bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat capacity extrema: Solution of the Ornstein–Zernike equation in the Percus–Yevick approximation for the truncated Lennard-Jones fluid
Authors: Freasier Ben C. , Bearman Richard J.
Published: July 2002

We have carried out a comprehensive study of a truncated Lennard-Jones (TRLJ) system (potential cutoff =2.5σ) in the Percus–Yevick (PY) approximation using Baxter’s equations and algorithms with some degree of novelty. We have determined the liquid–vapor phase diagram from the ‘‘energy equation of state,’’ have determined the spinodal curve for the compressibility equation of state, and have made calculations of the energy, compressibility, and virial pressures. We have calculated heat capacities, examined their extrema for the energy and compressibility equations, and have contrasted them with previously published simulation data. Our ‘‘energy equation of state’’ is defined self-consistently within the PY approximation and differs from the definition of other authors.
Direct, spectroscopic measurement of the rotational state distribution of HCl fragments from the vibrational predissociation of ν2=1 (HCl)2 produced by stimulated Raman excitation
Authors: Serafin Joseph , Ni Hong , Valentini James J.
Published: July 2002

Stimulated Raman excitation of (HCl)2 produces dimers with one quantum in the ν2 H–Cl stretch vibration. HCl fragments from predissociation of the dimers are detected by resonant multiphoton ionization (MPI). The HCl rotational state distribution shows a strong propensity to form the highest energetically accessible states.
Excited-state structure and photochemical ring-opening dynamics of 1,3,5-cyclo-octatriene from absolute resonance Raman intensities
Authors: Lawless Mary K. , Mathies Richard A.
Published: July 2002

Absolute resonance Raman cross sections are measured for 1,3,5-cyclo-octatriene (COT) in cyclohexane with excitation from 325 to 200 nm. These intensities and the absorption spectrum are modeled using a fully thermalized time-correlator theory to quantitate the excited-state equilibrium geometry displacements along 19 Raman-active normal modes. The resonance Raman spectra show significant intensity in low-frequency modes corresponding to planarization of the eight-membered ring. The 140 cm−1 twist-boat planarization (Δ=4.6) and the 339 cm−1 ring deformation (Δ=1.6) are particularly strong. However, no intensity is observed in modes which project onto the predicted disrotatory ring-opening motion, such as the nontotally symmetric CH2 twist fundamental or its overtone. Analysis of the fluorescence quantum yield (φF=2×10−6) gives an excited state lifetime on the order of ∼30 fs. These results show that ring planarization is the first step in the disrotatory ring opening of COT followed by rapid depopulation of the initially prepared state to a lower-lying excited electronic state upon which the actual ring opening occurs. Comparison of these results with the excited-state dynamics of other pericyclic systems suggests that pericyclic rearrangements occur only once a planar structure is established and that the bond rearrangement occurs predominantly on a low-lying, optically forbidden excited state.
Ordering in asymmetric poly (ethylene–propylene)–poly (ethylethylene) diblock copolymer thin films
Authors: Karim Alamgir , Singh Navjot , Sikka Mohan , Bates Frank S. , Dozier William D. , Felcher Gian P.
Published: July 2002

We have used neutron reflection and phase contrast microscopy to investigate the morphology and surface topology of thin films of nearly symmetric (f=0.55) and asymmetric (f=0.77), poly (ethylene–propylene)–poly (ethylethylene) (PEP–PEE) diblock copolymers (f being the PEP volume fraction) and have identified three important differences in their ordering properties. First, annealed films of the asymmetric diblocks do not form the lamellar microstructure found in symmetric diblocks; their structure can instead be modeled in terms of the hexagonal packing of PEE cylinders observed in bulk small-angle neutron scattering measurements. However, the cylinders show in-plane distortions, which we interpret in the context of nonintegral layering. These distortions are amplified at the surfaces where the PEE assumes lamellarlike form. Second, as-cast films of the asymmetric diblock are characterized by a microstructure lacking long-range order, pinned between strongly segregated PEE at both surfaces. These films can be equilibrated through annealing, leading to the well-ordered structures described earlier. The changes with annealing are surprising given that both PEP and PEE are well above their glass transitions at room temperature. Finally, the block asymmetry and the associated cylindrical structure in the interior are also manifested in the surface topology. Thin films of asymmetric PEP–PEE are smooth on a macroscopic scale unlike their symmetric counterparts, which form islands on the surface to accommodate films of nonintegral lamellar thicknesses.
p,T-dependence of self-diffusion in liquid hydrogen fluoride
Authors: Karger N. , Vardag T. , Lüdemann H.-D.
Published: July 2002

With the nuclear magnetic resonance (NMR)-spin echo technique, self-diffusion coefficients D of anhydrous liquid HF and DF have been obtained between the melting pressure curve and 373 K at a maximum pressure of 600 MPa. The distilling apparatus, made entirely of fluoropolymers, and the titanium autoclave used for the measurements are described. The isobars of D show a pressure independent activation energy of 9.92 kJ/mol for HF and 10.3 kJ/mol for DF. Deviations from Arrhenius behavior at the lowest temperatures could be described by a VTF-equation. The isotherms of self-diffusion show a monotonic, but slight decrease with rising pressure. The isotope effect DHF/DDF rises from 1.05 at 373 K to 1.20 at 200 K, which can be explained by higher hydrogen bond energies in the deuterated liquid. These findings are compared to results from similar measurements on water as well as apolar substances and discussed with respect to the topology of the hydrogen bond structure in liquid HF. Results from recent Monte Carlo-simulations are included in the discussion.
The solution of the time-dependent Schrödinger equation by the (t,t′) method: Multiphoton ionization/dissociation probabilities in different gauges of the electromagnetic potentials
Authors: Peskin Uri , Alon Ofir E. , Moiseyev Nimrod
Published: July 2002

A new approach is introduced for the calculation of multiphoton partial decay rates in ionizing and dissociative processes. The partial decay rates (partial resonance widths) are calculated by carrying out asymptotic analysis of a complex scaled resonance quasienergy solution of the time-dependent Schrödinger equation obtained when the acceleration, momentum, reduced momentum, or length gauges are used. The partial width analysis is made only after applying the proper complex-scaled transformations on the resonance solution. The recently developed (t,t′) algorithm [J. Chem. Phys. 99, 4590 (1990)] significantly reduces the computational effort when the Hamiltonian is represented in the momentum gauge or in the length gauge. The new approach is applied to a model Hamiltonian which describes the multiphoton detachment of Cl− in the ArF laser field.
Modeling of complex biological systems. I. Molecular dynamics studies of diglyceride monolayers
Authors: Peters Günther H. , Toxvaerd S. , Svendsen A. , Olsen O. H.
Published: July 2002

Molecular dynamics simulations of diglyceride monolayers at the air–water interface have been performed to study the dynamical behavior of these Langmuir layers at surface densities varying from 36.2 to 40.5 Å2/molecule. The monolayers are treated in full atomic detail, with the exception of methyl and methylene groups that are considered to be ‘‘anisotropic united atoms.’’ The presence of phase transitions are explored and correlated to the changes in the microstructure of the layer. Good agreement is found between the surface pressure–surface area (π−A) isotherms calculated in the simulations and determined in monolayer technique experiments. The diglyceride film shows a complex dynamical behavior during expansion. On expansion from a closely packed, highly ordered state, the film undergoes two phase transitions. The first transition, which occurs at 38.3 Å2/molecule, involves a ‘‘seesaw’’ mechanism of the sn-1 and sn-2 carboxyester groups. The latter forms the backbone of the diglyceride molecule at high surface pressure. This mechanism is governed by hydrophobic/hydrophilic forces. It is stabilized by a balance between inter- and intramolecular interactions. On expansion, the intermolecular interaction decreases, and at the first transition, the intramolecular interaction between the two chains are strong enough to cause an increase of the cross sectional area of the molecules. This swelling of the molecules is accompanied with an increase of intermolecular contributions. The second transition, which occurs at 39.8 Å2/molecule, is caused by tilting over nearest neighbors.
Spectral line shapes of damped quantum oscillators: Applications to biomolecules
Authors: Gu Yuangang , Widom Allan , Champion Paul M.
Published: July 2002

We present a full quantum mechanical treatment, using the quantum fluctuation–dissipation theorem, which is useful in describing the absorption line shape of a system composed of damped vibrational (harmonic) oscillators that are linearly coupled to an electronic excitation. The closed form expressions obtained from the model predict optical line shapes that are identical to standard treatments at high temperature or in the absence of damping. However, at low temperature, quantum corrections become important and the model predicts a skewed optical line shape that reflects the condition of detailed balance and differs significantly from the ‘‘Brownian oscillator’’ model of Yan and Mukamel [J. Chem. Phys. 89, 5160 (1988)]. We also find that quantum effects become observable in the line shape of the overdamped oscillator only when kBT/ℏω0≲ω0 /γ <1, which effectively depresses the temperature for crossover into the quantum regime. In Appendix D we discuss how the time correlator expressions derived for the line shape analysis can also be used to describe chemical reactions in the presence of quantum damping. The fact that the transition temperature for quantum behavior is depressed in the presence of strong damping may explain why the ‘‘classical’’ Arrhenius expression is often found to hold, even at temperatures where kBT<ℏω0. Finally, we explore the consequences of introducing a classical control variable (corresponding to slow conformational motions of a biomolecule), which is coupled to the optically active vibrational mode(s) of the embedded chromophore. This leads to a modulation of the Stokes shift and optical coupling in the system and results in a type of inhomogeneous broadening that has both a Gaussian and non-Gaussian component. The non-Gaussian broadening is found to be consistent with the highly skewed inhomogeneous line shape of deoxymyoglobin.
Valence-bond charge-transfer solvation model for nonlinear optical properties of organic molecules in polar solvents
Authors: Chen Guanhua , Lu Daqi , Goddard William A.
Published: July 2002

A simple model is developed for predicting solvation effects on the nonlinear optical properties of charge transfer organic materials such as 1,1 dicyano,6-(di-butyl amine) hexatriene. This model is based on the valence-bond charge-transfer (VB-CT) framework, using a continuum description of the solvent. The resulting VB-CT solvation model leads to analytic formulas for the absorption frequency (Eg), the polarizability (α), the hyperpolarizabilities (β,γ,δ), and the bond length alternation with only one solvent dependent parameter (ε, the dielectric constant of the solution). The theory involves just four solvent-independent parameters, V0, t, SF, and Q which are related to the band gap, bandwidth, geometry, and dipole moment of the CT molecule [plus a length (RDA) and force constant (k) derivable from standard force fields]. The results are in good agreement with experiment.
The power law aspect of solvation dynamics, based on the convolutionless generalized Langevin equation
Authors: Raineri Fernando O. , Friedman Harold L.
Published: July 2002

The familiar generalized Langevin equation (GLE1) of Mori has a variant, the convolutionless generalized Langevin equation (GLE2) of Tokuyama and Mori, for which we propose a very simple approximation to calculate time correlation functions. This new method, the reference frequency modulation approximation (RFMA), leads quite straightforwardly to known useful formulas. One is the power law of dynamic solvation due to Maroncelli, Kumar, and Papazyan. Another is a decay-time analog of the Powles–Glarum relation between single-particle and collective dielectric correlation times. A third application gives a relation between single-particle rotational time correlation functions of different tensorial rank recently used by Chang and Castner. The GLE2-RFMA method may be considered as the counterpart of the reference memory function approximation of GLE1 theory.
Double resonance spectra of J-coupled I=1/2 and S=3 nuclear spins
Authors: Capuani Silvia , De Luca Francesco , Maraviglia Bruno
Published: July 2002

An extension of the Cartesian product operators formalism for scalar coupled I=1/2 and S=3 nuclear spins is proposed in order to predict the behavior of the function modulating the I spin echo amplitudes in spin-echo double resonance (SEDOR) experiments. With such an approach the coupling constant can be evaluated quite independently of the S=3 quadrupolar line broadening and magnetic field inhomogeneity. The method, which can be generalized to a wide class of molecules, is tested by the experimental behavior of the hydrogen SEDOR modulation by the 10B nuclei in Na210B12H11SH.
Ab initio investigation of the vibrational structure of absorption and emission spectra of FCO
Authors: Krossner Th. , Perić M. , Vetter R. , Zülicke L.
Published: July 2002

Results of an ab initio study of the vibrational structure of several electronic spectral systems of FCO are presented. An interpretation of hitherto recorded spectra, as well as a prediction of features of yet unobserved electronic transitions is given. The absorption spectra observed in the uv spectral region are ascribed to the transitions involving the first excited states of 2A′ and 2A″ character. The emission spectrum obtained by Toby and Toby [J. Phys. Chem. 85, 4071 (1981)] is assigned to the transition X 2A′←2 2A″. Reassignment of some observed vibrational bands is proposed.
Absolute entropy of simple point charge model water by adiabatic switching processes
Authors: Tsao Li-Wen , Sheu Sheh-Yi , Mou Chung-Yuan
Published: July 2002

Direct molecular dynamics computation of the absolute entropy for simple point charge model water and ice have been carried out by adiabatic switching processes. A theoretical analysis shows that the average energy for nonadiabatic switching is always larger than the adiabatic energy. Due to the energy dissipation the deviation in entropy is positive for finite rate of switching. Two switching functions are used to examine the computational convergence and accuracy. Also, two reference integrable systems, ideal gas and ideal solid, are proposed as ends of the adiabatic switching. A proper reference system and efficient switching path are necessary for accurate computation of absolute entropy. Systematic and statistical errors of the results are discussed.
Studies on dynamics of Mu reaction with NO2 by muon spin resonance and relaxation techniques
Authors: Sugai T. , Sakamoto M. , Matsushita A. , Nishiyama K. , Nagamine K. , Kondow T.
Published: July 2002

The reaction of Mu with NO2 in a Kr atmosphere was studied by muon spin resonance and relaxation measurements at 300 K. It was found that the reaction proceeds via the following two steps: Mu+NO2→MuO+NO and MuO+NO2→MuNO3. In particular, the final product MuNO3 and the intermediate species MuO were successfully detected for the first time by the techniques of the muon spin resonance and the longitudinal relaxation under a high magnetic field of ∼2800 G. The rate constants of the Mu+NO2 and the MuO+NO2 reactions were determined to be (2.9±0.5)×1011 ℓ mol−1 s−1 and (4.5±0.1)×109 ℓ mol−1s−1 at 300 K. The isotope effect in the rate constant of the Mu+NO2 reaction in comparison with the H+NO2 reaction suggests that there are reaction paths with a slight energy barrier which is high enough to retard the H reaction, but not the Mu reaction. On the other hand, the rate constant of the MuO+NO2 reaction is smaller than 50% of those of the OH+NO2 and the OD+NO2 reactions. This isotope effect seems to originate from faster rotational motion of MuO and/or lower vibrational density of states at the transition state.
Influence of the range of attractive forces on vapor/liquid phase coexistence
Authors: Shukla K. , Rajagopalan Raj
Published: July 2002

The influence of the range of interparticle attractions on vapor/liquid coexistence in spherically symmetric systems has been investigated by mapping the pair potential on to the adhesive-hard-sphere model. Comparisons of our analytical predictions with recent Gibbs-Ensemble Monte Carlo simulations for the Yukawa potential show a very good agreement. The temperature range over which vapor/liquid coexistence is thermodynamically preferred shrinks as the range of attraction decreases, and for short ranges the coexistence curve flattens, consistent with the results of the simulations. Using the Sutherland potential model we also show that for very small ranges of attraction the liquid state is unstable down to very small temperatures and that a fluid/solid transition is preferred.
Direct measurement of rotational and vibrational relaxation in methane overtone levels by time-resolved infrared double-resonance spectroscopy
Authors: Klaassen Jody J. , Coy Stephen L. , Steinfeld Jeffrey I. , Abel Bernd
Published: July 2002

A time-resolved infrared double-resonance technique has been used to measure vibrationally and rotationally inelastic collision rates in ground and vibrational overtone levels of methane. A Raman-shifted Ti:sapphire laser is used to pump J=0 through 7 states in the 2ν3 and ν3+ν4 levels of 12CH4, and a tunable diode laser is used to probe the time-dependent level populations. Vibrational equilibration is observed among the octad, pentad, and dyad levels, with subsequent relaxation to the ground state. State-to-state rotational energy transfer rates are obtained in the ground and ν3+ν4 excited vibrational levels, and compared with theoretical predictions and with pressure-broadening measurements on the corresponding transitions. The probability of molecular reorientation in an inelastic collision is also inferred from the polarization dependence of the relaxation times. Parity-conserving and vibrational angular momentum propensity rules are inferred for the lower rotational levels of methane.
Solvent dynamical effects on electron transfer reactions
Authors: Zhu Jianjun , Rasaiah Jayendran C.
Published: July 2002

An integral equation [Rasaiah and Zhu, J. Chem. Phys. 98, 1213 (1993)] for the survival probabilities of electron transfer (ET) between thermally equilibrated reactants in solution is extended to include quantum effects on the ligand vibration and ET from a nonequilibrium initial state. We derive the kernel of the integral equation using a Green’s function technique and demonstrate that it is determined by the solvent dynamics, the relative contributions of ligand and solvent reorganization energies, and the barrier heights for electron transfer. The extension of the theory to ET from a nonequilibrium initial state modifies the integral equation to provide the survival probabilities for the reactants that are not necessarily kinetically of first order, but can be directly compared with experiment. The long time rate, however, shows a simple exponential time dependence that is analyzed in terms of a rate constant with a diffusive solvent controlled component and a remainder. The effect of solvent dynamics on the diffusive part is governed by the same factors that determine the kernel. We find that the fast diffusive mode (small relaxation time) affects the rate of ET reactions with high barriers, while the slow diffusive part (large relaxation times) influences the rate when the barriers are low. Quantum corrections to these effects are calculated using the semiclassical approximation. The theory is used to analyze the ET kinetics of betaine-30 in glycerol triacetate (GTA) over a 100° temperature range and the influence of the details of solvent dynamics on the rates of electron transfer is elucidated. An appendix discusses improved saddle point approximations for the rates of electron transfer reactions calculated using the golden rule.
Dissipative vibrational dynamics in a curve–crossing system
Authors: Kühn O. , May V. , Schreiber M.
Published: July 2002

The density matrix theory is utilized for the description of ultra fast optical properties and related vibrational wave packet dynamics of molecular systems in condensed media. As an example, optically induced vibrational wave packets in the so-called curve–crossing system are considered. Such a system goes beyond the standard treatment of optical phenomena since the vibrational wave packet moves in a double well potential and is subject to environmental influences like wave function dephasing and relaxation. The complete theoretical description has been carried out in a representation of the vibrational wave functions of the diabatic states which refer to the two coupled vibrational surfaces. Solving the corresponding density matrix equations by numerical methods allows us to incorporate the static coupling between the crossed surfaces in a nonperturbative manner. Standard projection operator technique is used to treat environmental contributions up to the second order. For the case of a bilinear coupling between the molecular system and the environment we determine the time development of an initially prepared Gaussian wave packet. Corresponding time-resolved spectra of a pump–probe configuration are also derived. The developed formalism is finally applied to the system of the coupled electronic states c′ 1Σ+u and b′ 1Σ+u of the N2 molecule.
Velocity relaxation of hot O(1D) atoms by collisions with rare gases, N2, and O2
Authors: Matsumi Yutaka , Shamsuddin Sayed Mohammed , Sato Yoshihiro , Kawasaki Masahiro
Published: July 2002

Speed and angular relaxation processes induced by collisions with He, Ar, Ne, Xe, N2, and O2 for the velocity of superthermal O(1D) photofragments have been studied by measuring the Doppler profiles of O(1D) as a function of the time delay between photolysis and probe laser pulses. The nascent O(1D) atoms, generated in the photodissociation of O2 by linearly polarized light at 157 nm, have a narrow kinetic energy distribution, centered at 9.8 kcal/mol and a large angular anisotropy. The O(1D) atoms are probed with a tunable vacuum-ultraviolet laser at approximately 115.2 nm. The time evolution of the speed distribution is extracted from the Doppler profiles for each bath gas. The anisotropy parameter distribution as a function of the speed at each time delay is also obtained. The distributions obtained indicate that (a) a collision with He reduces the speed of the O(1D) atom efficiently but only changes slightly the direction of its velocity and (b) a collision with Ne or Ar reduces its speed efficiently and almost randomizes the direction of the O(1D) atom velocity.
An accurate equation of state for fluids of linear homonuclear fused hard spheres
Authors: Maeso M. J. , Solana J. R.
Published: July 2002

A model relating the equation of state of linear homonuclear fused hard sphere fluids to the equation of state of the hard sphere fluid is derived from the pressure equation. The equation of state reproduces simulation data practically within their accuracy for diatomic and linear triatomic hard molecular fluids.
Laser-perturbed nuclear magnetic resonance spectroscopy and the conservation of parity
Authors: Harris Robert A. , Tinoco Ignacio
Published: July 2002

The effect of polarized incident light on NMR spectra is evaluated; shifts in energy levels and changes in population are calculated. For unoriented systems there is no effect of the amplitude of the field of the light on NMR spectra. The effect of light intensity on NMR spectra produces negligible changes in chemical shifts; fractional changes of less than 10−6 are calculated for light intensities of 10 W cm−2. The fundamental concept of conservation of parity is used to obtain general conclusions about the effect of light on NMR spectra of optical isomers. The NMR spectrum of a molecule in right circularly polarized light must be the same as its mirror image molecule in left circularly polarized light. A recent report [Warren et al., Science 255, 1683 (1992)] gives experimental results which are not consistent with any of these conclusions.
Microsolvation of aromatic molecules by argon: Structure of fluorene–(Ar)2 from high-resolution ultraviolet spectroscopy
Authors: Sussmann R. , Zitt U. , Neusser H. J.
Published: July 2002

We present the high-resolution UV spectrum of the 000(S1←S0) transition in fluorene–(Ar)2 recorded by mass-selective resonance-enhanced two-photon ionization with a pulsed narrow-band laser system (Δν=140 MHz). The measured b-type rotational structure of the strongest band at 33 692.51 cm−1 in fluorene–(Ar)2 allows the exclusion of a (2‖0) structure for this band with the two Ar atoms placed on one side of the substrate. The observation of a b-type rotational transition is proof of a specific opposite sided (1‖1) structure with the two Ar atoms placed above (and below) the central five-membered ring and one remaining sign ambiguity concerning the short in-plane axis coordinate.
Thermal recovery and spectral diffusion of photochemical holes in polymorphic systems
Authors: Schellenberg P. , Friedrich J. , Kikas J.
Published: July 2002

We present comparative investigations of doped benzophenone in its glass and crystal phases by using thermal cycling hole burning techniques. Chlorin and s-tetrazine were used as probe molecules. We measured the distribution of reactive barriers and the spectral diffusion behavior in both phases. The distribution in the glass is broad and is well described by a Gaussian. In the crystal, there are rather well-defined barriers, yet they are different for different sites. The respective kinetics is exponential and its temperature dependence is Arrhenius-like in contrast to the glass phase. Thermally induced spectral diffusion broadening in the glass phase is strong and shows the pattern well known from other organic glasses. In the crystal, it is much weaker, yet it is not completely absent. The experiments seem to indicate a threshold temperature ∼40 K.
A cage model of liquids supported by molecular dynamics simulations. II. The stochastic model
Authors: Polimeno Antonino , Moro Giorgio J.
Published: July 2002

The cage variables describing solute–solvent interactions have been introduced in part I of the work. Moreover, the equilibrium distribution and the relaxation times for both solute and cage variables were derived from the analysis of a molecular dynamics (MD) simulation of liquid argon. In the second part of the work, a stochastic model for the cage is developed on the basis of these informations. The model is characterized by a particular choice for the set of independent stochastic variables and for the evolution operators describing each elementary process. Accurate solutions are derived numerically, while analytical solutions are obtained by separating the cage frame rotations from fast variables like the solute velocity. The calculation of correlation functions allows the comparison with MD results. A substantial agreement is found except for the displacement between solute and cage center. The examined cage model, because of its simple structure, allows a straightforward analysis of the effects of the solvent cage and, in particular, of the distribution of cage frequencies describing the dispersion in the strength of solute–solvent interactions.
Theoretical study of the electronic states of AlB
Authors: Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

The singlet, triplet, and quintet states of AlB below about 30 000 cm−1 are studied theoretically to facilitate spectroscopic investigations and for comparison with analogous calculations on the Al2 and B2 molecules. The ground state of AlB is X 3Σ− with a dissociation energy of 1.96±0.06 eV. The A 3Π state is computed to lie only 610 cm−1 above the ground state. Since transitions from the (2)3Σ− state to both the X 3Σ− and A 3Π states are predicted to be relatively strong, these transitions in the region of 17 000–18 000 cm−1 should be an excellent means of characterizing AlB and of determining the X−A separation. The adiabatic ionization potential to form the X 2Σ+ ground state of AlB+ is estimated to be 7.05 eV. Overall the spectroscopy of AlB is much more similar to Al2 than B2.
Molecular dynamics simulation of molten silica at high pressure
Authors: Vessal B. , Amini M. , Akbarzadeh H.
Published: July 2002

Molecular dynamics simulation of molten silica has been undertaken for an extensive range of pressures using a recently derived potential model. Linear behavior is observed, in plotting (Z−1)V2 vs density square, which is consistent with experimental results on compressed fluids. An increase in the medium range order and the number of higher coordinated silicons and oxygens is observed under pressure.
On the Monte Carlo method for sticky disks
Authors: Borštnik B.
Published: July 2002

It is shown that the finding of Stell [J. Stat. Phys. 63, 1203 (1991)] that sticky systems are not thermodynamically stable has for the consequence that custom tailored Monte Carlo methods, which sample the configurational phase space in an apparently successful way, break down if the methods are generalized in such a way that several particles instead of one particle are moved simultaneously to new position. Due to diverging integrals which are supposed to determine the probability of clusters with maximal connectivity, one can expect the system would always proceed toward the state of maximal degree of aggregation without respect to the strength of the potential.
Rotational excitation in scattering of hyperthermal NO from Pt(111)
Authors: Wiskerke A. E. , Taatjes C. A. , Kleyn A. W. , Lahaye R. J. W. E. , Stolte S. , Bronnikov D. K. , Hayden B. E.
Published: July 2002

Rotational excitation of NO scattered from Pt(111) has been measured for incoming energies from 0.3 to 1.6 eV. For an initial energy of 0.3 eV a clear rotational rainbow, which we assign to O-end collisions, is visible at superspecular exit angles. This is very surprising, since NO–Pt(111) is a chemisorption system with a binding energy of 1 eV. Sharp, pronounced rainbows are visible in the range of incoming kinetic energies where the initial sticking coefficient is high. For an initial energy of 0.3 eV the initial sticking coefficient is 0.9, and at this energy the clearest rotational rainbow is observed. In contrast, at an initial energy of 1.6 eV the sticking coefficient is 0.5 and no rotational rainbow is observed. At subspecular exit angles the distributions are indistinguishable from a Boltzmann distribution at all the energies investigated, and show a clear energy dependence. Boltzmann-type distributions at high incident energy indicate a thorough redistribution of the available energy, although they cannot be explained in terms of a simple statistical model. The scattering results are interpreted as indicating a competition between direct scattering from the repulsive wall and indirect scattering via the deep potential well. Most of the direct scattering can be assigned to O-end collisions with the surface, where the binding energy is expected to be much smaller. Indirect scattering becomes more important at higher energies as more molecules are able to escape the well promptly.
‘‘Dynamical’’ versus ‘‘statistical’’ rotational distributions in hyperthermal NO–Pt(111) scattering
Authors: Taatjes C. A. , Wiskerke A. E. , Kleyn A. W.
Published: July 2002

Rotational distributions from NO–Pt(111) scattering have been reported [Wiskerke et al., J. Chem. Phys. 102, 3835 (1995)]. At lower incoming energies (<1 eV) clear rotational rainbows are seen, but the distributions for higher energies approach Boltzmann distributions with apparent temperatures far exceeding the surface temperature. We compare here the NO–Pt(111) scattering distributions to the predictions of a simple statistical model. The model assumes randomization of the available energy, subject to (partial) conservation of parallel linear momentum and angular momentum about the surface normal. Some characteristics of the rotational and angular distributions which arise from such a statistical energy repartitioning are discussed and compared to experimental results. It is seen that a combination of peaked angular distributions and Boltzmann-type rotational distributions independent of the scattering angle are reproduced by a simple statistical calculation with partial conservation of parallel linear momentum. For the NO–Pt(111) system, it is shown that a complete description of the high-energy scattering requires specifically dynamical assumptions. The transition from ‘‘dynamics’’ to ‘‘statistics’’ most likely arises from a combination of increased averaging, resulting from a competition between scattering via different regions of the potential energy surface, and a weakening of the rainbow features, perhaps due to the onset of chattering collisions.
Free jet infrared spectroscopy of (28SiF4)2 in the 10 μm region
Authors: Urban R.-D. , Takami M.
Published: July 2002

The rotation–vibration spectra of 28SiF4 dimer have been studied near the ν3 band of monomer 28SiF4. A well-resolved parallel band has been observed 17.5 cm−1 below the monomer band origin. Two perpendicular bands, separated by 2.0 cm−1 and located 4.3 cm−1 above the origin for the lower component, show equidistant Q-branch peaks as only resolved lines. The observed band structure indicates that the threefold degeneracy of the vibrational motions in SiF4 is all removed. A C2h structure with a pair of SiF bonds in an antiparallel position is proposed.
Formation of NO(A 2Σ+) by the neutralization reaction between NO+ and SF−6 at thermal energy
Authors: Tsuji Masaharu , Ishimi Hiroaki , Nakamura Masafumi , Nishimura Yukio , Obase Hiroshi
Published: July 2002

An optical spectroscopic study has been made of the ion–ion neutralization reaction between NO+(X 1Σ+:v″=0) and SF−6 in the flowing afterglow. Only the NO(A 2Σ+–X 2Πr) emission from v′=0 was excited, indicating that no energy is deposited into the vibration of NO(A). The rotational distribution of NO(A:v′=0) was expressed by a single Boltzmann rotational temperature of 600±50 K. The average fraction of the total available energy deposited into rotation of NO(A) was evaluated to be only 1.9%. Most of all excess energy was expected to be partitioned into translation of the products due to a strong mutual Coulombic attractive force between NO+ and SF−6. The observed vibrational and rotational distributions were less excited than statistical prior ones, indicating that the reaction dynamics is not governed by a simple statistical theory. The mechanism of the selective excitation of NO(A) in the ion–ion neutralization reaction was discussed.
VB resonance theory in solution. II. I2−■I+I− in acetonitrile
Authors: Bianco Roberto , Hynes James T.
Published: July 2002

The electronic structure in solution theory developed in the preceding article is applied to the molecular ion I2−■I+I− reaction system in the dipolar, aprotic solvent acetonitrile, which illustrates in detail the implementation of the general theory. A two-dimensional, nonequilibrium free energy surface in the nuclear separation and a difference solvent coordinate is constructed via solution of a nonequilibrium solvation, nonlinear Schrödinger equation. The reduction to a single important solvent coordinate—from a manifold of three solvent coordinates—is motivated by an examination of the equilibrium solvation path and an analysis of the harmonic nonequilibrium fluctuations around this path. The evolving solute electronic structure over the basis of two orthogonal valence bond diabatic states—approximately corresponding to −II and II−—is discussed. Comparisons with the limiting Born–Oppenheimer and self-consistent approximations for the solvent electronic polarization are made, with the former proving to be more accurate, and the latter giving a qualitatively inaccurate picture of the electronic structure near the equilibrium geometry. The validity of the dielectric image approximation is also examined. The polarization force associated with the charge shift in the reaction system and important for the system vibrational relaxation is also calculated.
Activated surface diffusion: Are correlated hops the rule or the exception?
Authors: Georgievskii Yuri , Kozhushner Mortko A. , Pollak Eli
Published: July 2002

The migration of adsorbed atoms on crystal surfaces is considered. To describe the adatom motion one often uses a generalized Langevin equation (GLE). The time-dependent friction, which enters the GLE, is caused by the interaction with the crystal excitations. However, the explicit form of the time-dependent friction is not well known. We show that if the damping is associated with acoustic phonons and the coupling of the adatom with the surface is not too strong, then the friction is Ohmic. An explicit expression for the friction coefficient is given in terms of the basic physical parameters of the crystal and in terms of the activation energy of the adatom on the surface. We find that usually the diffusion occurs in the intermediate damping regime. In this regime nearest neighbor hops are most probable and transition state theory gives an excellent estimate for the rate of escape and for the diffusion coefficient. Using the recently developed turnover theory for surface diffusion [Phys. Rev. E 49, 5098 (1994)] we derive explicit expressions for the evolution of the time-dependent site distribution and compare it with an experiment where correlated hops have been observed. We conclude that even when the motion is one-dimensional, correlated hops are to be expected only for sufficiently high temperatures or for physisorbed atoms.
Classical trajectories on simple model potentials for N2–Kr: Comparison with relaxation and other data
Authors: ter Horst Marc A. , Jameson Cynthia J.
Published: July 2002

We compare the ability of six N2–Kr potential energy surfaces to predict experimental interaction second virial coefficients, diffusion coefficients, mixture viscosity, thermal conductivity, and nuclear magnetic resonance (NMR) rotational relaxation cross sections. These include a previously published empirical surface derived from fits to molecular beam experiments and various model potentials of the Tang and Toennies (TT) type. The TT type potentials differ in the set of dispersion coefficients employed. Two sets are obtained from published ab initio calculations, another from combining rules and one from empirical considerations. The repulsive parameters have been obtained from published results of a charge overlap combining rule. A variation of the TT model suggested by Aziz is also used to further investigate the effect of the repulsive wall anisotropy on the rotational relaxation cross sections. Forty-five effective cross sections that determine the bulk transport and relaxation phenomena have been calculated by classical trajectories for temperatures ranging from 100 to 800 K. The sensitivity of the NMR-derived cross sections to the various characteristics of the anisotropy of the potential (such as the anisotropy in the well depth, in the high repulsive wall, in the low repulsive wall, and at V=0) are examined. The empirical anisotropic LJ(12,6) surface of Rotzoll provides the best agreement with the diffusion, viscosity, thermal conductivity, and NMR relaxation experimental results.
Free and pendular-like rotation: Orientational dynamics in hard ellipsoid fluids
Authors: Tang Shuang , Evans Glenn T.
Published: July 2002

Rotational motions in molecular fluids may exhibit a coherence arising from collision-free propagation or from highly correlated collisions. Information on both of these behaviors are derived from the analysis of orientational and angular momentum relaxation. In the present study, the rotating body is taken to be a hard ellipsoid, immersed in a hard sphere fluid, and its dynamics are analyzed within a framework of kinetic theory. When the fluid bath is dilute, collisions are regarded as uncorrelated events and orientational relaxation has elements of the Fokker–Planck and Gordon diffusion models. When the correlated collisions are dominant, the rotational dynamics are frustrated. In this limit the rotational diffusion coefficient vanishes, the hard sphere shear viscosity diverges and their product is well behaved and yields a relationship in close accord to the Stokes–Einstein–Debye relation.
Comparison between molecular orbital and surface integral calculations of the exchange energy for the homonuclear dimer ions He+2, Li+2, and Be+2
Authors: Chang T. C. , Tang K. T.
Published: July 2002

The exchange energy, defined as half of the energy splitting of the gerade and ungerade states of a molecular ion, is investigated by molecular orbital calculations and by the surface integral method. Numerical results of He+2, Li+2, and Be+2 are obtained through restricted Hartree–Fock, unrestricted Hartree–Fock and localized Heitler–London type calculations. The results of the surface integral method are analytical expressions which depend only on the ionization energy. While the approaches are completely different, the results are generally in good agreement. Discrepancies are highlighted with the hope of promoting further interplay of these complementary methods.
Chain correlations in symmetric binary polymer blends
Authors: Garas George E. , Kosmas Marios K.
Published: July 2002

The many chain problem of two polymers is studied through the summation of the series of the one-loop diagrams between different polymer chains. The free energy of the polymer blend is found revealing that chain correlations affect the phase separation behavior of polymer chains of finite lengths N. The parameter χeff related with the scattering form factor is found to overestimate the composition and molecular weight dependence of the strength of the interactions of the two different polymers expressed by the generalized Flory–Huggins parameter χ. The composition dependences of both parameters are affected by the proximity to the critical region in accord with recent neutron scattering experiments and in the manner Monte Carlo simulations indicate. These dependences become stronger as the system comes close to the critical point either from the one phase or from the two phase region. The dependence of χeff on the molecular weight is not monotonic and is related with the composition of the system. The critical value of χ is found to present a positive deviation of the order of N−1/2 from the mean field prediction taking the classical value χc=2/N only at the limit of large molecular weights.
Double-resonance spectroscopy of the high Rydberg states of HCO. I. A precise determination of the adiabatic ionization potential
Authors: Mayer Eric , Grant Edward R.
Published: July 2002

We report the first spectroscopic observation of the high Rydberg states of HCO. Individual lines in a system of vibrationally autoionizing Rydberg series converging to the (010) state of HCO+ are rotationally labeled in a double-resonance excitation scheme that uses resolved levels in the (010) A′ vibronic component of the 3pπ 2Π Rydberg state as intermediates. Observed high-Rydberg structure extends from the adiabatic ionization threshold—which falls just below the principal quantum number of 12 in the vibrationally excited series—to the (010) vertical threshold. Elements of a single series extending from n=12 to 50, for which the total angular momentumless spin can be assigned as N=1, are extrapolated to obtain a vertical convergence limit with respect to the 3pπ 2Π(010)A′ N′=0 intermediate state of 20 296.9±0.3 cm−1. Referring this transition energy to the ground state, and subtracting the precisely known fundamental bending frequency of the cation, establishes the adiabatic ionization potential corresponding to the transition from HCO 2A′(000) J″=0, K″=0 to HCO+ J+=0 1Σ+(000). The result is 65 735.9±0.5 cm−1 or 8.150 22±0.000 06 eV.
Thermal electrons in liquids with high polarizability
Authors: Atrazhev V. M. , Iakubov I. T.
Published: July 2002

The density dependence of the excess electron mobility μ(N) and the ground state energy V0(N) are described within the framework of the pseudopotential theory for electrons in liquids with high polarizability. The theory is based on the experimentally known parameters only such as the scattering length of an isolated atom, the atomic polarizability and the radial distribution function of a liquid. Both μ(N) and V0(N) as functions of the liquid density are calculated for liquid Ar, Kr, and Xe. The calculated nonmonotonic μ(N) and V0(N) dependencies are compared with experimental data.
Cluster–surface interaction at high kinetic energy. I. Electron emission
Authors: Hendell E. , Even U.
Published: July 2002

The electron emission resulting from a collision of accelerated clusters and a solid surface was measured. The charged, mass selected clusters were accelerated to high velocities (3–100 km/s) in a linear accelerator. The quantum yield for electron emission as well as their kinetic energy distribution were measured. These measurements allow an estimate of kinetic to internal energy conversion.
Spectral signatures of critical transition structures derived from temperature-dependent IR spectra
Authors: Hurst Scott M. , Wilcox C. F. , Bauer S. H.
Published: July 2002

We describe a modified configuration of the temperature drift (TD) technique for recording infrared spectra that arise from vibrationally excited states. TD is a differential measurement that records decrements in IR absorptions due to modest increases in temperature. In particular, it permits identification of oscillators that are coupled to large amplitude, low frequency motions associated with transition structures, such as transitions to free rotations about single bonds that are hindered at ambient temperatures. Selected portions of the spectra of two species were recorded to demonstrate the feasibility of locating spectral features of critical transition structures.
The quasi-minimal residual algorithm applied to complex symmetric linear systems in quantum reactive scattering
Authors: Karlsson Hans O.
Published: July 2002

The solution of systems of linear equations Ax=b with complex symmetric coefficient matrix A of size N, typically appearing in quantum-reactive scattering problems, is discussed. The quasiminimal residual (QMR) method is introduced to solve the complex symmetric linear system and is compared to the generalized minimal residual (GMRES) method. The methods are applied to two different chemical problems: the initial state-selected reaction probability for the H2+OH→H +H2O reaction, and the cumulative reaction probability for the isomerization of ketene, both with N≳104. It is shown that the QMR method behaves more favorably, i.e., converges faster, than the GMRES for large N, especially when high accuracy is needed.
Orbital alignment during cage-exit of open-shell photofragments: F in solid Ar and Kr
Authors: Kizer K. S. , Apkarian V. A.
Published: July 2002

The statistical theory for sudden cage-exit [J. Zoval and V. A. Apkarian, J. Phys. Chem. 98, 7945 (1994)] is extended to orbitally degenerate photofragments, specifically treating the case of F atoms in solid Ar and Kr. It is shown that the experimental energy-dependent quantum yields of photodissociation of F2 are only compatible with the p hole on the F atom being completely aligned parallel to the cage wall during the sudden exit. Although relative quantum yields and energy thresholds are well predicted, the calculated absolute quantum yields are a factor of ∼2 smaller than the experimental values.