bluuebunny/crossref_metadata_2025 huggingface dataset:
Supercooling of aqueous solutions subjected to different thermal treatments
Authors: Benmore C. J. , Soper A. K.
Published: July 2002

In a recent paper by H. Nakayama and C. Hayashi [Thermochim. Acta 253, 77 (1995)] it has been suggested that the structural organization of water molecules around a hydrophobic molecular ion changes when the solution is subjected to different thermal treatments. In this letter we report a series of neutron diffraction measurements that show no significant difference in the total structure factor of the solution for the case of tetrabutylammonium chloride in deuterated water.
The high-pressure range of the reaction CH(2Π)+CO+M⇒HCCO+M
Authors: Fulle D. , Hippler H. , Striebel F.
Published: July 2002

The pulsed laser photolysis (PLP), time-resolved saturated laser-induced fluorescence (SLIF) technique has been used to study the reaction CH+CO+M⇒HCCO+M (1) in the temperature range between 300–800 K and at total pressures between 4 and 160 bar helium. The CH radicals have been generated using UV-multiphoton dissociation of CHBr3 at 248 nm. In the investigated temperature and pressure range, the usual falloff behavior of the reaction has been found. Falloff curves have been constructed and a high-pressure limiting rate constant with a weak negative temperature dependence of k1,∞=1.7×10−10(T/300 K)−0.4 cm3 molecule−1 s−1 has been extracted. This high-pressure limiting rate constant has been treated in terms of the statistical adiabatic channel model. Isobar Arrhenius curves have been constructed for pressures between 0.01 and 100 bar, including the formation of C2O+H at high temperatures.
Proton transport along water chains in an electric field
Authors: Drukker Karen , de Leeuw Simon W. , Hammes-Schiffer Sharon
Published: July 2002

Proton transport along water chains is thought to be essential for the translocation of protons over large distances in proteins. In this paper the real-time nonequilibrium quantum dynamics of proton transport along chains of three or four water molecules is simulated using the multiconfigurational molecular dynamics with quantum transitions method. A linearly increasing external electric field is applied to the water chain to model the field exerted by a protein, and restraints are applied to the oxygen atoms to model the structural constraints of the protein. The simulations indicate that fluctuating electric fields and structural constraints strongly affect the dynamics of proton transport along water chains. In addition, quantum mechanical effects such as hydrogen tunneling and nonadiabatic transitions play an important role under certain nonequilibrium conditions.
Molecular size and luminescence decay influenced by energy transfer by the exchange mechanism
Authors: Taen Shoji
Published: July 2002

The decay law has been investigated theoretically when electronic excitation energy transfer is efficient. The concept of a nonvanishing closest donor–acceptor distance has been introduced into the theory of Inokuti and Hirayama for energy transfer by exchange interaction. A previous report of Hara and Gondo on the same subject based on Inokuti–Hirayama’s theory has been examined and some mistakes have been found in the theory and experiment. Careful treatment of equations yields a decay law different from that of Hara and Gondo. The inclusion of the closest donor–acceptor distance modifies the decay form significantly.
Green’s function calculation of electron spin polarization. I. Gyroscopic model formalism
Authors: Doktorov A. B. , Boiden Pedersen J.
Published: July 2002

A gyroscopic model representation of chemically induced electron spin polarization (CIDEP) is advantageous in high magnetic fields where the ST0 approximation is valid. It reduces the dimension of the set of basic equations and separates different physical behaviors into separate quantities. We generalize the gyroscopic model to cover arbitrary recombination and dephasing rates of the radicals. By application of the Green’s function technique we derive general fundamental expressions for the stationary polarization without introducing any of the usual simplifying assumptions of the radical pair system, such as spherical symmetry or short range interactions. These expressions show which Green’s functions and what values are needed for an actual calculation of CIDEP. The expressions can be applied to a wide class of radical pair systems and they form an exact foundation for the development of systematic and consistent approximation schemes. The general results are applied to an assumed delta function exchange interaction and an exact expression for polarization generated in free diffusion is derived. The applicability of this expression to short range exchange interactions is investigated.
Submicron confocal Raman imaging of holograms in multicomponent photopolymers
Authors: Kagan C. R. , Harris T. D. , Harris A. L. , Schilling M. L.
Published: July 2002

We report submicron chemical imaging of optical holograms in multicomponent photopolymers using a confocal scanning Raman microscope. The microscope is sensitive to the submicron, ∼1% concentration variations of the polymeric components that form refractive index modulations (Δn) responsible for hologram diffraction. Concentration variations are established by both small molecule diffusion and polymer matrix swelling during hologram writing. Both density and composition variations contribute to Δn. These measurements demonstrate that submicron Raman microscopy is applicable to multicomponent organic, inorganic, and hybrid materials as a route to correlate materials chemistry/morphology with their physical properties.
Formation of droplets with a very narrow size distribution in mixtures subjected to shear flow. I. Shear rate dependence
Authors: Hashimoto Takeji , Matsuzaka Katsuo , Fujioka Kiyotoshi
Published: July 2002

The phase-separated structures of a semidilute solution of off-critical polymer mixtures composed of polystyrene (PS) and polybutadiene (PB) in a common solvent of dioctylphthalate (DOP) under steady shear flow are investigated by light scattering as a function of shear rate γ̇. Light scattering patterns show multiple-order scattering maxima up to the 13th order or more in a weak shear regime, revealing formation of droplets with surprisingly uniform size having a relative standard deviation of droplet radius σR∼0.03. The average droplet radii R̄x parallel and R̄z perpendicular to the flow direction are proportional to γ̇−1, with an average aspect ratio v̄R≡R̄x/R̄z of 2 independent of γ̇ in the weak shear regime. The result obtained in the average particle size is in good agreement with that predicted by Taylor’s theory for a single droplet case, but the extreme uniformity in size distribution would not be trivial and be worth it for future theoretical considerations.
A comparison between the CISD[TQ] wave function and other highly correlated methods: Molecular geometry and harmonic vibrational frequencies of MgH2
Authors: Tschumper Gregory S. , Schaefer Henry F.
Published: July 2002

The linear MgH2 molecule was selected as a model system to compare the total electronic energy (ETOT), equilibrium bond length (re), and vibrational frequency (ωn) predictions from six different configuration interaction (CI) and coupled-cluster (CC) methods using three large correlation consistent basis sets. The three CI procedures employed incorporated all single and double substitutions (CISD) as well as all triple and quadruple substitutions (CISDTQ) or limited triple and quadruple substitutions (CISD[TQ]). The remaining three CC schemes included all single and double excitations (CCSD) as well as all triple excitations (CCSDT) or a perturbative approximation of the triple excitations [CCSD(T)]. Within the frozen core approximation employed in the study, the CISDTQ method constituted a full CI wave function. With the largest basis set this approach included 1.79 million configuration state functions and predicted re=1.711 Å, ω1=1602 cm−1, ω2=438 cm−1 and ω3=1628 cm−1. At the equilibrium geometry predicted by each method, agreement with the CISDTQ properties was observed to improve systematically in the following manner for all three basis sets: ETOT: CISD≪CISD[TQ]≈CCSD<CCSD(T)<CCSDT≈CISDTQ, re: CISD≪CCSD<CISD[TQ]<CCSD(T)<CCSDT≈CISDTQ, ωn: CISD≪CCSD<CISD[TQ] ≈CCSD(T)<CCSDT≈CISDTQ. With the largest basis set, ETOT was also computed after the Mg–H bond had been stretched to 3.0 Å. At this nuclear configuration the CISD[TQ] wave function outperformed the CC methods and recovered 99.8% full CI correlation energy while including over 100 times fewer configurations in the CI expansion. At this stretched geometry, agreement with the full CI correlation energy improved as follows: ETOT: CISD≪CCSD≪CCSD(T)≈CCSDT <CISD[TQ]≈CISDTQ.
Photoelectron distributions from femtosecond pump/probe excitation with chirped probe pulses
Authors: Meyer S. , Meier C. , Engel V.
Published: July 2002

We calculate photoelectron distributions obtained from the two-photon pump/probe ionization of Na2. A first ultrashort pulse prepares a vibrational wave packet in the (2)1∑u+ electronic state. The dynamics is probed by time-delayed ionization caused by a chirped femtosecond laser pulse. Time-resolved photoelectron spectra are determined for different chirp parameters. It is shown that the spectra are very sensitive to the choice of the chirp. In particular the width can be selectively modified and the in- or outward motion of the wave packet can be distinguished by using an appropriately chirped electric field. An analytical expression for the photoelectron spectrum explains its dependence on the chirp parameter.
Selective excitation of the ion pair surface in the intracluster Ca–HCl* harpoon reaction
Authors: Lawruszczuk R. , Elhanine M. , Soep B.
Published: July 2002

The excited state reaction of calcium with hydrogen chloride has been investigated in the specific conditions of a van der Waal complex formed in a supersonic jet after laser ablation of the metal. The reaction channel leading to ground state calcium chloride has been specifically studied in this work, by laser induced fluorescence. A very high vibrational distribution has been observed for CaCl with a maximum at v=30 and extending up to the energetic limit at v∼60. This high v population distribution has been modeled with the direct interaction direct repulsion model and corresponds to an immediate energy release occurring at the transition state, i.e., at the level of the ion pair ground state Ca+(2S), HCl− surface. This results from the observation of a continuous action spectrum for the formation of the high levels of CaCl after excitation of the complex in good agreement with the direct excitation of the ground state ion pair potential. It suggests that the potential energy surface promoting the ground state formation is different from the one leading principally to excited CaCl products already observed.
Vibrational relaxation of ClO2 in water
Authors: Poulsen J. Aa. , Thomsen C. L. , Keiding S. R. , Tho/gersen J.
Published: July 2002

Photodissociation of ClO2 in aqueous solution at 400 nm results in the formation of ClO+O and Cl+O2. ClO and O geminately recombine to ClO2 in the electronic ground state (2B1), formed with an initial vibrational energy of ≈2.5 eV. In this paper the vibrational relaxation of ClO2(2B1) in aqueous solution is studied by femtosecond transient absorption spectroscopy in the spectral range 234 to 1024 nm. The measured transient absorption of the vibrationally relaxing ClO2 molecules is compared with the transient absorption calculated for relaxation in the asymmetric stretch as well as the symmetric stretch and bending modes. The calculations of the absorption spectra pertaining to the asymmetric stretch are based on a harmonic potential derived from the experimentally determined fundamental vibrational energy, whereas that of the symmetrical vibrations are based on ab initio potentials. An excellent agreement is obtained by assuming that the vibrational relaxation predominantly occurs in the asymmetric stretch with a 9.5 ps relaxation time. A weak spectral feature in the ultraviolet part of the spectrum is assigned to vibrational relaxation in the symmetric stretch and bending modes, indicating a coupling between the asymmetric and symmetric modes.
Density functional study of the NO dimer using GGA and LAP functionals
Authors: Duarte H. A. , Proynov E. , Salahub D. R.
Published: July 2002

The nature of the ON-NO bonding in the NO dimer still remains a challenge for currently available theoretical and experimental methods. Most of the theoretical studies reported so far predict a singlet cis ground state. However, the fully optimized geometry of the NO dimer may favor a triplet ground state, depending on the approximate method used. In this work we explore in detail the electronic structure of the fully optimized trans- and cis-NO dimer including a vibrational analysis in different electronic states, using several exchange-correlation functionals within the Kohn-Sham DFT method. The recently developed LAP exchange-correlation schemes that use the Laplacian of the density and the self-consistent kinetic energy density, improves significantly the results. The N-N bond distance is in better agreement with the experimental results, and the triplet/singlet gap is smaller, however, still predicting a triplet ground state. The nature of the electronic ground state is discussed in detail. We explored the possibility that in such a system the singlet ground state may be well approximated as a broken spin symmetry state with localized magnetic moments on each NO monomer aligned antiferromagnetically. A KS broken symmetry solution was thus obtained. However, the energy lowering due to the symmetry breaking was not sufficient to reverse the triplet/singlet energy ordering. The LAP functional avoids symmetry breaking out to larger distances than does the generalized gradient approximation (GGA). Although progress has been steady, the existing exchange-correlation functionals are not yet able to describe fully all aspects of this demanding system.
The G 1Πg state of Li27 revisited: Observation and analysis of high vibrational levels
Authors: Urbanski K. , Antonova S. , Lyyra A. M. , Li Li , Ji B.
Published: July 2002

CW optical–optical double resonance has been employed to study the upper portion of the G 1Πg state of Li27. The use of fluorescence and ion detection enabled the observation of vibrational levels ranging from v=20 to 48 and rotational levels ranging from J=1 to 25. For optical–optical double resonance experiments the Franck–Condon factors between these levels and useful A 1Σu+ state levels (levels that can be reached from the thermally populated levels of the ground state) were exceedingly small. The sensitive ionization detection made possible observation of this upper, anharmonic region of the potential. From the data a Rydberg–Klein–Rees potential curve was constructed which represents over 99% of the potential well depth. A C5 coefficient has been calculated and represents the first experimental determination of a long range coefficient for a doubly excited state in diatomic lithium.
Thermodynamics of water-cubic ice and other liquid-solid coexistence in nanometer-size particles
Authors: Johari G. P.
Published: July 2002

When contributions from the interfacial energy become significant and comparable to the bulk energy, liquid and crystalline phases can coexist at a temperature much lower than the usual melting point. A formalism for this coexistence is given, and thermodynamic conditions for the melting of nanometer-size cubic ice crystals are derived when both the ice and water are at an equilibrium vapor pressure. By using the approximate values of surface energy and the enthalpy and entropy of melting, it is shown that nanometer-size water droplets can coexist with cubic ice particles of about the same size at temperatures in the 150–180 K range. The unusually large decrease in the temperature of a liquid-solid phase equilibrium is expected to be a general phenomenon in the nanometer-size films, clusters, and particles of materials.
Depolarized light scattering in the isotropic phase of liquid crystals
Authors: Shibata Tetsuya , Matsuoka Tatsuro , Koda Shinobu , Nomura Hiroyasu
Published: July 2002

The polarized and depolarized Rayleigh light scattering spectra for the isotropic phase of p-methoxybenzylidene-n-butylaniline (MBBA) and 4-cyano-4-penthylbiphenyl (5CB) were measured in order to investigate the coupling between translational and orientational modes of molecular motion. The Rayleigh dip was observed in depolarized light scattering spectra of isotropic phase of both liquid crystals in the wide temperature ranges. The Rayleigh dip spectrum was attributed to the coupling between orientational and translational mode of molecular motions in anisotropic molecules. Data analysis based on the Alms’ theory (J. Chem. Phys. 67(12), 5779 (1977)) leads to the evaluation of the coupling parameter R. The temperature dependence of the coupling parameter of both liquid crystals near the phase transition point was discussed.
A multitechnique maximum entropy approach to the determination of the orientation and conformation of flexible molecules in solution
Authors: Berardi R. , Spinozzi F. , Zannoni C.
Published: July 2002

We present a maximum entropy method that allows the simultaneous analysis of different types of experimental data in order to obtain conformational information on flexible molecules in solution. We consider various NMR observables (dipolar, quadrupolar, J-couplings, nuclear Overhauser enhancements), and dielectric and neutron scattering techniques, and we express them using a common formalism in terms of orientational–conformational order parameters. We then show how these observables can be inverted in structural information allowing for continuous or discrete internal degrees of freedom and for any available prior information. We demonstrate the potentialities of the method on simulated 1HNMR, 2HNMR and dielectric data for some terminally halogenated alkyl chains and show the improvement in conformational analysis obtained by simultaneously analyzing different and complementary data sets.
Smoluchowski equation with a sink term: Analytical solutions for the rate constant and their numerical test
Authors: Berezhkovskii A. M. , D’yakov Yu. A. , Zitserman V. Yu.
Published: July 2002

Smoluchowski equation with a sink term is widely used as a model of a rate process in a slowly relaxing environment. Two approximate solutions for the rate constant obtained for a steeply growing sink are tested numerically using an exponential sink. Both analytical solutions are in a good agreement with the numerical results over a wide range of the problem parameters (environment relaxation rate). They show how the rate constant Γ decreases when the viscosity η of the environment increases. If the dependence is approximated by the fractional power law, Γ∝η−α, the exponent α is always less than unity and depends on η. It tends to zero for fast relaxation of the environment (small η) and increases when the relaxation slows down (η grows).
Molecular dynamics simulations of the homogeneous nucleation of UF6 and SF6 molecules: Effects of the intramolecular vibrational relaxations on the nucleation rates
Authors: Tanimura Shinobu , Yasuoka Kenji , Ebisuzaki Toshikazu
Published: July 2002

The effects of flexibility in the homogeneous nucleation processes of UF6 and SF6 molecules from vapor phase were investigated by classical molecular dynamics (MD) simulations. We performed MD simulations using a flexible-molecule model and compared the results with those obtained from a rigid-molecule model. We took into account the flexibility of molecules in MD simulations by a harmonic intramolecular potential. We found that the nucleation rate in the flexible model of the UF6 molecule was about twice as large as that in the rigid model of UF6. This acceleration in nucleation rate was attributed to the flow of the condensation heat into the intramolecular vibrations. On the other hand, the nucleation rates in rigid and flexible models of SF6 were almost the same because the flow of the condensation heat into the intramolecular vibrations in the flexible model of SF6 was negligibly small. In order to confirm the reliability of the classical intramolecular vibrational model in the present work, we estimated the intramolecular vibrational relaxation times of the flexible UF6 and the flexible SF6 molecules in the gas phase using the same MD simulation technique as used for the nucleations. The intramolecular vibrational relaxation times obtained from the MD simulations were in good agreement with those from the experimental data in cases of both UF6 and SF6 molecules.
A comprehensive study of the phase diagram of symmetrical hard-core Yukawa mixtures
Authors: Caccamo C. , Costa D. , Pellicane G.
Published: July 2002

The phase diagrams of hard-core Yukawa mixtures (HCYM), constituted of equal sized hard spheres interacting through an attractive Yukawa tail, are determined by means of Gibbs Ensemble Monte Carlo (GEMC) simulations, Semi-grand Canonical Monte Carlo (SGCMC) simulations, and through the modified hypernetted-chain (MHNC) theory. Freezing lines are obtained according to an approach recently proposed by Giaquinta and co-workers [Physica A 187, 145 (1992); Phys Rev. A 45, 6966 (1992)] in which an analysis of multiparticle contributions to the excess entropy, Δs, is performed, with the determination of the Δs=0 locus. Liquid–vapor coexistence, determined through GEMC simulations, turns out to be favored when the strength ratio ν of unlike to like particle interaction, is close to 1. For lower ν’s, liquid–vapor coexistence is favored at low densities, and liquid–liquid coexistence, determined through SGCMC simulations, at high densities. The liquid–vapor binodal shifts downward in temperature and flattens when ν decreases, with a decrease of the critical temperature. At ν=0.9 a triple point can be identified from the intersection of the freezing line with the binodal line; at ν=0.7, instead, the binodal ends on the line of liquid–liquid (consolute) critical points, the intersection of the two lines thus identifying the “crossover” density and temperature between the two equilibrium regimes which correspond to the critical end point of the mixture. We find that, for not too high densities, consolute equilibrium can be also explored through GEMC simulations; the results for liquid–liquid coexistence obtained through this method and SGCMC simulations compare quite satisfactorily with each other. The trend of the liquid–vapor binodal to disappear for relatively weak unlike interactions is discussed in connection with the disappearance of liquid–vapor equilibrium which occurs in one component hard-core Yukawa fluids characterized by very short ranged attractive forces. The latter behavior has been conjectured to be relevant for the onset of crystallization in protein solutions; the implications of the present results, which are obtained in the context of a two component, albeit rough, modelization of a realistic solution, are discussed. In agreement with similar results obtained by Giaquinta et al., we finally find that the Δs=0 locus not only brings the signature of the freezing transition, but also of structural rearrangements preluding to other phase equilibria; in fact, the Δs=0 line turns out to be coincident to a high accuracy with the line of consolute critical points and with the gas branches of the liquid–vapor binodals.
Valence distribution and structural distortions in MMX chain compounds
Authors: Borshch S. A. , Prassides K. , Robert V. , Solonenko A. O.
Published: July 2002

We here present an analysis of valence distributions and Peierls-type distortions in one-dimensional halogen-bridged chains MMX. In those systems which result from the linking of metal dimers, the phenomenological chain and cluster models show that Peierls distortions can be associated with internal degrees of freedom in dimers and not with the displacements of bridging halogen atoms. On the other hand, our semiempirical quantum-chemical band calculations suggest that the role of internal Peierls mode in Pt2(dta)4I chains can be played by the twisting distortion of dta ligand. The resulting distribution of valence states is –M+2–M+3–X–M+3–M+2, in agreement with some experimental data.
Conformation of a polymer chain near the solvent critical region. II. Monte Carlo simulation
Authors: Vasilevskaya Valentina V. , Khalatur Pavel G. , Khokhlov Alexei R.
Published: July 2002

Using the Monte Carlo (MC) calculations, we study the conformational behavior of a two-dimensional (2D) single flexible-chain polymer dissolved in a monoatomic solvent. It is shown that near the solvent critical region the polymer chain can contract. Such a behavior is observed if the radius of fluctuations of solvent density is smaller than the natural size of the N-unit chain with excluded volume interaction [R∼Nν, where ν≈3/(d+2) is the Flory–Edwards exponent]. On the other hand, the chain goes back to the initial swollen state when the solvent correlation length becomes larger than R. Under these conditions, the polymer chain is effectively confined in a large solvent droplet. We find that the strongly fluctuating solvent can induce significant conformational changes only if there is a rather strong attraction between polymer segments and solvent particles. In this case, the average chain size is a nontrivial function of polymer–solvent attraction: At rather weak affinity of polymer and solvent, the average chain size grows with the increase of this attraction; with further increase of affinity of polymer and solvent, the chain begins to contract. Thus, in the case of high affinity of polymer and solvent, the polymer chain can undergo the complex coil–globule–coil transition. In general, the results of the MC simulations are in reasonable agreement with those obtained from the self-consistent integral-equation calculations.
Phase measurement in a collinear pump probe experiment: Application to molecular dynamics studies in liquids
Authors: Bousquet Bruno , Canioni Lionel , Sarger Laurent
Published: July 2002

We propose a novel approach of nonlinear resonant spectroscopy based on a collinear and degenerated pump probe experiment. This particular scheme leads to an oscillating time-resolved signal superimposed to the usual transient absorption signal. We have performed both amplitude and phase analysis of this oscillating signal as a function of the pump probe delay for different laser wavelengths. As an example, results on the resonantly excited dye molecule hexamethylindotricarbocyanine iodide (HITCI) in ethylene glycol at room temperature have been exploited in the framework of the phenomenological Brownian oscillator model. The corresponding numerical simulations have been carried out with a special emphasis on the phase sensitivity versus pump probe delay and detuning. The chromophore dynamical parameters obtained from phase variations of our signal are compared with other available spectroscopic studies.
Fluorescence interference noise in a two-site system: Excitation transfer dynamics from intensity fluctuations
Authors: Szöcs V. , Kauffmann H. F.
Published: July 2002

The optical response of a two-site system driven by a pair of optical pulses in an interferometric set up has been studied theoretically by applying the density operator formalism. The one-exciton approach is taken for modeling two coupled two-level systems (TLS), the external field is presented semiclassically and bath-induced dissipative processes are included. In the δ-pulse limit the population of the excited state has been formulated to the lowest order perturbation expansion in the external field. In the limit of slow luminescence the interferogram of time-integrated total fluorescence has been calculated for pulses with constant relative phase. For phase-randomized pulses the variance of the correlated fluorescence signal as a function of the pulse delay allows direct interrogation of coherent transients and dephasing processes. Our analysis follows the principle of coherence observation by interference noise, COIN [O. Kinrot, I. Sh. Averbukh, and Y. Prior, Phys. Rev. Lett. 75, 3822 (1995)], but is a generalization of this concept to expand on electronically interacting TLS. The theoretical results demonstrate that analysis of fluorescence interference fluctuations may provide a powerful diagnostic tool for probing the initial quantum coherence of energy transfer, i.e., excitation oscillations by employing fs-fluorescence correlation measurements in stable interferometric configurations.
A measure of the average cooperativity of a binding system
Authors: Ben-Naim A.
Published: July 2002

A new measure of the average cooperativity of a binding system is suggested. Existing measures, suggested earlier by Wyman, and by Minton and Saroff, do not adequately account for all types of cooperativities in systems with more than two binding sites. The newly suggested measure captures the average cooperativity at any stage of the binding process. It is shown that this average can change dramatically, even its sign, during the span of a binding process.
Adsorption and photon-stimulated desorption of CCl4 on an Al(111) surface investigated with synchrotron radiation
Authors: Chen Jin-Ming , Liu Ru-Gun. , Hsu Yao-Jane , Yang Su-Chien , Liu Yuen-Chung , Lee Yuan-Pern , Wen Ching-Rong , Chuang T. J.
Published: July 2002

Adsorption and desorption of CCl4 molecules on an Al(111) surface at 90 K are characterized with photoemission spectroscopy (PES) and photon-stimulated ion desorption (PSID) techniques following valence-level and core-level excitations. Results of valence-level and Cl(2p) core-level PES spectra indicate that CCl4 dissociates partially upon adsorption on an Al(111) surface at submonolayer coverage and that molecular CCl4 adsorbs to form multilayers at large exposures. The dissociation upon adsorption of CCl4 on an Al surface at 90 K is likely mediated by the charge-transfer process. The Cl+ desorption threshold at ∼18.5 eV in valence-level PSID spectra may originate from the 5t2→7a1* (C–Cl antibonding orbital) transition consistent with the Menzel–Gomer–Redhead (MGR) mechanism. The total-electron yield (TEY) spectrum and the Cl+ PSID spectrum of solid CCl4 following the Cl L-edge excitation are clearly dissimilar. The enhanced desorption yield of Cl+ ions is detected at the Cl 2p→7a1* excitation, compared to the Cl 2p→8t2* and Cl 2p→Rydberg state excitations. Cl(2p) core-level excitations yield much greater desorption of ions compared with direct valence-band excitation. Based on resonant photoemission spectra, core resonant excitations decay predominantly via spectator Auger transitions, whereas shape resonance excitation is followed by normal Auger decay. Enhanced Cl+ ion desorption from solid CCl4 following Cl 2p→7a1* excitation is interpreted in terms of the rapid desorption via a repulsive surface which is directly related to spectator electrons localized in antibonding orbitals.
Size disparity and lower critical solution temperature: A critical investigation of free-volume disparity
Authors: Chhajer Mukesh , Gujrati P. D.
Published: July 2002

By critically examining a simple model system of equilibrium polymerization that is athermal in the traditional sense, we demonstrate that many of the consequences of the free-volume disparity induced by size disparity are inconsistent with its current understanding. Despite its traditional name, the model is not truly athermal because of the compressibility. The resulting energetics endows the model system with a very rich and complex behavior. The analytical results that are obtained in a mean-field approximation show how and when an upper critical solution temperature, a lower critical solution temperature (LCST) and an immiscibility loop may occur. We suggest that it is the difference in the thermal volume-expansion coefficients rather than the difference in free volumes of the coexisting phases (and not of the components) that plays a central role in determining the phenomenon of LCST and may be used to provide for its quantitative characterization. Too much or too little of free volume disfavors LCST; hence, it occurs only in a finite range of the pressure. Moreover, for the phenomenon of LCST to occur, the size disparity should be larger than some critical value that also depends on the pressure. A line of theta points is found in a four-parameter phase space characterizing the simple model. The model also enables us to understand how a variety of phase diagrams including hourglass emerges, despite the fact that the conventional mechanism for hourglass in not present.
Crossover approach to scaling behavior in dilute polymer solutions: Theory and simulation
Authors: Lue L. , Kiselev S. B.
Published: July 2002

We develop a crossover theory for dilute polymer solutions, analogous to crossover theories for critical phenomena in simple fluids. In this theory, a critical degree of polymerization N* is found, which plays a similar role as the Ginzburg number in second-order phase transitions. To test the predictions of this theory, we perform Monte Carlo simulations of polymer chains composed of rigidly bonded hard spheres of various diameters and chain lengths. Various properties of these chains were analyzed, including the end-to-end distance distribution and mean-square radius of gyration. We find that the approach to the asymptotic scaling regime displays two types of crossover behavior, depending on the value of the model parameter ū, which is a measure of the strength of the monomer–monomer excluded volume interaction: (i) ū<1 and (ii) ū>1. In case (i), the system exhibits crossover from a Gaussian chain to the Kuhnian chain, as the degree of polymerization increases. In case (ii), the system exhibits crossover from the rigid rod to a Kuhnian chain. Our crossover theory is found to work well for polymers with ū>1 only near the asymptotic scaling regime. However, for ū<1, the theory works well in all regimes.
Time-dependent wave packet study of the N++H2 reaction
Authors: Russell Caroline L. , Manolopoulos David E.
Published: July 2002

The time-dependent wave packet method has been used to calculate initial state selected reaction probabilities, reaction cross sections, and rate constants for the N++H2 reaction on the potential energy surface of Wilhelmsson, Siegbahn, and Schinke [J. Chem. Phys. 96, 8202 (1992)]. In addition to providing results that can be used to test more approximate theories, these calculations are used to shed light on a number of key issues concerning the reaction, including the correct value of the reaction endothermicity, the reactivity of excited H2 rotational states, and the lifetimes of the intermediate NH2+ complexes that are formed in collisions of N+ with H2(v=0) and H2(v=1). We also show that an earlier quasiclassical trajectory study of the reaction on the same potential energy surface predicted the wrong cross-section behavior in the threshold region as a result of an incorrect treatment of product zero-point energy.
Pendular state spectroscopy of an asymmetric top: Parallel and perpendicular bands of acetylene-HF
Authors: Moore D. T. , Oudejans L. , Miller R. E.
Published: July 2002

We report here a comprehensive theoretical and experimental study of the pendular state spectroscopy of a near-prolate asymmetric top molecule and consider the specific case of the acetylene-HF binary complex in a static electric field. The experiments were carried out using the optothermal detection method and the spectra were recorded in an electric field of sufficient magnitude to give rise to substantial orientation in a number of states. The calculated and experimental spectra are compared for different vibrational band types and polarization configurations. In this way we have identified the states that give the best orientation and thus are well suited to photodissociation studies. The hybridized orientational probability distributions are presented for the relevant states.
Stochastic resonance in the absence and presence of external signals for a chemical reaction
Authors: Yang Lingfa , Hou Zhonghuai , Xin Houwen
Published: July 2002

A catalytic reduction of NO with CO on Pt(100) surface is adopted to study its response under random perturbation. Noise-induced oscillations and noise-induced frequency shifts have been observed when the system works in the vicinity of the oscillatory region and meanwhile is subjected to random modulation of its feeding speed. Stochastic resonance behavior can be recognized from the noise-induced peak in the power spectrum even though in the absence of external signals. The numerical results have been obtained near supercritical Hopf bifurcation points, but are not confined to the classification of bifurcation. When the system falls into bistable regions, noise can help an external weak signal to induce state-to-state transitions and also shows a stochastic resonance behavior except for the case that the system has an isolated bifurcation scheme.
Diffusion-assisted long-range reactions in confined systems: Projection operator approach
Authors: Seki Kazuhiko , Barzykin Alexander V. , Tachiya M.
Published: July 2002

The diffusion-assisted long-range reversible reaction equation is solved for the pair survival probability using a projection operator method in terms of the diffusion propagator in the absence of reaction. For a localized (delta function) reaction sink, the well-known analytical solution is immediately reproduced from the operator expression. It is emphasized that the mean reaction time approach, often used to approximate the overall reaction rate, is not adequate for a nonequilibrium initial condition. The general operator solution for a delocalized sink is shown to reduce to a closed matrix form, provided the propagator has a discrete spectrum of eigenmodes. The matrix solution is exact and applies for an arbitrary functional form and strength of the reaction sink. Although matrices of infinite dimensions are involved, they can be truncated at a certain finite dimension to attain any prescribed precision. Convergence of the truncated matrix solution is fast and often only a few of the lowest eigenmodes are sufficient to obtain quantitatively reasonable results. Several long-range reaction models are analyzed in detail revealing the breakdown of the widely used closure approximation obtained as a first-order Padé approximation of the operator solution.
High-resolution N14 NMR in polycrystalline solids
Authors: Ermolaev Konstantin , Fung B. M.
Published: July 2002

The N14 NMR spectra of solids are usually very broad due to the presence of large quadrupole coupling constants. However, even partial excitation of the whole spectrum can give valuable information. With magic angle spinning (MAS), the spectrum consists of a number of peaks, but normally the centerband cannot be readily distinguished from the spinning sidebands. Multiple-pulse methods of sideband elimination, such as TOSS and PASS, cannot be used for N14 because of its short spin–spin relaxation time. On the other hand, the sidebands can be eliminated by systematic data treatment. First, the signal-to-noise ratio (S/N) is enhanced by co-adding all the peaks in a MAS spectrum in a periodic way. Then, several spectra obtained at different spinning rates are added or multiplied together to identify the centerband. In the centerband region of the spectrum obtained from the addition method, the residual sidebands can be distinguished from the weak signals by the use of logical or digital filtering. Results obtained by using these methods to treat the spectra of two mixtures of KNO3, Pb(NO3)2, and NH4Cl are shown. The experimental requirements are not very stringent, the S/N ratio is good, and peaks covering a large range of chemical shifts can be readily observed with high-resolution.
Resonance enhanced multiphoton ionization spectroscopy of jet cooled C3212S2 and C3212S32S from 45 500 to 48 000 cm−1
Authors: Beatty A. S. , Shiell R. C. , Chang D. , Hepburn J. W.
Published: July 2002

The (1+1) resonance enhanced multiphoton ionization spectra of jet-cooled CS2 were obtained between 45 500 and 48 000 cm−1 (220–208 nm) for the two isotopomers C3212S2 and C3212S32S. With the use of different expansion gases which resulted in varying degrees of vibrational cooling and the comparison of the two isotopomer spectra, a partial assignment of the C̃ 1B2–X̃ 1Σg+ transition was obtained. The electronic origin of this transition is confirmed to lie at 46 248.7 cm−1 and values for the predissociation lifetimes for the upper state for 34 vibrational bands and isotopomer shifts of seven vibrational levels of the B21 state are presented. The lifetimes of the Σ0 bands were found to be larger than those of corresponding Π1 and Δ2 bands. A simulation of the spectrum, which used the harmonic approximation, is in qualitative agreement with the band positions and shifts, but quantitative disagreement between the values leads us to conclude that a normal mode analysis is not appropriate to describe the low vibrational levels of the B21 state. The results presented increase our understanding of the low-lying predissociating levels of CS2, which will be of use in future photofragment studies.
Green’s function calculation of electron spin polarization. II. Approximation schemes
Authors: Doktorov A. B. , Neufeld A. A. , Pedersen J. Boiden
Published: July 2002

We present two approximation schemes to the previously derived Green’s function method that utilizes a gyroscopic representation of the spin state. First a consistent approximation scheme is developed in which the exact equations are expanded in terms of the small parameter lx/d, where lx is the decay length of the exchange interaction and d is the distance of closest approach. A general and explicit expression, correct to first order in the expansion parameter, is derived for spherical symmetric systems. Secondly, we introduce a modified kinematic approximation which for the first time accounts for recombination and dephasing processes. We show that for spherically symmetric systems the results of the modified kinematic approximation is equivalent to the first order results. This equivalence constitutes the first formal proof of the validity of a kinematic approximation. The derived expression depends only on the magnitude and decay length of the exchange interaction, the recombination and dephasing rate constants, and on the free Green’s function. The problem of calculating electron spin polarization (CIDEP) is thus reduced to a calculation of the free Green’s function, which describes the relative motion of the radicals in the absence of recombination.
Rearrangements and tunneling splittings of protonated water dimer
Authors: Wales David J.
Published: July 2002

Stationary points and rearrangement mechanisms are characterized for protonated water dimer with a variety of basis sets and both Møller–Plesset and density functional theory to describe electron correlation. The results are consistent at each level of theory and suggest that this molecule will exhibit interesting tunneling splitting patterns which should be resolvable experimentally.
The H+N2O→OH(2Π3/2,v′,N′)+N2 reaction at 1.5 eV: New evidence for two microscopic mechanisms
Authors: Brouard M. , Burak I. , Gatenby S. D. , Hart D. , Minayev D.
Published: July 2002

The product state-resolved dynamics of the photon-initiated reaction H+N2O→OH×(2Π3/2,v′,N′)+N2 has been studied using Doppler-resolved laser induced fluorescence (LIF) at a mean collision energy of 143 kJ mol−1 (≡1.48 eV). Nascent OH(v′=0,1) rovibrational population measurements indicate that only a small fraction of the available energy is channeled into the internal modes of the OH reaction products, as is consistent with previous work at other collision energies. State-resolved angular scattering distributions have been determined and are found to depend sensitively on product OH rovibrational quantum state. For the v′=0 products, the angular scattering distributions are forward-backward peaking at low N′, changing to sideways peaking at high N′. OH products born in the v′=1,N′=6 state possess forward-backward peaking angular scattering distributions, similar to the OH(v′=0) products born with intermediate N′. In addition to these findings, the experiments have allowed the precise determination of the OH quantum state-resolved distributions of kinetic energy releases and, hence, by energy balance, of internal energies accessed in the N2 co-products. The product state-resolved kinetic energy disposals are found to broaden somewhat, and to favor higher kinetic energy disposal, as the internal energy of the OH is increased. The internal energies accessed in the OH and N2 products are therefore (anti-)correlated. More interestingly, the kinetic energy distributions are bimodal, particularly for OH(v′=0) fragments born in high N′, and for those born in v′=1. This finding is attributed to the operation of two microscopic reaction mechanisms, which are probably associated with H atom attack at the two ends of the NNO target molecule. The results are discussed in the light of previous experimental and theoretical work.
Uniform semiclassical calculation of the direct part of the photodissociation cross section of water
Authors: Hüpper Bruno , Eckhardt Bruno
Published: July 2002

The photodissociation cross section of water in the first absorption band naturally splits into two parts, a rather dominant and broad background, and a series of isolated resonances. We focus here on the semiclassical calculation of the background term. We compare various semiclassical methods of calculation, including the reflection approximation, the Wigner–Weyl series, Airy function approximations, and higher-order approximations in the time domain, and find that only the latter gives quantitative agreement. They are numerically efficient as they can be reduced to a 2-D spatial integration together with a Fourier transform.
Simulations of crystal growth from Lennard-Jones melt: Detailed measurements of the interface structure
Authors: Huitema H. E. A. , Vlot M. J. , van der Eerden J. P.
Published: July 2002

Simulations of crystal growth from the melt are performed, using the Lennard-Jones potential. Growth of the (100), (111), and (110) faces of the fcc crystal from its melt are simulated. The measured growth rates show that the kinetic coefficient of the (100) face is about twice as high as that of the (111) and (110) faces, which have comparable kinetic coefficients. In order to perform measurements of the interface structure during growth, an order parameter is defined that discriminates between solid and liquid like particles on the basis of the symmetry properties of the local environment. The measurements show that the interface width for the three orientations is similar and does not increase appreciably with undercooling. The difference in occupation fraction in the layers in the interfacial region explains the larger kinetic coefficient found for the (100) face.