bluuebunny/crossref_metadata_2025 huggingface dataset:
Complex trajectory method in semiclassical propagation of wave packets
Authors: Boiron Marc , Lombardi Maurice
Published: July 2002

We propose a semiclassical wave packet propagation method relying on classical trajectories in a complex phase space. It is based on the Schrödinger wave equation and the usual expansion with respect to ℏ, except that the amplitude of the wave packet is taken into account at the very zeroth order, unlike in the usual WKB method where it is treated as a corrective or first order term. Formally, it amounts to making both the wavelength and the width of the wave packet tend to zero with ℏ. The action and consequently the classical trajectories derived are complex. This method is tested successfully in many cases, analytically or numerically, including the bounce and even the splitting of the wave packet. Our method appears to be much more accurate than the WKB method while less computationally demanding than the Van-Vleck formula. Moreover, it has a particularly interesting property: the singularities (caustics) of the usual semiclassical theories do not appear in this formalism in all cases tested.
Kinetic theory of band shapes in molecular spectra of gases: Application to band wings
Authors: Filippov N. N. , Tonkov M. V.
Published: July 2002

A kinetic theory of spectral band shapes is developed which accounts for the effects of line mixing and finite duration of collisions. The theory is based on a projection operator technique with a nonorthogonal metric. The roles and hierarchy of various approximations commonly employed are analyzed. It is shown that the relaxation operator must be defined before the initial correlations are suppressed so that the double sum rule for the matrix elements of this operator is conserved. This conservation is important for the calculations of the wings of band shapes. Rotovibrational band shapes for some specific cases of Raman and infrared absorption spectra of gases are considered. The expressions obtained relate the band wing profiles with the spectral density of molecular torque arising from binary collisions.
Clear-cut experimental observation of the capillary wave to elastic wave crossover in concentrated polymer solutions
Authors: Huang Q. R. , Wang C. H. , Deng N. J.
Published: July 2002

A detailed experimental study of the surface laser light scattering (SLLS) spectrum of the polyisobutylene (PIB)/decane solution over a wide range of polymer concentration is carried out. In the dilute solution, the SLLS spectrum is found to be described adequately by a scattering theory from a capillary wave. As the polymer concentration increases, the solution viscoelasticity is found to play an important role in affecting the dynamics of surface waves. By comparing experiment with theory, a clear-cut capillary wave to elastic wave evolution in the PIB/decane solution is found. The nature of the quasielastic peak that appears in the SLLS spectrum of the viscoelastic polymer solution is delineated.
Cooperativity in binding of proteins to DNA. II. Binding of bacteriophage λ repressor to the left and right operators
Authors: Ben-Naim A.
Published: July 2002

In continuation of part one [J. Chem. Phys. 107, 10242 (1997)] we further examine the sources of ligand–ligand correlations in binding of proteins to DNA. We present four reasons for expecting that, in contrast to the ligand–ligand interactions, the correlations between ligands in these systems can be long ranged and nonadditive. In view of these findings we reexamine a specific set of experimental data on the binding of λ repressor to the corresponding operators.
Coupled cluster response calculations of two-photon transition probability rate constants for helium, neon and argon
Authors: Hättig Christof , Christiansen Ove , Jo/rgensen Poul
Published: July 2002

The implementation of two-photon transition moments is described for the hierachy of coupled cluster models CCS, CC2 and CCSD and applications are reported for the two-photon transition probability rate constants for transitions from the ground state to the first excited Se1 state in helium and to the lowest De1 and Se1 states in neon and argon. At the CCSD level the rate constant for the Se1 transition in argon is in good agreement with the experimental rate constant and also with a recent MCSCF calculation.
Calorimetric study of proton tunneling in solid 5-bromo-9- hydroxyphenalenone and deuteration-induced phase transitions in its deuteroxy analog
Authors: Matsuo Takasuke , Kohno Kenichi , Inaba Akira , Mochida Tomoyuki , Izuoka Akira , Sugawara Tadashi
Published: July 2002

The heat capacities of 5-bromo-9-hydroxyphenalenone (BHP) and its deuteroxy derivative (BDP) were measured at temperatures between 2 and 310 K. The heat capacity of BHP is a smooth function of temperature and that of BDP has two peaks at 21.3 and 33.9 K. By analyzing the data on BHP, a tunnel splitting of (64±2) cm−1 associated with the quantum mechanical motion of the hydrogen atom in the intramolecular O–H–O hydrogen bond was derived along with the Debye temperature 60.8 K and two Einstein temperatures 131.4 (nondegenerate) and 210.4 K (doubly degenerate). The enthalpy change of 225 J mol−1 and entropy change of 6.8 J K−1 mol−1 were determined for the total thermal effects associated with the two phase transitions in BDP. The value of the transition entropy is consistent with the twofold disorder in the high temperature phase. The tunneling energy and transition enthalpy satisfy an inequality demanded, on the assumption that the potential energies experienced by the proton and deuteron are the same, by the occurrence in BDP and nonoccurrence in BHP of the phase transitions.
Influence of the solvent on the conformation of a chain molecule
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

Effects of the solvent on the conformation of a polymeric chain molecule are examined by using a set of polymer–solvent integral equations for correlation functions for the polymer and the solvent. Solutions of the integral equations are used for computing the polymer–solvent distribution, chain conformations, and scaling properties associated with polymer swell and collapse in good and poor solvents. The variation of chain properties with the solvent density and the solvent quality is examined for chains having up to 100 bonds.
The (1)1Πg state of 39K2 revisited
Authors: Russier I. , Aubert-Frécon M. , Ross A. J. , Martin F. , Yiannopoulou A. , Crozet P.
Published: July 2002

Vibrational levels up to v=107 of the (1)1Πg state of 39K2, which dissociates to K(4s)2S1/2+K(4p)2P3/2, have been observed in high-resolution Fourier transform records of C 1Πu→(1)1Πg fluorescence. From a revised analysis of this system, we have constructed a rotationless Rydberg–Klein–Rees (RKR) potential curve which extends to R=40.3 Å. Vibrational and rotational constants have been calculated from this curve. The parameters describing the Coulombic interactions between K(4s)+K(4p) atoms are given in a Hund’s case (c) basis. From this work, the dissociation energy De for the (1)1Πg state is found to be 1290.30±0.02 cm−1, leading to a new value of Te=16 203.25±0.09 cm−1 for this state.
Temperature, pressure, and perturber dependencies of line-mixing effects in CO2 infrared spectra. II. Rotational angular momentum relaxation and spectral shift in Σ←Σ bands
Authors: Rodrigues R. , Boulet C. , Bonamy L. , Hartmann J. M.
Published: July 2002

The Energy Corrected Sudden approach is used in order to deduce collisional parameters and to model infrared quantities in Σ-Σ bands of CO2-He and CO2-Ar mixtures at room temperature. Measurements are first used for the determination (from a fit) of the rotational angular momentum relaxation time and of some parameters representative of the imaginary part of the relaxation operator. It is shown that line-broadening data as well as absorption in both the wing and central part of the ν3 and 3ν3 bands lead to consistent determinations. The model is then used for detailed analysis of line-mixing effects. The influences of pressure, of the band spectral structure, and of the collision partner are studied. Differences between the effects of collisions with He and Ar are pointed out and explained.
Complete binary collision approximation for the gas transport coefficients via the time correlation formulation
Authors: Alavi Saman , Snider R. F.
Published: July 2002

The connection between the low density limit of the time correlation equations for the transport coefficients and the solution of the Boltzmann equation to lowest order approximation appear to have been made in essentially two different ways. Either the time correlation function is evaluated by using the time dependent (linearized) Boltzmann equation, or by utilizing a resummation of an expansion in the reciprocal of a convergence parameter. As well, the connection is often made to only the lowest order solution of the Boltzmann equation, ignoring the possible importance of higher order moments (Sonine polynomials) in the solution of the Boltzmann equation. The present work uses a projection operator method and a subsequent binary collision expansion of the time correlation function to retain all contributions to the transport coefficient from binary collisions. This explicitly avoids an expansion in a divergent parameter and reproduces all Sonine polynomial contributions to the transport coefficient. Gas transport coefficients for a binary mixture are obtained in a similar manner.
Vibrational corrections to static and dynamic hyperpolarizabilities of pure liquids: Calculations on methanol
Authors: Norman Patrick , Luo Yi , Ågren Hans
Published: July 2002

The methanol molecule is used as a test case for studying the simultaneous action of solvation and vibrational effects on static and dynamic hyperpolarizabilities within the reaction field model. The calculations indicate that the medium can play a significant role for the vibrational correction. The calculated third-order optical nonlinear susceptibilities of methanol in gas and liquid phases are in fair to good agreement with their experimental counterparts.
Shear induced diffusive mixing in simulations of dense Couette flow of rough, inelastic hard spheres
Authors: Zamankhan Piroz , Tafreshi Hooman Vahedi , Polashenski William , Sarkomaa Pertti , Hyndman Caroline L.
Published: July 2002

Large-scale numerical simulations of a system of inelastic, rough, hard spheres of volume fraction φs=0.565, which are initially distributed randomly in a Couette geometry, show clear evidence of the movement of the particles in directions transverse to the bulk motion. This behavior of the aforementioned system, which has been considered as a model for a granular fluid, is consistent with recent experimental observations [Hsiau and Hunt, J. Fluid Mech. 251, 299 (1993)]. Based on the results obtained, there are indications that a bounded rapid granular flow could be a diffusive system at volume fractions even higher than 0.56. This finding contradicts earlier computer experiments [Campbell, J. Fluid Mech. 348, 85 (1997)] which found a rapidly flowing granular material is a diffusive system except at large solids concentrations (i.e., φs>0.56).
Coupling-constant dependence of the density functional correlation energy
Authors: Joubert Daniel P. , Srivastava G. P.
Published: July 2002

Coupling-constant integration of the mutual Coulomb interaction of a many-electron system formally gives the density functional exchange-correlation energy. The exchange-energy scales linearly with the coupling-constant, but the dependence of the correlation energy on the coupling-constant is not known. We present accurate numerical results for the coupling-constant dependence of the density functional correlation energy for two-electron systems. For the systems studied, we find that the coupling-constant dependence of the correlation energy can be represented accurately by a cubic polynomial or a two parameter Padé approximation for coupling constants between zero and one. We also find that the recently published approximate correlation energy of Perdew, Burke and Ernzerhof [Phys. Rev. Lett. 77, 3865 (1996)] follows the exact dependence remarkably well.
Energy corrected sudden calculations of linewidths and line shapes based on coupled states cross sections: The test case of CO2–argon
Authors: Thibault F. , Boissoles J. , Boulet C. , Ozanne L. , Bouanich J. P. , Roche C. F. , Hutson J. M.
Published: July 2002

The accuracy of the energy-corrected sudden (ECS) formalism for line shape calculations is investigated, using coupled states calculation for CO2–Ar collisions on the recently developed “single repulsion” potential of Hutson et al. [J. Chem. Phys. 107, 1824 (1997); 105, 9130 (1996)]. Inelastic cross sections σ0(L→0,E)≡QL′(E) are calculated using the MOLSCAT program, and then averaged over Maxwell–Boltzmann kinetic energy distributions to give the thermally averaged “basic rates” QL′(T) needed in the ECS formalism. The ECS linewidths for low initial J, Ji⩽16, are sensitive only to the low-L basic rates, for which the CS calculations are converged; comparing them with directly calculated CS linewidths thus gives a stringent test of the ECS model, and it works well (within 10%). However, for higher Ji lines and for band shape calculations, basic rates for higher L are needed for convergence. These are obtained by an extrapolation procedure based on experimental data, using an exponential power law and the adiabaticity factor recently suggested by Bonamy et al. [J. Chem. Phys. 95, 3361 (1991)] ECS calculations using the resulting basic rates are designated “extrapolated CS-ECS calculations,” and are found to give accurate results for high-J linewidths, for near-wing absorption and for band profiles over a very wide range of perturber pressures (up to 1000 atm).
Three-dimensional simulation of hexagonal phase of a specific polymer system under shear: The dynamic density functional approach
Authors: Zvelindovsky A. V. M. , van Vlimmeren B. A. C. , Sevink G. J. A. , Maurits N. M. , Fraaije J. G. E. M.
Published: July 2002

The first three-dimensional (3D) simulation of meso-phase formation in a specific polymer system—55% aqueous solution of the triblock polymer surfactant (EO)13(PO)30(EO)13—under simple steady shear is performed. The method is based on dynamic mean-field density functional theory. The hexagonal phase is investigated. The simulations reproduce recent experimental observations on the same polymer system.
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Studies of low-temperature electronic energy transfer and nuclear spin-state changing collisions
Authors: Van Marter Todd , Heaven Michael C.
Published: July 2002

Energy transfer between I(2P1/2) and O2(X) was examined in a Laval nozzle expansion at temperatures near 150 K. The rate constant for electronic energy transfer was found to be kb(150)=(7.0±0.7)×10−12 cm3 s−1, which was a factor of 1.5 greater than a previous low-temperature determination [T. Van Marter, M. C. Heaven, and D. Plummer, Chem. Phys. Lett. 260, 201 (1996)]. Laser induced fluorescence (LIF) detection of I(2P1/2) via the 5p46s(2P3/2)−5p5(2P1/2) transition was demonstrated. The 5p5(2P1/2)F=2 and F=3 hyperfine sublevels were resolved in the LIF spectrum, permitting observation of the kinetics associated with transfer between F=2 and 3. I(2P1/2) was generated by 498 nm photolysis of I2. The nascent population distribution of the hyperfine levels was found to be nonstatistical, with N(F=2)/N(F=3)=1.3±0.1. Collisions with O2(X) caused F=3⇔F=2 transfer. Analysis of the F=2 and 3 decay kinetics yielded hyperfine transfer rate constants of k(2⇒3)=(2.6±0.3)×10−11 and k(3⇒2)=(1.9±0.2)×10−11 cm3 s−1 (T=150 K). Both the electronic energy transfer and the hyperfine transfer rate constants are of relevance to supersonic oxygen iodine lasers, as these devices operate on the P1/22F=3⇒2P3/2F=4 transition, in the presence of a relatively high pressure of O2.
Quantum control of nuclear wave packets by locally designed optimal pulses
Authors: Ohtsuki Y. , Kono H. , Fujimura Y.
Published: July 2002

A new approach to locally design a control pulse is proposed. This locally optimized control pulse is explicitly derived, starting with optimal control formalism, and satisfies the necessary condition for a solution to the optimal control problem. Our method requires a known function, g(t), a priori, which gives one of the possible paths within the functional space of the objective functional. A special choice of g(t)≡0 reduces the expression of the control pulse to that derived by Kosloff et al. For numerical application, we restrict ourselves to this special case; however, by combining an appropriate choice of the target operator together with the backward time-propagation technique, we apply the local control method to population inversion and to wave packet shaping. As an illustrative example, we adopt a two-electronic-surface model with displaced harmonic potentials and that with displaced Morse potentials. It is shown that our scheme successfully controls the wave packet dynamics and that it can be a convenient alternative to the optimal control method for wave packet shaping.
Analysis and transition probabilities of the A 1Σ+→X 1Σ+ system of KH excited by the 4880 Å line of the argon ion laser
Authors: Camacho J. J. , Poyato J. M. L. , Pardo A. , Reyman D.
Published: July 2002

The fluorescence spectrum of KH induced by the 4880 Å line of an argon ion laser has been analyzed. This work extends previous observations on potassium hydride in visible region by using this excitation line. Along with the principal fluorescence series for the A 1Σ+→X 1Σ+ band system, corresponding to the excitation transition, v′=7, J′=6←v″=0, J″=5, we analyzed a very interesting satellite rotational and vibrational structure induced by collision. The radiative transition probabilities for the A 1Σ+→X 1Σ+ band system of KH have been calculated by using hybrid potential energy curves for the X 1Σ+ and A 1Σ+ states and transition dipole moment function from the radiative lifetimes of different vibrational levels (v′=5−22 in the A 1Σ+ state) reported by Giroud and Nedelec. The transition probabilities and lifetimes are in good agreement with the corresponding observed measurements usually within the experimental uncertainty. Collision-induced rotational and vibrational energy transfer in the A 1Σ+ state has been investigated. From the rotational and vibrational satellite structure of some bands, cross sections for rotational and vibrational energy transfer have been determined.
Collision induced deactivation of PH2(Ã 2A1;v2′=1,0) and PH2(X̃ 2B1;v2″=1) by diatomic molecules
Authors: Nguyen Xuan Chieu , Margani Alessandro
Published: July 2002

Deactivation of the bending mode v2′=1 and v2′=0 vibrational levels of PH2(Ã 2A1), and of the v2″=1 level of ground state PH2(X̃ 2B1) due to collisions with the diatomic molecules H2, N2, CO, and NO has been investigated. The Parmenter and co-worker’s, the Thayer and Yardley’s, and the collision complex theories have been used to rationalize the PH2(Ã 2A1) quenching data. Explanations for the deviations of the quenching data due to these molecular quenchers from the variation trend found, in a previous work, for the rare gas quenchers have been proposed. For the vibrational relaxation of PH2(X̃ 2B1;0,1,0), the data interpretation is based essentially on the theories by Schwartz, Slawsky, and Herzfeld–Tanczos, and Moore.
Quantum chemical analysis of the energy of proton transfer from phenol and chlorophenols to H2O in the gas phase and in aqueous solution
Authors: Schüürmann Gerrit
Published: July 2002

Proton transfer energies of phenol and 14 chlorophenols with H2O as a base are analyzed in the gas phase and in solution using quantum chemical methods at the semiempirical and ab initio level of computation. The effect of aqueous solution was accounted for by applying the density functional theory (DFT) implementation of the conductor-like screening model (COSMO) as well as semiempirical continuum-solvation models. The results reveal substantial and systematic overestimations of the free energies of proton transfer as derived from experimental solution-phase pKa data. This can be traced back to both deficiencies in the current model parameterization as well as to limitations of the underlying gas-phase quantum chemical models, which is further illustrated by additional complete-basis-set (CBS) calculations for the proton transfer reaction with phenol. In contrast, the relative pKa trend is reflected well by COSMO-DFT calculations with correlation coefficients (adjusted for degrees of freedom) of 0.96. Decomposition of the dissociation energy in aqueous solution into a gas-phase term and a term summarizing the solvation contributions provides new insights into the effect of solvation on proton transfer energies, and yields mechanistic explanations for the observed differences in the gas-phase and solution-phase acidity orders of various subgroups of the compounds.
Structural instability of the diamond C(111) surface induced by hydrogen chemisorption
Authors: Su C. , Lin J.-C.
Published: July 2002

The low energy electron diffraction technique was used to study the hydrogen chemisorption induced structural instability on the diamond C(111) surface. From the quantitative analysis of diffraction spots intensity on the as-dosed, partially desorbed, and annealed hydrogenated C(111) surfaces, the correlation between the (1×1)↔(2×1) phase transformation, hydrogen coverage, and surface temperature is shown. Thermal treatment with partial hydrogen desorption on the fully hydrogenated C(111) surface induces a (1×1)–(2×1) reconstruction with the observable half-order spots intensity (I1/2) emerging only after heating the substrate to 1270 K. Conversely, thermal annealing of the partially hydrogenated C(111) surface without desorbing H causes the size shrinking of the (2×1) domains as well as the relaxation of the hydrogenated domains. The temperature effect of I1/2 summarized from both thermal studies reveals that the (2×1) domain instability originated from the relaxation of the hydrogenated domains at elevated temperatures. In addition, the H chemisorption behavior on C(111) at different surface temperatures suggests that the terrace edges could be the preferential sites for the initial H adsorption and the growth of the hydrogenated domains might predominantly start from the terrace boundaries at a surface temperature as low as 125 K. The present study also allows us to tentatively propose that there might exist a low-temperature chemisorption state in addition to the hydrogenated metastable state as suggested by the sum-frequency generation spectroscopy and theoretical studies. A possible mechanism for the hydrogen chemisorption induced structural transformation is also discussed.
Calculation of crystal and molecular structures of hexachlorobenzene C6Cl6
Authors: Thiéry M-M. , Rérat C.
Published: July 2002

Crystal and molecular structures of hexachlorobenzene C6Cl6 were investigated by molecular-packing analysis with a computed dynamical model. This model includes thermal motions and molecular deformations. Several crystalline structures with monoclinic symmetry P21/c have been found by the calculation. Factors which influence the stability of these structures are investigated. The lowest-energy structure, labeled I, agrees with the experimental one. Calculated crystal and molecular parameters agree with data determined by x-ray diffraction at 296 K. While no temperature-induced phase transition has been found from 296 to 124 K at ambient pressure, a phase transformation has been evidenced numerically at 0.43 GPa and 296 K. The monoclinic pressure-induced phase, stable above 0.43 GPa, is labeled II.
Polarized fluorescence of polyatomic fragments produced through photodissociation of polyatomic molecules in the gas phase
Authors: Blokhin A. P. , Gelin M. F. , Kalosha I. I. , Polubisok S. A. , Tolkachev V. A.
Published: July 2002

A combined theoretical and experimental study is carried of the polarized emission of polyatomic products produced through photodissociation of polyatomic molecules. A general approach, based on the formalism of dissociation kernels and orientational correlation functions, is developed to predict anisotropy of the fluorescence of photoproducts. We consider the most general case of asymmetric top parent and product molecules. The rotational predissociation effect is taken into account. Various kinds of photoreactions are studied: those when fragments after dissociation are in the electronically excited states and those when fragments are in the ground electronic states so that additional laser pulse is necessary to excite their fluorescence. Particular attention is concentrated on those practically important extreme cases, when predissociation times and lifetimes of the electronically excited states of photoproducts are short or long as compared to the averaged period of free rotation. The steady state polarized fluorescence of radicals produced through dissociation of several disulfides into two identical radicals is measured. The results are interpreted in the framework of the free recoil model (FRM). In this model, photoproducts are assumed to experience no torque and fly apart freely, so that the only origin of the fragment rotation is rotation of the parent molecule. Predictions of the impulsive model (IM), in which fragments are supposed to suffer instantaneous torque due to the rupture of the chemical bonds of the parent molecule, are demonstrated to disagree strongly with our experimental data. This gives an additional confirmation of the validity of the FRM in describing dissociation of polyatomic molecules into polyatomic fragments. The FRM can therefore be invoked to estimate interrelation between the characteristic times, governing the processes of dissociation and emission, and the averaged period of free molecular rotation. Also, the FRM can be used for the determining orientations of the absorption and emission dipole moments in the reference frames of the parent and product molecules.
Infrared transition intensities in acetylene: An algebraic approach
Authors: Abbouti Temsamani M. , Champion J.-M. , Oss S.
Published: July 2002

The two-dimensional algebraic model for degenerate bending modes has been extended to include interaction terms with stretching modes and to provide a detailed description of the dipole operator for linear molecules. An algebraic mixed model is used to investigate, for the first time, the absolute infrared intensities in 12C2H2 up to 10 000 cm−1. Intensities of hitherto unmeasured vibrational bands are also given.
Wave packet dynamics of IBr predissociation
Authors: Hussain Ahmed N. , Roberts Gareth
Published: July 2002

The delayed photofragmentation of IBr following perturbative excitation to the B(0+) state by a sub-100 fs laser pulse has been studied theoretically within a time-dependent framework. The principal aim of this work is to provide an interpretation of time-resolved experiments of IBr predissociation over a range of initial energies [M. J. J. Vrakking, D. M. Villeneuve, and A. Stolow, J. Chem. Phys. 105, 5647 (1996)]. Calculations of the time dependence of individual quasistationary vibrational levels of the B(0+) diabatic potential and B′(0+) adiabatic potential, and coherent superposition states of the diabatic vibrational levels, have been carried out to determine the quantized molecular evolution over intersecting bound and repulsive diabats. It is found that the dissociation probability varies as a function of energy within the B(0+) well, giving vibrational state-specific decays that range from below 1 ps to greater than 12 ps. The vibrational lifetimes are interpreted in terms of the degree of resonance between B(0+) diabatic levels and those of the excited B′(0+) Born–Oppenheimer state that arises from the diabatic curve crossing, expressed via the shapes of the diabatic and adiabatic wave functions in the region of the crossing point. To connect the vibrational dynamics with experiments, 1+2 pump–probe transient ionization signals and the frequency-resolved absorption cross sections have been computed. The former are interpreted in terms of their corresponding power spectra calculated by the maximum entropy method, which reveal the vibrational beat processes responsible for the quasibound time evolution monitored experimentally. An iterative comparison of these calculations with experiment in principle allows the shapes of the diabatic and adiabatic potential curves to be mapped out over a wide energy range from the dissociation asymptote to the diabatic crossing point, and provides a strong indication of the distance variation of the off-diagonal elements of the Hamiltonian matrix that couple the two diabatic excited states.
State-resolved collisional energy transfer in highly excited NO2. II. Vibrational energy transfer in the presence of strong chemical interaction
Authors: Abel Bernd , Lange Norbert , Reiche Florian , Troe Jürgen
Published: July 2002

The state-resolved collisional self-relaxation of highly (optically) excited NO2 (Eint≈18 000 cm−1) in a thermal cell has been probed directly using time-resolved optical double resonance spectroscopy. The thermally averaged state-to-state cross sections have been derived from a master equation analysis of the kinetic traces. Rovibrational energy transfer (intramolecular V–V,V–T,R) was found to be more than an order of magnitude less efficient than pure rotational energy transfer (R–T,R–RT) within a vibrational state. The obtained cross sections for vibrational energy transfer are discussed with respect to the different relaxation mechanisms of the molecule, i.e., direct “fast” relaxation NO2(νi)+NO2→NO2(νf)+NO2 and complex forming collisions NO2(νi)+NO2→N2O4→NO2(νf)+NO2, and compared with high pressure recombination rates k∞. The experiments show that the observed collisions are closer to the impulsive than to the complex forming limit. In addition, we have discussed the magnitude of the experimental relaxation rates in terms of excited state couplings and the influence of vibronic chaos on the relaxation of highly excited NO2.
Spectral diffusion induced by energy transfer in doped organic glasses: Delay-time dependence of spectral holes
Authors: den Hartog F. T. H. , van Papendrecht C. , Silbey R. J. , Völker S.
Published: July 2002

A new effect in doped organic glasses, which we refer to as “energy transfer (ET)-induced spectral diffusion (SD),” ET→SD, has recently been reported by us. In this process “extra” SD, in addition to “normal” SD in glasses, is triggered by the energy balance released on “downhill” ET. Quantitative aspects of the ET→SD process have been investigated by means of time-resolved hole-burning experiments on free-base chlorin (H2Ch) in polystyrene (PS) presented here. The “effective” homogeneous linewidth Γhom′ was determined as a function of delay time td (10−5−103 s), temperature (1.2 to 4.2 K) and concentration (c=1×10−5 to 6×103 M), at various excitation wavelengths within the S1←S0 0-0 band. Γhom′ as a function of temperature was found to obey the relation Γhom′=Γ0′+aT1.3, characteristic for glasses, and we present an analysis of the residual linewidth Γ0′ and the coupling constant a. In this analysis we determined (i) the separate contributions to Γ0′ arising from the fluorescence lifetime, ET, and ET→SD, (ii) the separate contributions to a arising from “pure” dephasing, “normal” SD, and “extra” spectral diffusion caused by ET→SD. The contributions of ET→SD to Γ0′ and a prove to be proportional to the concentration and to the logarithm of the delay time (∝c log td).
Why is there no low-temperature phase transition in NaOH?
Authors: Bessonette Paul W. R. , White Mary Anne
Published: July 2002

Although NaOH and NaOD exhibit parallel polymorphism at high temperatures, NaOD exhibits a low-temperature phase transition to a hydrogen-bonded antiferroelectric phase and no comparable transition has been found in NaOH. Measurements of NaOH by dielectric relaxation and adiabatic calorimetry were undertaken to determine if proton disorder becomes frozen in NaOH at low temperatures. No evidence for relaxation in NaOH was found from calorimetry or dielectric measurements. A comparison of the low-temperature heat capacities of NaOH and NaOD showed that NaOH has excess heat capacity, likely due to the existence of tunneling levels, and this was satisfactorily fit to a two-level Schottky anomaly. Thus, hydrogen-atom ordering in NaOH appears to take place through a more gradual process at low temperatures, rather than a low-temperature phase transition as in NaOD. The difference in the behaviour of NaOH and NaOD likely is associated with oxygen-oxygen distances that are slightly longer in NaOH than in NaOD, owing to the different nature of higher-temperature dynamical disorder (classical double-well potential for OD− and tunneling for OH−).
New closure for the Ornstein–Zernike equation
Authors: Martynov G. A. , Sarkisov G. N. , Vompe A. G.
Published: July 2002

A closure for the Ornstein–Zernike equation was proposed on the basis of an analysis of the properties of bridge functionals. The closure allows one to calculate the two-particle distribution function g(r) of the Lennard-Jones fluid with a relative error that does not exceed 1%–2% at all densities and temperatures. At the same time, the thermodynamic parameters of fluid in the same approximation are calculated with larger error. It has been shown that these facts are due to those additional errors that are entered by the formulas establishing the linkage between g(r) and thermodynamic parameters.
Fluid transport properties by equilibrium molecular dynamics. II. Multicomponent systems
Authors: Dysthe D. K. , Fuchs A. H. , Rousseau B. , Durandeau M.
Published: July 2002

The Green-Kubo formalism for evaluating transport coefficients by molecular dynamics has been applied to multicomponent mixtures of flexible, multicenter models of linear and branched alkanes and nitrogen and helium in the gas phase and in the liquid phase. Simulation results on binary systems are summarized and trends in prediction using simple but realistic molecular models are shown. New simulation results of N2–n-pentane agree with experiment with a maximum deviation of 36%, the greatest error being for pure n-pentane. Methodological aspects of simulating multicomponent systems with trace components are studied, varying the system size and molecular interaction potentials. It is shown that mixtures are treated representatively even when only one to two molecules of a species are present in the simulated system, unless there is an extreme degree of self-association. It is demonstrated that molecular dynamics may predict quantitatively (7% and 11% deviation) the viscosity of a seven component “synthetic” natural gas.
Ab initio investigation of the Renner–Teller effect in the ground electronic state of HCCD+
Authors: Perić M. , Ostojić B. , Radić-Perić J.
Published: July 2002

Results of an ab initio treatment of the Renner–Teller effect in the ground electronic state of HCCD+ are presented. The model Hamiltonian employed involves four nuclear degrees of freedom correlating at linear geometry with two degenerate bending vibrations. Computations of vibronic energy levels and wave functions are carried out by means of a variational approach. The results of calculations are compared with those for symmetric isotopic species HCCH+ and DCCD+.
Density functional, single and multireference perturbation theory study of the reaction Σg3O2+HOCH2CH2⋅→HOO⋅+HOCH=CH2, modeling an important step in tropospheric benzene oxidation
Authors: Ghigo Giovanni , Tonachini Glauco
Published: July 2002

The one-step transformation of hydroxycyclohexadienyl radical into phenol by O2 is modeled by the title hydrogen-abstraction reaction, which converts the simplest β-hydroxy radical to an enol. The reaction is studied by different quantum-mechanical methods, to assess which level of theory is simultaneously reliable and affordable enough to investigate relatively large aromatic systems. Density functional theory (DFT(B3LYP)), unrestricted Mo/ller–Plesset perturbation theory to the 2nd order (UMP2), and complete active space multiconfiguration self-consistent field (CAS-MCSCF) optimizations are first carried out to determine stable and transition structures. Then, more accurate energetics are determined by spin-projected single-reference PMP4//UMP2 calculations (which are compared with coupled cluster CCSD(T)//UMP2 results), and by two multireference second-order perturbation methods (MR-PT2), based on CAS-MCSCF wave functions and structures. With an (11,9) active space and the 6-311G(d,p) basis set, the MR-PT2 estimates for the energy barrier and reaction energy are: 14.5 and −12.1 kcal mol−1 (CAS-PT2), and 8.3 and −13.4 kcal mol−1 (MC-QDPT2). These estimates fall between the DFT(B3LYP)/6-311G(d,p) (3.3 and −19.1 kcal mol−1) and PMP4/6-311G(d,p) values (17.2 and −10.7 kcal mol−1). Single-point energy computations using larger basis sets are also discussed. The DFT(B3LYP) method tends to underestimate the barrier for H abstraction; the PMP4 barrier is likely to represent an upper bound, given that the single-reference perturbation expansion does not converge very efficiently. For extensions of the study to aromatics, DFT could be deemed to be an acceptable compromise between reliability and feasibility.
Quantum proton transfer coupled to a quantum anharmonic mode
Authors: Karmacharya Rakesh , Schwartz Steven D.
Published: July 2002

Many model studies of proton tunneling in condensed phase employ a reaction coordinate that is coupled to a bath of harmonic oscillator modes. The nature of the coupled modes and the effect of the coupling parameters on reaction rate is an active area of investigation. Recent experimental results from the Fleming group showed that the spectral density for solvation can be temperature dependent [A. Passino, Y. Nagasawa, and G. R. Fleming, J. Chem. Phys. 107, 6094 (1997)]. Translated from the Langevin picture, this result implies that bath modes are anharmonic, or that a different set of harmonic modes are needed at each temperature. In addition, calculations of proton transfer rates have shown that quantum dynamics can be significantly affected by the variation of spectral densities in the low frequency regime [D. Antoniou and S. D. Schwartz, J. Chem. Phys. 109, 5487 (1998)]. We report a study of proton transfer in which the reaction coordinate is coupled to a Morse oscillator with nonlinear coupling. Comparison with the case of coupling to a harmonic oscillator shows that coupling a Morse oscillator to the reaction coordinate leads to enhanced tunneling. We compare our results with those reported in an earlier study [Y. Dakhnovskii, B. Bursulaya, and H. J. Kim, J. Chem. Phys. 102, 7838 (1995)], where the rate of proton tunneling coupled to a one-dimensional classical anharmonic mode was studied.
Vibrational contributions to solute molecular properties obtained through a semiclassical model employing ellipsoidal cavities
Authors: Norman Patrick , Macak Peter , Luo Yi , Ågren Hans
Published: July 2002

A recently proposed semiclassical model for simulating optical properties of solutions and liquids is extended to encompass vibrational polarizabilities. Taking account of both the electronic and vibrational contributions it is shown that the size of an ellipsoidal cavity of pure liquids can be determined in a way that gives consistency between the predicted microscopic and macroscopic properties. It is found that a good estimate of the vibrational polarizability of the pure liquid can be obtained from experimental data on the molecular, gas phase, infrared (IR) spectrum. The influence of the cavity shapes on the liquid susceptibilities is examined using pure liquid ethanol as a test system.
Inelastic scattering of atoms and molecules from liquid crystal surfaces
Authors: Waclawik E. R. , Goh M. Cynthia , Donaldson D. J.
Published: July 2002

Collisions of an atomic beam of He atoms with the surface of an aligned nematic liquid crystal film (4′-pentyl-4-cyano biphenyl, 5CB) initiate a transient change in the alignment of the surface region of the film. The effect is greater when the He atoms approach perpendicular to the alignment director of the sample than when the approach is parallel to the director. The relaxation of the system back to its initial alignment has a temperature dependence which suggests that rotational diffusion is the relaxation mechanism. Laser induced fluorescence spectra of I2 molecules scattered from the isotropic phase and from the nematic phase of 5CB indicate that molecules scattered from aligned 5CB are more internally excited than those scattered from an isotropic 5CB sample. In total, these results indicate that the dynamics of collisions at the gas–liquid crystal interface are strongly dependent upon the molecular order in the liquid crystal.
Investigations of pH-dependent domain structure of lead arachidate Langmuir-Blodgett films by means of x-ray specular and diffuse scattering and atomic force microscopy
Authors: Geue Th. , Schultz M. , Englisch U. , Stömmer R. , Pietsch U. , Meine K. , Vollhardt D.
Published: July 2002

Langmuir-Blodgett (LB) multilayer systems are found to be inhomogeneous in morphology and structure. The most characteristic feature of these multilayer films is the appearance of three-dimensional domains in which the molecules arrange themselves as a result of the film transfer onto the substrate. A series of lead arachidate (PbA2) LB multilayers have been prepared as model systems to study the dependence of film morphology on the pH value in the subphase. Atomic force microscopy (AFM) and two different x-ray scattering methods [specular (XSR) and diffuse (XDS) x-ray reflectometry] have been used to investigate the intrinsic interface properties, such as the vertical electron density profile and lateral distribution of head groups and chains, as well as the microscopic description of the interface structure, thus providing an overall picture of the investigated multilayers. With AFM, discrete height variations of domains with minimum step widths of one double layer independent of the salt concentration in the films were observed. The lateral domain size shows a dependence on pH. It was found to be maximum at pH=4.2 (pure acid) but minimum at pH 7.0 (maximum salt content). The AFM pictures were treated by a statistical analysis to extract quantities that can be compared with the x-ray results. A considerable number of Bragg maxima were observed in XSR. The vertical correlation length LZ was calculated from the angular width of Bragg maxima along 2θ and was found to vary with pH value. It follows, in general, the tendency of the domain sizes, being maximum at pH=4.8 and minimum at pH=7.0, respectively. The lateral correlation length LX has been evaluated via XDS from the half widths Δω measured by rocking the sample across a fixed 2θ. It decreases for increasing pH. LX was compared with the respective quantity of the AFM analysis. Estimated by XDS, the correlation lengths for inner interfaces and domains complement one another with the lateral length scales resulting from AFM analysis of the surface.
Model for the electrorheological effect in flowing polymeric nematics
Authors: Rodrı́guez R. F. , Reyes J. A. , Manero O.
Published: July 2002

An analytical model to study the response of a polymeric nematic confined in a rectangular cell, to a dc electric field is presented. The effect of a pressure-driven plane Poiseuille flow and its competition with the electric field is explicitly considered. For the final stationary state where the induced reorientation of the director has already occurred, an aligned structure with a greatly enhanced viscosity (electrorheological effect) is produced. For this same state the first normal stress difference is calculated as a function of position and of the applied field. For this quantity, regions of negative and positive values develop along the direction of the velocity gradient and an increase in the electric field or the flow causes an augmented effect in its profiles. The net force between the plates is also calculated. As the Reynolds number increases, it is found that it also changes sign from positive to negative, and the effect is more pronounced as the strength of the electric field increases. Finally, the paper is closed by discussing the scope and limitations of the model and methods employed.
The translational energy of desorbing products in NO and N2O decomposition on Pd (110)
Authors: Ohno Yuichi , Kimura Kazushi , Bi Ming , Matsushima Tatsuo
Published: July 2002

The angular and velocity distributions of desorbing N2, N2O, and NO molecules were studied in the decomposition of NO and also N2O on a palladium (110) surface by means of angle-resolved thermal desorption combined with cross-correlation time-of-flight techniques. The results were quite similar in both cases although the desorption of N 2, N2O, and NO concurrently peaked at 490 K in the NO decomposition and, in the N 2O decomposition, the desorption of N 2 and N2O peaked at 140 K. The desorption of N2O and NO showed a cosine angular distribution and a Maxwellian velocity distribution at the surface temperature. On the other hand, the N2 desorption collimated sharply at ±41°–43° off the surface normal in a plane along the [001] direction. Then the velocity distributions of N2 involved two hyperthermal components with the mean translational energy of 0.47 and 0.22 eV, respectively. A mechanism for the inclined N2 desorption was proposed to be due to a highly exothermic reaction of N2O(ad)→N2(g)+O(ad) and the strong repulsive force operative on the product N2 from the surface.
Accurate computation of quantum densities of states and RRKM rate constants for large polyatomic molecules: The STAIR method
Authors: Wadi Hicham , Pollak Eli
Published: July 2002

A new method is given for the computation of quantum mechanical microcanonical densities of states of large molecules. The method is based on the observation that for large molecules with many vibrational degrees of freedom, the complex time partition function dephases rapidly allowing for a good short-time approximation. The short-time approximation up to third order gives an Airy function expression for the thermal density of states at a given temperature T. The microcanonical density of states is then deduced from the thermal density. The input needed for the method is the first three moments of the Hamiltonian at a series of temperatures, which adequately cover the energy range of interest. These may be computed using quantum Monte Carlo methods. The method is tested for a harmonic model of trans-stilbene, a separable anharmonic model of cyclopropane, and a separable anharmonic model of a system with 50 degrees of freedom. The short-time Airy method is found to give accurate estimates for the density of states, the integrated density of states, and RRKM microcanonical rate constants.
Negative coupling during oscillatory pattern formation on a ring electrode
Authors: Christoph J. , Otterstedt R. D. , Eiswirth M. , Jaeger N. I. , Hudson J. L.
Published: July 2002

Pattern formation during the oscillatory electrodissolution of Co was studied using a Co ring electrode with a small reference electrode at a short distance in the center of the ring. Traveling pulses as well as source points (one-dimensional target patterns) were observed. These findings could be reproduced using a reaction-migration equation (RME) the coupling function of which was derived for the particular geometry from basic potential theory and was found to become negative for increasing distance. During pattern formation (i.e., for inhomogeneous potential distribution) the potential drop across the double layer could actually exceed the external applied voltage giving direct evidence for negative coupling.