form a heteropolar covalent bond. In this process, the binding carbon evolves from an sp2- to an sp3-hybridized electronic structure. Significant contributions from Al 3s and 3p atomic orbitals are found in the frontier molecular orbitals in aluminum/polyene complexes. This results in the fact that despite the presence of sp3 sites due to Al–C bonds, which reduces π conjugation along the chain, a large degree of delocalization in π levels is maintained. Our calculations are discussed in relation to experimental ultraviolet photoelectron spectra (UPS) taken during initial stages of aluminum deposition on oxygen-free films of trans-polyacetylene oligomers.
Thermophysical properties of the lanthanide sesquisulfides. IV. Schottky contributions, magnetic, and electronic properties of ε-phase Yb2S3 and Lu2S3
Authors: Gruber John B. , Shaviv Roey , Westrum Edgar F. , Burriel Ramón , Beaudry Bernard J. , Palmer Paul E.
Published: July 2002

The heat capacities of ε-phase Yb2S3 and Lu2S3 have been determined from 6 to 350 K and their thermodynamic properties evaluated. The resolution of the Schottky and magnetic properties by evaluation of the lattice heat capacity is shown to be in accord with spectroscopically determined energy levels. The lattice heat capacity of Yb2S3 was determined by means of the Komada–Westrum phonon distribution model. Excess heat-capacity contributions were thus evaluated and analyzed as Schottky and magnetic heat capacities. A phase transition associated with magnetic ordering was detected in the heat capacity of Yb2S3 near 7 K with an entropy content of 0.68R. The entropies at 298.15 K are 22.77R and 19.74R for Yb2S3 and for Lu2S3.
Wetting and layering in critical binary fluid mixtures
Authors: Bonn D. , Kellay H. , Wegdam G. H.
Published: July 2002

We present an ellipsometric study of wetting and adsorption at the liquid–vapor interface of cyclohexane–methanol and cyclohexane–deuterated methanol systems. Due to the first-order character of the wetting transition and the hysteresis associated with it, either a very thin or a thick film of the heavy phase intrudes between the lighter phase and the vapor. In the cyclohexane–methanol mixture, the thin film grows in layers with a thickness on the order of the molecular diameter rather than continuously; a series of first-order layering transitions takes place at the interface. In the deuterated system, no clear layering effects are observed. As the hydrogen bonds in the latter system are somewhat weaker, this suggests that hydrogen bonding plays a role in the observed layering effects. Measurements above the critical temperature TC allow for the conclusion that the thin film corresponds to the critical adsorption state. Residual layering effects may persist above TC for the cyclohexane–methanol system.
Thermodynamic fluctuations and chemical chaos in a well-stirred reactor: A master equation analysis
Authors: Geysermans P. , Nicolis G.
Published: July 2002

The stochastic analysis of a chemical system obeying to mass-action kinetics and giving rise to deterministic chaos is carried out using the master equation approach. Quantitative comparison with the predictions of the deterministic description based on the rate equations shows that the system’s attractor and the probabilistic properties are robust toward fluctuations.
Measurements of C3H+3, C5H+3, C6H+6, C7H+5, and C10H+8 dissociative recombination rate coefficients
Authors: Abouelaziz H. , Gomet J. C. , Pasquerault D. , Rowe B. R. , Mitchell J. B. A.
Published: July 2002

The rate coefficients α for the dissociative recombination (DR) of the aromatic cyclic ions C6H6+ and C10H8+ have been measured in a flowing afterglow experiment at room temperature using Xe+ and Kr+ as precursor ions. C+ was also used as a precursor ion and this allowed the DR rate coefficients for C3H3+, C5H3+, and C7H5+ to be determined. The values obtained (in units of 10−6 cm3 s−1) at 300 K are, respectively, α(C3H3+)=0.7, α(C5H3+)=0.9, α(C6H6+)=1.0, α(C7H5+)=0.7, α(C10H8+)=0.3 with an uncertainty estimated to be ∼30%. A discussion is given concerning the relationship between the DR rate and the structural complexity of the ions.
High resolution, jet-cooled infrared spectroscopy of (HCl)2: Analysis of ν1 and ν2 HCl stretching fundamentals, interconversion tunneling, and mode-specific predissociation lifetimes
Authors: Schuder Michael D. , Lovejoy Christopher M. , Lascola Robert , Nesbitt David J.
Published: July 2002

An extensive series of near-infrared absorption spectra are recorded for jet-cooled (6–14 K) hydrogen chloride dimer (HCl)2. Both ΔKa=0 and ΔKa=±1 bands are observed for both the free (ν1) and bonded (ν2) HCl stretches; all three chlorine isotopomers (H 35Cl–H 35Cl, H 35Cl–H 37Cl, and H 37Cl–H 37Cl) are observed and analyzed for K″a ≤ 2. The slit jet spectrum extends significantly the previous cooled cell infrared study of this complex and provides a measure of tunneling splittings for Ka=0 and 1 for each of the HCl ground (v=0) and excited (v=1) states. Mode specific vibrational predissociation is observed via analysis of the absorption line shapes, with Lorentzian contributions to the line profiles of Δν1≲1.6 MHz and Δν2=5.1±1.2 (2σ) MHz full width at half-maximum for ν1 and ν2 excitation, respectively. Stronger coupling in (HCl)2 of the bonded (ν2) vs free (ν1) HCl vibration to the dissociation coordinate is consistent with the comparable trends observed in other hydrogen bonded dimers. Quantum mechanical variational calculations on an electrostatic angular potential energy surface are used to model the internal HCl rotor dynamics using a coupled rotor formalism; analysis of the internal rotor eigenfunctions provides direct evidence for large amplitude ‘‘geared’’ internal rotation of the HCl subunits.
Pressure isotherms, phase transition, instability, and structure of tethered polymers in good, Θ, and poor solvents
Authors: Carignano M. A. , Szleifer I.
Published: July 2002

The conformational and thermodynamic behavior of chain molecules tethered to a planar surface are studied for a variety of solvent qualities with a recently developed single-chain mean-field theory. The lateral pressure isotherms calculated from the theory for chains of n=50 segments show very good quantitative agreement with the recent molecular dynamics simulations of Grest, without the use of any adjustable parameter, for good and Θ solvents. The behavior of the pressure isotherms is analyzed in terms of a virial expansion and it is shown that the regimes where there is scaling of the pressure with surface coverage σ are very narrow for this chain length. Moreover, comparisons with analytical self-consistent field (SCF) theory show good agreement only when the parabolic density profile is used in the full virial equation. In the bad solvent regime the pressure isotherms for grafted chains show negative values of the pressure and also a negative compressibility for some range of surface coverages for temperatures below the Θ temperature. This indicates the possibility of microphase separation in this regime of temperature and surface coverages.

For chains with translational degrees of freedom there is a first-order phase separation at temperatures below the Θ temperature into a very dilute and more concentrated polymer phase. From results of two different chain lengths it seems that there is a universal coexistence phase diagram in the scaling variables n0.5(T/Θ−1) vs nσ. In the bad solvent regime it is found that the chains are highly collapsed for very low surface coverage. As the surface coverage is increased the chains tend to stretch laterally in order to gain as much contact as possible with the other polymer chains. This is in contrast to the good solvent regime where the chains always stretch perpendicular to the surface in order to avoid as much contact as possible with the other polymer chains. In the intermediate surface coverage regime it is found that the density profiles look parabolic-like for good solvents becoming more steplike as the quality of the solvent becomes poorer, in agreement with recent neutron scattering experiments. For very high surface coverages the density profiles in the good solvent regime also look steplike.
Pressure broadening and collisional narrowing in OH(v=1←0) rovibrational transitions with Ar, He, O2, and N2
Authors: Schiffman A. , Nesbitt David J.
Published: July 2002

Line shapes are measured for OH(v=1←0) transitions in the presence of Ar, He, O2, and N2 as a function of N rotational, spin–orbit, and λ doublet states. Pressure broadening coefficients for all transitions and buffer gases are determined from fits of the observed line shapes to the Voigt profile. The dependencies of the observed broadening coefficients on the OH quantum levels are discussed and compared with previous pressure broadening studies in HF and NO. The observed OH line shapes are interpreted in terms of their impact on the determination of mesospheric and stratospheric OH populations, temperatures, and quantum state distributions from OH nightglow and dayglow emission. In the case of OH+Ar, evidence for Dicke narrowing is presented and narrowing coefficients are reported from fits to a ‘‘hard collision’’ model.
New formulation for data analysis in the quasielastic light scattering with the sinusoidal electric field and its application to the spherical polyions in aqueous solutions
Authors: Imaeda Takashi , Kimura Yasuyuki , Ito Kohzo , Hayakawa Reinosuke
Published: July 2002

A new formula for the data analysis in the quasielastic light scattering with the sinusoidal electric field (QELS-SEF) was derived from the time-averaged autocorrelation function. This time-average process exactly corresponds to the actual experimental procedure which is performed with a correlator used in the QELS-SEF measurement. The new formula was applied to the power spectrum of the polystyrene latex solution in the heterodyne mode of QELS-SEF, and then was found to fit quite well to the experimental data. By this fitting procedure, we revealed the dependences of the electrophoretic mobility μ and the apparent diffusion constant Dapp of the polystyrene latex on the frequency and amplitude of the applied ac electric field. Thus, the frequency dispersion or relaxation of Dapp was observed in the same frequency region as the dielectric relaxation.
Dynamic characterization of liquid crystalline polymers under flow-aligning shear conditions
Authors: Maffettone P. L. , Marrucci G. , Mortier M. , Moldenaers P. , Mewis J.
Published: July 2002

The dynamic response of liquid crystalline polymers at high shear rates has been studied by measuring shear and normal stresses during transients following a step in the shear rate. Stepwise increase and decrease as well as flow reversals have been investigated. A solution of poly(benzylglutamate) (PBG) in m-cresol is used in the experiments. One purpose of the work is to perform a comparison with the theoretical predictions that apply to the case of spatially uniform situations. The latter are expected to prevail at high shear rates, when the tumbling character of the nematic phase is effectively suppressed. The results indicate profound differences with respect to previous experiments at lower shear rates. A somewhat surprising result is that the behavior at high shear rates appears to be dominated by the viscous, rather than the elastic stress.
Random paths and fluctuation–dissipation dynamics for one-variable chemical systems far from equilibrium
Authors: Vlad Marcel O. , Ross John
Published: July 2002

A random path integral representation of the Ross–Hunt–Hunt thermodynamic and stochastic theory is given for chemical reactions far from equilibrium in the case of constant-step and one-variable processes. An explicit analytical expression for the chemical Lagrangian is presented. A connection is made between the thermodynamic fluctuation–dissipation regimes characteristic to the process and the chemical Lagrangian. The path integral formalism is used to prove the validity of fluctuation regression hypothesis and to derive two variational principles for the most probable and average paths, respectively. The most probable path corresponds to the absolute maximum of the Lagrangian and the average path corresponds to the minimum value of the information gain obtained by observing a certain average path. For nonlinear regimes these two variational principles generally give distinct results; they are identical only in the vicinity of a stable steady state. An eikonal approximation is suggested for evaluating time-dependent probability distributions which reduces the integration of Master Equations to two quadratures. The suggested eikonal approximation leads to a proportionality between the species-specific free energy of the system and the extremal value of the time integral of the chemical Lagrangian. This relationship is similar to the expression of the mechanical action in terms of the Lagrangian in classical mechanics. Most results derived in this paper for one variable can be extended to multivariable systems. Finally a comparison is made with other stochastic approaches to nonequilibrium thermodynamics.
The possible crossover effects of NaNO3 confined in porous media: From bulk to clusters
Authors: Mu R. , Jin F. , Morgan S. H. , Henderson D. O. , Silberman E.
Published: July 2002

Differential scanning calorimetry (DSC) and Raman spectra are reported for NaNO3 bulk and for NaNO3 confined in porous silica with pore radii, rp=2.5, 5, 10, 20 nm. Raman spectra are also given for a 6 M solution of NaNO3. The melting transition for the confined NaNO3 exhibits a 1/rp dependence where rp is the pore radius for rp≳5 nm. No melting transition is observed for NaNO3 confined in 2.5 nm pores. Above this pore size, their appears to be a deviation in the melting transition dependence on rp. The internal modes observed in the Raman spectra for the confined material are in agreement with those of the bulk solid except for a feature observed on the low frequency side of the ν1 band. The external TO mode observed at 100 cm−1 and the librational mode at 175 cm−1 for NaNO3 both decrease in intensity and broaden as rp decreases and both bands disappear at rp=2.5 nm. An additional peak at 70 cm−1 not observed in the solution or bulk NaNO3 spectra appears in the spectra of confined NaNO3 and increases in intensity as rp decreases. We assign this band to a new phase of NaNO3 which is stabilized by the surface hydroxyl groups of the porous silica. For NaNO3 confined in pores, rp≤2.5 nm, we suggest that NaNO3 exists as disordered aggregates.
Interaction between surfaces in ethanol: Adsorption, capillary condensation, and solvation forces
Authors: Wanless Erica J. , Christenson Hugo K.
Published: July 2002

When two surfaces in a condensable vapor are brought together, the vapor will capillary condense in the narrow gap between the surfaces. The surface forces apparatus, (SFA) has been used to study this condensation process with mica surfaces in ethanol vapor close to saturation. In particular, the critical surface separation at which the condensed bridge forms has been quantified. For thin adsorbed films (≤2 nm), the results are not consistent with a model of a liquid bridge formed by adsorbed films thickening under the influence of van der Waals forces. Instead, nucleation from vapor in the gap between the surfaces is possibly contributing to the formation of the bridge. The short-range interaction of mica surfaces in near-saturated ethanol vapor is also presented. The contact adhesion is much smaller than in nonpolar liquids due to the shielding of the ionic components of the adhesion. The solvation force is found to be similar to that in nonpolar liquids, except that the innermost minima are deeper. This is attributed to the amphiphilic nature of the ethanol molecule.
Kohn variational principle for a general finite-range scattering functional
Authors: Brown David , Light John C.
Published: July 2002

The Kohn variational principle (KVP) has been used to compute both the R and the log-derivative matrices, which are formally inverses of one another. We show that the KVP for these matrices are special cases of a KVP for a more general functional which can be derived by imposing more general boundary conditions on the trial function space. This more general matrix, which we denote Z, can then be used to compute the S-matrix in a procedure analogous to that for R and Y. This approach is demonstrated for the Eckart barrier problem. Our studies suggest that within the framework presented, the log derivative case presents some computational advantage.
The thermodynamic stability of clathrate hydrate. III. Accommodation of nonspherical propane and ethane molecules
Authors: Tanaka Hideki
Published: July 2002

The thermodynamic stability of clathrate hydrates encaging nonspherical molecules has been investigated by examining the various components of the free energy of cage occupancy. In the present study, we develop a method to treat clathrate hydrates of nonspherical guest molecules by extending a generalized van der Waals and Platteeuw theory [Tanaka and Kiyohara, J. Chem. Phys. 98, 4086 and 8110 (1993)]. The potential energy surfaces for the rotational motions of nonspherical propane and ethane molecules are examined. The free energy associated with the nonspherical nature of the guest molecules is evaluated as a perturbation from the reference system encaging spherical guests. The vibrational free energy of both guest and host molecules is divided into harmonic and anharmonic contributions. The latter term that is essential in the free energy of the hindered rotation for the guests is evaluated and thermodynamic properties are found to be in good agreement with the measured values. The dynamical behaviors are investigated by calculating the reorientational and velocity correlation functions. The occupation rates are calculated by grandcanonical Monte Carlo simulations and are in reasonable agreement with those from the direct calculation.
A study of the kinetics of swelling in cylindrical polystyrene gels: Mechanical behavior and final properties after swelling
Authors: Hakiki Abdelkrim , Herz Jean E.
Published: July 2002

Mechanical properties of cylindrical polystyrene gels were investigated both by the kinetics of swelling and uniaxial elastic modulus. These gels were prepared by specific chemical reactions using well-defined difunctional and precursor polymers. From the data of the kinetics of swelling we determined the cooperative diffusion coefficient of the gel and the related mesh size. Experimental results were found to be consistent with the theory of Tanaka et al. Elastic moduli were interpreted on the basis of the phantom and affine models.
Electronic state distribution of Xe+* formed by excitation transfer from He(2 3S) to Xe+(2P3/2) at thermal energy
Authors: Tsuji Masaharu , Kaneko Nobuo , Ishimi Hiroaki , Nishimura Yukio
Published: July 2002

The formation process of Xe+* in the He afterglow reaction of Xe has been studied by observing Xe ii lines in the ultraviolet and visible regions. Sixty one Xe+* states in the 13.86–19.49 eV range were excited by the He(2 3S)+Xe+(5p5 2P03/2) excitation-transfer reaction. It was found that Xe+* was not formed by the He(2 3S)+Xe(6s 3P02) Penning type reaction and the He++Xe(6s 3P02) charge-transfer reaction. There were some unclassified Xe ii lines, which occupied 14% of the total production of Xe+*. Most of them were attributed to Xe+ transitions from unknown high energy Xe+* states in the 18–19.5 eV range. The electronic state distribution of individual Xe+* levels has been determined by taking account of radiative cascade for low lying electronic levels. The He(2 3S)+Xe+(2P03/2) reaction expressed no resonant character. The electronic state of Xe+* was distributed more widely than those of Ar+* and Kr+* in the He(2 3S)+Ar+(3p5 2P03/2), He(2 3S)+Kr+(4p5 2P01/2), and He(2 3S)+Kr+(4p5 2P03/2) reactions. The excitation mechanism of rare gas cations due to collisions between a rare gas metastable atom and a rare gas ion is discussed. The lack of the excitation processes of Kr+* and Xe+* by the He++Kr(3P02) and He++Xe(3P02) reactions was attributed to the absence of near-resonant Kr+(5p) and Xe+(6p) states whose excitation satisfies the selection rule of Δl=±1.
Pump–probe spectroscopy of the hydrated electron: A quantum molecular dynamics simulation
Authors: Schwartz Benjamin J. , Rossky Peter J.
Published: July 2002

Quantum nonadiabatic molecular dynamics simulations are used to directly compute the transient absorption spectroscopy following photoexcitation of equilibrium hydrated electrons. The calculated spectral transients are found to be in excellent agreement with ultrafast traces measured in recent transient spectral hole-burning experiments [Barbara and co-workers, J. Chem. Phys. 98, 5996 (1993); J. Phys. Chem. 98, 3450 (1994)], indicating that the computer model correctly captures the underlying physics. The model transients are dissected into ground state bleach, excited state absorption, and stimulated emission spectral components, each of which is examined individually and analyzed in terms of the microscopic solvent response following photoexcitation. Although there is no distinct spectral hole, bleaching dynamics are found to play an important role in the overall transient spectroscopy. The excited state absorption spectrum undergoes a complex evolution due to solvation dynamics which alters both the frequencies and the oscillator strengths of the relevant quantum transitions. Calculated excited state emission from the electron is characterized by an enormous dynamic Stokes shift as well as an overall spectral narrowing in time. In combination, these three components allow the assignment of features of the measured ultrafast spectroscopic transients in terms of specific details of the microscopic solvent response.
A theoretical study of the vibrational excitation of NO/Ag(111)
Authors: Gates G. A. , Darling G. R. , Holloway S.
Published: July 2002

We present a model study of the vibrational excitation of NO molecules scattering from a Ag(111) surface using time-dependent wave packet methods. The molecule–surface interaction is described by a two-dimensional potential energy surface in the vibrational and center-of-mass coordinates. Surface motion is modeled by a single atomic oscillator. This model reproduces the experimentally observed increase in the vibrational excitation as a function of both increasing initial translational energy of the molecule and surface temperature. The surface temperature only enters via the Boltzmann weighting of initial surface vibrational states. We simplify the simulation to a two-dimensional surface–mass model, where the gas–surface interaction is analogous to a classical ‘‘cube’’ model. It is shown that the relative velocity of the incoming molecule, with respect to the surface, causes the observed dependence of the vibrational excitation on surface temperature. The savings in computational time achieved with this model allows a study of the effect of the potential topology and surface mass on the temperature dependence of the vibrational excitation.