bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics and dynamics of the dissociative chemisorption of oxygen on Ir(111)
Authors: Davis J. E. , Nolan P. D. , Karseboom S. G. , Mullins C. B.
Published: July 2002

The initial dissociative chemisorption probability, S0, of O2 on Ir(111) has been investigated with molecular beam techniques and electron energy loss spectroscopy (EELS). The adsorption process on the clean surface occurs by distinct dynamical mechanisms. At incident kinetic energies, Ei, of 0.1 eV and below, the dissociative chemisorption probability decreases with increasing kinetic energy, indicating the dominance of a trapping-mediated mechanism. A decrease in the value of S0 with increasing surface temperature, Ts, is also characteristic of this regime. This temperature dependence reflects the participation of a physically adsorbed state and molecularly chemisorbed state in the dissociation scheme. Additionally, the dependence of S0 on incident angle, θi, in the low kinetic energy regime exhibits near normal energy scaling. At high kinetic energy (Ei>0.1 eV), the initial dissociative chemisorption probability rises with increasing Ei indicating that translational energy is effective in surmounting a potential barrier to adsorption. Direct access of a molecularly chemisorbed state followed by dissociation, rather than direct access of the dissociated state, is hypothesized to be the primary initial adsorption step. Several observations support this mechanism, including a temperature dependence in the high kinetic energy regime and no observed increase in oxygen saturation coverage with increasing kinetic energy. In addition, EEL spectra show that molecularly chemisorbed states of oxygen are formed on the Ir(111) surface at Ts<70 K after exposure to a 1.36 eV beam and partial saturation of the atomic overlayer. Attempts to identify molecularly chemisorbed oxygen at low coverages were unsuccessful and limited by the experimental setup which provides cooling of the iridium crystal to only ∼68 K.
Comment on “Structure and spectroscopy of (HCN)n clusters: Cooperative and electronic delocalization effects in C–H⋯N hydrogen bonding” [J. Chem. Phys. 103, 333 (1995)]
Authors: Stone A. J. , Buckingham A. D. , Fowler P. W.
Published: July 2002

King and Weinhold assert in J. Chem. Phys. 103, 333 (1995) that the description of the hydrogen bond in terms principally of electrostatic interactions is incapable of describing the cooperative properties of hydrogen-bonded systems. However, provided that induction is included in the electrostatic interactions, the cooperative energy and dipole moment of HCN clusters can be quantitatively described.
Condensation of supersaturated vapors on monovalent and divalent ions of varying size
Authors: Seto Takafumi , Okuyama Kikuo , de Juan L. , Fernández de la Mora J.
Published: July 2002

Vapor nucleation induced by seed ions of controlled size and charge is studied for several monovalent positive and negative ions as well as divalent cations. Ions from dissolved salts are transferred into a gas via electrospray, purified by differential mobility selection, and introduced into a condensation nucleus counter of the mixing type using supersaturated dibutyl phthalate vapors. The number of drops nucleated on the ions is then measured as a function of the vapor supersaturation S. None of the anions tested is activated. Singly charged cations with mobilities Z from 0.48 up to 0.93 cm2/V/s are activated at values of ln S 30% smaller than predicted by Thomson’s model. All doubly charged cations tested (Z from 0.46 to 1.08 cm2/V/s) fall very near the Thomson curve, independently of their size or of whether the double ionic charge is localized on one metal atom or on two separate monovalent sites.
Dynamic heterogeneities and cooperativity in a lattice model glass
Authors: Heuer Andreas , Tracht Ursula , Spiess Hans W.
Published: July 2002

The dynamic heterogeneities of glass-forming materials are elucidated for a simple model glass, for which we choose a generalized version of the well-known facilitated kinetic Ising model, originally proposed by Fredrickson and Andersen. The fluctuations within the heterogeneous distribution of reorientation rates are quantified in terms of a rate memory parameter by comparing appropriately chosen two-time and four-time correlation functions. Qualitatively, this parameter expresses after how many relaxation processes a typical relaxator has forgotten its dynamic history. In agreement with most experimental observations the rate memory parameter is close to its minimum value, indicating that the fluctuations within the heterogeneous distribution are on the same time scale as the relaxation processes itself. Upon increasing the “cooperativity parameter” in the glass model the rate memory parameter decreases. The same tendency is observed for decreasing temperature. The apparent dependence of the rate memory parameter on cooperativity is rationalized. Furthermore the results are compared with previous simulations of the facilitated kinetic Ising model.
A storage ring for polar molecules
Authors: Katz Daniel P.
Published: July 2002

An electrostatic storage ring for polar molecules is proposed, including an analysis of the ring dynamics, loading, and losses for experimentally practical parameters. It is shown that storage lifetimes on the order of 103–104 s can be expected for methyl fluoride under reasonable operating conditions. A design for the storage ring is presented along with potential applications to molecular beam steering, molecular collision physics, and surface deposition and lithography. Other approaches to molecular storage are also briefly discussed.
Femtosecond time-resolved two-photon ionization spectroscopy of K2
Authors: Schwoerer H. , Pausch R. , Heid M. , Engel V. , Kiefer W.
Published: July 2002

We investigated the coherent motion of vibrational wave packets in the |B〉 1Πu state of the potassium dimer applying two color pump/probe spectroscopy with a sub 100 fs time resolution. Special interest was paid to the ionization probe step which was analyzed carefully by varying the probe energy over a wide range. Time-dependent quantum calculations explain the experimental outcomes by introducing a nonconstant transition dipole moment between the |B〉 and the ionic state |X+〉 and by taking into account the excitation of long lived autoionizing Rydberg states.
Two-pulse coherent control of electronic branching in Li2 photodissociation
Authors: Abrashkevich Dmitri G. , Shapiro Moshe , Brumer Paul
Published: July 2002

Control over the product branching ratio in the photodissociation of Li2 into Li(2s)+Li(2p) and Li(2s)+Li(3p) channels is explored computationally using the pump–pump coherent control scenario. Extensive control over the Li(3p)/Li(2p) branching ratio is demonstrated as the delay time between the two pulses is varied. The pulse width dependence is examined and better control is found to result from a narrow pump pulse which excites a superposition of only two levels, followed by a broad dissociation pulse.
Renormalization-group corrections to an approximate free-energy model for simple fluids near to and far from the critical region
Authors: Lue Leo , Prausnitz John M.
Published: July 2002

We combine liquid-state theory with a renormalization-group method to develop an equation of state for simple fluids that is valid near and far from the critical point. The resulting equation of state has nonclassical critical exponents close to those experimentally observed. Far away from the critical point, the equation of state reduces to that obtained by liquid-state theory. This formalism is applied successfully to square-well fluids, and to three real fluids: methane, carbon dioxide, and n-butane.
Density functional crystal orbital study on the normal vibrations and phonon dispersion curves of all-trans polyethylene
Authors: Hirata So , Iwata Suehiro
Published: July 2002

Optimized structural parameters and frequencies of the infrared- and Raman-active vibrations are obtained for all-trans polyethylene by using the analytical energy gradient scheme in the density functional crystal orbital formalism. The Slater–Vosko–Wilk–Nusair (SVWN), the Becke–Lee–Yang–Parr (BLYP), and the Becke3–Lee–Yang–Parr (B3LYP) functionals are used with the 3-21G and 6-31G* basis sets. The frequencies calculated with the 6-31G* basis set are found to be in better agreement with the observed frequencies than those calculated with the 3-21G basis set regardless of the exchange-correlation functionals used. The root mean square errors between the calculated and observed frequencies are 21, 20, and 15 cm−1 for the SVWN/6-31G*, the BLYP/6-31G*, and the B3LYP/6-31G* calculations, respectively. Optical branches of the phonon dispersion curves are calculated at the SVWN/6-31G* level by adopting a C7H14 unit as a reference unit cell. The calculated phonon dispersion curves are in reasonable agreement with the curves experimentally determined and with the curves obtained with an empirical force field except for the skeletal stretching branches. Inelastic neutron scattering (INS) spectrum is also calculated by using the force field derived at the SVWN/6-31G* level. The overall intensity profile of the observed INS spectrum is well reproduced by the present calculations in which the effects of the Debye–Waller factors and the phonon wings are taken into account.
Implementation of a quantum algorithm on a nuclear magnetic resonance quantum computer
Authors: Jones J. A. , Mosca M.
Published: July 2002

Quantum computing shows great promise for the solution of many difficult problems, such as the simulation of quantum systems and the factorization of large numbers. While the theory of quantum computing is fairly well understood, it has proved difficult to implement quantum computers in real physical systems. It has recently been shown that nuclear magnetic resonance (NMR) can be used to implement small quantum computers using the spin states of nuclei in carefully chosen small molecules. Here we demonstrate the use of a NMR quantum computer based on the pyrimidine base cytosine, and the implementation of a quantum algorithm to solve Deutsch’s problem (distinguishing between constant and balanced functions). This is the first successful implementation of a quantum algorithm on any physical system.
Ferroelectric properties of pyridinium periodate
Authors: Pająk Z. , Czarnecki P. , Wąsicki J. , Nawrocik W.
Published: July 2002

Polycrystalline pyridinium periodate has been studied over a wide range of temperature by differential thermal analysis, proton nuclear magnetic resonance (NMR) spectra and relaxation and complex dielectric permittivity measurements. Two solid-solid phase transitions at 210 and 323 K have been revealed and the pyridinium cation reorientation in all three phases characterized. The polar properties of the intermediate phase have been demonstrated. The upper phase transition temperature appears to be the Curie point of a new ferroelectric compound.
First-principles calculation of the conformation and electronic structure of polyparaphenylene
Authors: Miao M. S. , Van Camp P. E. , Van Doren V. E. , Ladik J. J. , Mintmire J. W.
Published: July 2002

In this article, an all-electron first-principles total energy calculation with Gaussian-type functions for the wave functions, for the exchange correlation potential, and for the charge density has been applied for single chains of polyparaphenylene (PPP). A local-density approximation within a helical band structure approach has been used. The calculated torsional potential shows a minimum at the torsion angle of 34.8°. The internal coordinates were optimized in the equilibrium conformation and are in good agreement with experimental and other theoretical results. The calculated direct band gap is 2.54 eV compared with the experimental result from UPS spectra of 3.4 eV for the gas phase. The band structure strongly depends on the conformation which suggests that the electronic properties can be modified in a wide range through doping or addition of side groups.
Semiclassical simulations of multidimensional Raman echoes
Authors: Mukamel Shaul , Piryatinski Andrei , Chernyak Vladimir
Published: July 2002

A high-temperature and a weak-nonlinearity (low-temperature) semiclassical expansion are developed for computing two-dimensional vibrational Raman spectroscopies, and applied to an exactly solvable Brownian-oscillator model. The origin of photon echoes is discussed using phase-space-wave-packets. Impulsive and semi-impulsive echoes are shown to satisfy different phase-matching conditions, and are generated in different directions.
Efficient collisional vibrational relaxation of triplet state molecules: Pyrazine deuteration and methylation effects
Authors: Wu Fei , Weisman R. Bruce
Published: July 2002

To further explore the collisional vibrational relaxation of triplet state polyatomic molecules, studies of pyrazine have been extended to its fully deuterated isotopomer (pyrazine-d4) and methyl derivative (2-methylpyrazine). The lowest triplet states of these compounds were prepared with 5596 and 5149 cm−1, respectively, of vibrational excitation through pulsed optical excitation followed by intersystem crossing. Collisional vibrational energy loss was then monitored using the refined competitive radiationless decay (CRD) method. The average energy losses per gas kinetic collision were measured as a function of donor vibrational energy for relaxation by He, Ne, Ar, Kr, Xe, H2, D2, CO2, H2O, CH4, and CH3F. Deuteration is found to cause very little change in the efficient energy loss behavior measured earlier in T1 pyrazine-h4. By contrast, methylation significantly increases the average energy lost per collision and also seems to lower the energy threshold for rapid relaxation. It is deduced that vibronic coupling between T1 and nearby triplet states is largely responsible for the high susceptibility to collisional energy loss. Many other molecules with closely spaced excited states may be expected to show collisional vibrational relaxation that is significantly enhanced by electronic excitation.
Predicting nonstatistical unimolecular reaction rates using Kramers’ theory
Authors: Guo Yin , Shalashilin Dmitrii V. , Krouse Justin A. , Thompson Donald L.
Published: July 2002

A method for computing unimolecular reaction rate constants in the IVR-limited regime is presented. It is based on Kramers’ energy diffusion theory, with the reaction coordinate taken as the subsystem and the rest of the vibrational modes as the bath. Applications to some bond fission reactions demonstrate that the method accurately predicts the rate constants for wide range of energies by using the result of a dynamical calculation of the reaction rate at a single energy to determine the friction coefficient. Examination of the energy exchange in the reaction coordinate provides a qualitative understanding of the validity of the approach for treating unimolecular reactions. Thus, the method provides a practical means of calculating reaction rates in the IVR-limited regime at considerable savings of computer time than that required by standard classical trajectory calculations.
Nonadiabatic molecular dynamics simulations of the photofragmentation and geminate recombination dynamics in size-selected I2−⋅(CO2)n cluster ions
Authors: Margulis C. J. , Coker D. F.
Published: July 2002

In this paper we investigate the photodissociation, geminate recombination and relaxation dynamics in size-selected I2−⋅(CO2)n cluster ions using an electronically nonadiabatic molecular dynamics method and a model Hamiltonian gained from diatomics-in-ionic systems (DIIS). These theoretical studies make contact with recent time resolved pump-probe and photoelectron detachment experiments. Our studies reveal a rich excited state dynamics in which various competing electronic relaxation channels as well as vibrational relaxation influence the recovery of signal in these experiments.
Electron-pair densities of group 14, 15, and 16 atoms in their low-lying multiplet states
Authors: Koga Toshikatsu , Matsuyama Hisashi , Dehesa J. S. , Thakkar Ajit J.
Published: July 2002

Electron-pair intracule (relative motion) h(u) and extracule (center-of-mass motion) d(R) densities are studied for the 3P, 1D, and 1S multiplet states of five group 14 atoms with p2 configurations, the 4S, 2D, and 2P multiplets of five group 15 atoms with p3 configurations, and the 3P, 1D, and 1S multiplets of five group 16 atoms with p4 configurations. For all 15 atoms, it is confirmed that a lower total energy E in a high angular momentum state is always accompanied with the decrease of the electron–nucleus attraction energy Ven which is greater than the increase in the electron–electron repulsion energy Vee, as discussed in the literature for a few light atoms. In all the cases, the difference in the radial intracule densities 4πu2h(u) clarifies that for a multiplet with a higher angular momentum, the probability of finding a pair of electrons separated by a short/long distance is larger/smaller, leading to a greater Vee and a smaller average interelectronic distance 〈u〉. The Fermi hole effect is not important for the relative stability of the multiplets examined. For the multiplets arising from the pn electronic configurations, the differences in the radial extracule densities 4πR2d(R) are found to be almost isomorphic with the corresponding intracule ones: In a multiplet with a high angular momentum, the average center-of-mass radius 〈R〉 of an electron pair is always smaller, and two electrons are more likely to be at opposite positions with respect to the nucleus. Accordingly, the valence p orbital in a more stable multiplet is more tight, resulting in a lower electron–nucleus attraction energy Ven for all the 15 atoms.
Connection between back-reaction boundary conditions and approach to equilibrium for double square wells
Authors: Weaver David L.
Published: July 2002

One-dimensional diffusion with back-reaction at a boundary is calculated and compared with diffusion in an asymmetrical double well. Good agreement is found between exact analytical or numerical results and a suggested mean equilibrium time approximation. The results are applied to a model for the helix-coil transition in linear biopolymers and a reasonable time scale for this process results.
Vibrational structure in the vinylidene anion photoelectron spectrum: Closing the gap between theory and experiment
Authors: Stanton John F. , Gauss Jürgen
Published: July 2002

Photoelectron spectra of the vinylidene anion exhibit rich vibrational structure, including several transitions of the neutral that lie well above the barrier for rearrangement to acetylene. Most of these features have been assigned, and are largely consistent with vibrational energy levels estimated by ab initio calculations. Notable exceptions include the 602 and 611 vibronic transitions which involve the CH2 rocking coordinate that ultimately leads to isomerization. Very poor agreement between theory and experiment has been attributed to a neglect of vibrational anharmonicity in the former, a hypothesis that has not been verified computationally. This issue forms the focus of the present work, in which high-level calculations and an explicit determination of the anharmonic force fields of both the anion and neutral are used to predict the vibrational energy levels. Excellent agreement is achieved for both 602 and 611, as both calculated values fall within the experimental error bars.
Ab initio investigation of the vibronic spectrum involving the two lowest-lying electronic states of HCCO
Authors: Schäfer Boris , Perić Miljenko , Engels Bernd
Published: July 2002

The results of ab initio calculations of the vibronic spectra involving both lowest-lying states X2A″ and A2A′ of the ketenyl radical HCCO are presented. Potential energy surfaces are computed by means of the coupled cluster method with perturbative triple corrections [CCSD(T)]. A recently developed ab initio approach for the treatment of the Renner–Teller effect in tetra-atomic asymmetric molecules is applied to calculate the vibronic energy levels.
Potential energy surface and quasiclassical trajectory studies of the N(2D)+H2 reaction
Authors: Pederson Lisa A. , Schatz George C. , Ho Tak-San , Hollebeek Timothy , Rabitz Herschel , Harding Lawrence B. , Lendvay György
Published: July 2002

We present a global potential energy surface for the 1A″ state of NH2 based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio (multireference configuration interaction) results. Extensive quasiclassical trajectory calculations are performed on this surface to study the N(2D)+H2/D2 reaction dynamics. Comparison is made with calculations on the lower level [first order configuration interaction (FOCI)] surface of Kobayashi, Takayanagi, Yokoyama, Sato, and Tsunashima (KTYST). We find a saddle point energy of 2.3 (1.9) kcal/mol for the perpendicular approach for the second order configuration interaction (SOCI) (SOCI with Davidson correction) surfaces, and a collinear stationary point energy of 5.5 (4.6) kcal/mol. The ordering of these stationary points is reversed compared to the corresponding FOCI results, and the only true reaction path on our surface is perpendicular. The primary reaction mechanism is determined to be C2v insertion to produce short lived (100–300 fs) NH2 intermediates. Angular distributions are found to be primarily forward–backward symmetric, with a slight bias towards backward scattering at low energies. Decay of the NH2’s occurs before energy is fully randomized, so the product vibrational distributions are a little hotter than statistical—with vibrational population ratios NH(v″=1)/NH(v″=0)=0.8 and ND(v″=1)/ND(v″=0)=0.9 (near threshold). These ratios, and other aspects of the vibrational product distributions are in excellent agreement with recent laser induced fluorescence studies.
Kinetic energy analysis of O(3P) and O2(b 1Σg+) fragments produced by photolysis of ozone in the Huggins bands
Authors: O’Keeffe Patrick , Ridley Trevor , Lawley Kenneth P. , Maier Robert R. J. , Donovan Robert J.
Published: July 2002

The velocity profile of O2(b 1Σg+,v=0) produced by photolysis of O3 in the Huggins band region at 351.4 nm has been measured using a delayed pulsed field extraction time-of-flight technique confirming that this fragment is formed by single-photon absorption. The velocity profile of O(3P0) produced by photolysis at 322.64 nm has also been obtained. O(3P0) fragments are shown to be produced in coincidence with O2(X 3Σg−), O2(a 1Δg), and O2(b 1Σg+). The relative contribution of each of these channels to the total O(3P0) signal is reported and it is shown that spin-forbidden channels dominate the dissociation process at this wavelength. Two quite different primary crossings of the initially excited state are suggested to account for the change in the relative contribution of these three channels when photolysis is changed from resonance with a vibronic band of ozone to an off-resonance wavelength. The determination of the anisotropy parameter, β(v), for these dissociation processes suggests that the transition dipole responsible for all of the Huggins band absorption has B2 symmetry.
Theoretical study of an isotope effect on rate constants for the CH3+H2→CH4+H and CD3+H2→CD3H+H reactions using variational transition state theory and the multidimensional semiclassical tunneling method
Authors: Kurosaki Yuzuru , Takayanagi Toshiyuki
Published: July 2002

Vibrationally adiabatic ground-state (VAG) potential curves for the CH3+H2→CH4+H (I) and CD3+H2→CD3H+H (II) reactions have been calculated at the QCISD(T,full)/cc-pVTZ//MP2(full)/cc-pVTZ level of theory. It has been found that both the barrier height and barrier width of the VAG potential curve for reaction II are slightly smaller than those for reaction I. Thermal rate constants for reactions I and II have been calculated with the obtained VAG potentials using variational transition state theory combined with the multidimensional semiclassical tunneling method. Rate constants for reaction II have been predicted significantly larger than those for reaction I especially at low temperatures. This computational result qualitatively explains the experimental observation of Momose et al. [J. Chem. Phys. 108, 7334 (1998)] that reaction I does not occur but reaction II occurs at 5 K in solid parahydrogen.
Dielectric beta relaxations in the glassy state of salol?
Authors: Wagner Hermann , Richert Ranko
Published: July 2002

The recently observed possibility to suppress the β-relaxation intensity of o-terphenyl by annealing at temperatures below the glass transition guided us to ask, whether the absence of a dielectric β process in many glass-forming materials, e.g., salol (phenyl salicylate), is a matter of the slow cooling rates usually employed to enter the glassy state. In order to assess this issue, we have quenched liquid salol to well below Tg at a rate of dT/dt=−490 K/min. Opposed to the case of cooling rates around −5 K/min or slower, this highly quenched sample displays a symmetric dielectric relaxation peak near f=103 Hz with an appreciable relaxation strength, Δε=6×10−3. This novel feature of salol disappears irreversibly after a temperature excursion towards the glass transition at Tg=220 K.
Vibrational levels and statistical analysis of the X(1Σg+) ground state of CS2
Authors: Brasen G. , Demtröder W.
Published: July 2002

Single rovibronic levels in the perturbed B21(Δu1) state of CS2 molecules in a cold molecular beam were excited by ultraviolet (UV) photons from a frequency doubled single mode continuous wave (cw) dye laser. The dispersed fluorescence spectrum, detected by a liquid nitrogen (LN) cooled charge coupled device (CCD) array behind a monochromator allowed the determination of vibrational term values in the X(1Σg+) state from the (0,00,0) level up to 20 000 cm−1. Based on calculations including Fermi resonances, most of the measured vibrational bands could be assigned to several vibronic ground state level progressions. A statistical analysis of the positions of all measured levels shows for the nearest neighbor distances and the Δ3 statistics a pure Poissonian distribution. This indicates that no strong correlation between neighboring vibrational levels nor long range interactions are present and therefore no chaotic behavior could be found in contrast to the cases of NO2 and SO2.
Physical origin of the high reactivity of subsurface hydrogen in catalytic hydrogenation
Authors: Michaelides A. , Hu P. , Alavi A.
Published: July 2002

In the catalytic hydrogenation of hydrocarbons, subsurface hydrogen is known experimentally to be much more reactive than surface hydrogen. We use density functional theory to identify low-energy pathways for the hydrogenation of methyl adsorbed on Ni(111) by surface and subsurface hydrogen. The metastability of subsurface hydrogen with respect to chemisorbed hydrogen is mainly responsible for the low activation barrier for subsurface reactions.
On the role of dielectric friction in vibrational energy relaxation
Authors: Ladanyi Branka M. , Stratt Richard M.
Published: July 2002

The phrase “dielectric friction” tends to bring to mind the drag force exerted by a polar liquid on some translating ion or rotating dipolar molecule, but the underlying idea is far more general. Any relaxation process taking place in a polar environment, including those involving solvation and vibrational relaxation, has the potential to be strongly affected by the special dynamics associated with Coulombic forces. Indeed, there is considerable evidence that vibrational energy relaxation is noticeably accelerated in hydrogen-bonding solvents. What is less clear is precisely how electrostatic forces achieve the accelerations they do and to what extent this phenomenon relies on specifically protic solvents. We explore this issue in this paper by using classical molecular dynamics to study the vibrational population relaxation of diatomic solutes with varying levels of polarity dissolved in both dipolar and nondipolar aprotic solvents. We find that the conventional analysis based on partitioning the force autocorrelation function can be usefully extended by adapting an instantaneous perspective; distinguishing between the purely equilibrium effects of the instantaneous liquid structure surrounding a solute and the solely nonequilibrium effects of the relaxation dynamics launched from those initial conditions. Once one removes the powerful influence of electrostatic forces on the liquid structure, either by simple normalization or by looking at the “force-velocity” autocorrelation function, the subsequent dynamics (and therefore the mechanism) of the relaxation is revealed to be dominated by short-ranged repulsive forces, even under the most polar circumstances. The main rate-enhancing effect of Coulombic forces seems to be an equilibrium electrostriction: The solvent is simply ordered around the solute in such a way as to amplify the repulsive forces. At least in our examples, the slowly varying character of Colombic forces actually makes them quite ineffective at any kind of direct promotion of vibrational energy relaxation.
Hydrogen bond and cooperative effect in the reactions of HOCl with HCl on water clusters
Authors: Xu Si Chuan
Published: July 2002

The reactions of HOCl with HCl on water clusters have been theoretically investigated. Ab initio calculations indicate that hydrogen bond and cooperative effect play an essential role in the reactions; that the reaction barrier of HOCl with cyclic (H2O)n.HCl cluster is the least at n=3 and that the ionization of HCl and HOCl on ice surfaces may not be complete but partly at very low temperatures. Two cases a and b of the model reactions are considered for detailed analysis. On the surface of ice, the barrier energies are about 4 and 6 kcal/mol for cases a and b, respectively, at the MP2//HF/6-31G(d) level, which is close to an experimental estimation. This study suggests a similar previously reported mechanism that the heterogeneous reaction of HOCl with HCl on ice is catalyzed at the stratospheric conditions through structure catalysis and hydration that enhances ion character of species.
On the drift mobility of a molecular polaron in the presence of Coulomb traps
Authors: Rackovsky S. , Scher H.
Published: July 2002

We study the drift mobility of a molecular polaron in the presence of an external applied field and Coulomb traps. The model is based on one previously developed for geminate recombination of photogenerated charge carriers. It is shown that the unusual combination of Poole–Frenkel-like field dependence and non-Arrhenius temperature dependence of the mobility, measured experimentally in molecular films, is well reproduced by this model. Our key result is that this nearly universal experimental behavior of the mobility arises from competition between rates of polaron trapping and release from a very low density of Coulomb traps.
Two-dimensional vibrational spectroscopy. I. Theoretical calculation of the nonlinear Raman response function of CHCl3
Authors: Hahn Sangjoon , Park Kisam , Cho Minhaeng
Published: July 2002

The two-dimensional Raman response function of CHCl3 is theoretically considered with interpretations of each peak in terms of the associated vibrational transition pathways. In order to numerically calculate the 2D Raman spectrum, ab initio calculations of necessary quantities, such as the first- and second-order derivatives of the molecular polarizability with respect to vibrational coordinates and cubic potential anharmonic coefficients, were carried out by using the basis set 6-311++G(2df,2pd) at the Hartree–Fock level. Quantitative comparison between the two nonlinear response functions associated with the mechanical and electronic anharmonicities shows that the 2D Raman response from the high-frequency intramolecular vibrational modes of CHCl3 is mainly determined by the mechanical (potential) anharmonicity contributions. On the other hand, it is found that the two distinctive contributions originating from the mechanical and electronic anharmonicities interfere in the low-frequency region of the 2D spectrum. Overall, it is suggested that the high-frequency 2D Raman spectrum could provide a map of the mechanical anharmonic mode couplings. We briefly discuss how the 2D Raman spectrum can be used to elucidate the potential energy hypersurface and in turn to study the intramolecular vibrational energy redistribution process.
Nematic-isotropic interfaces in semiflexible polymer blends
Authors: Drovetsky Boris Y. , Liu Andrea J. , Mak C. H.
Published: July 2002

We present calculations of the anisotropy of the tension of interfaces separating an isotropic phase rich in a flexible polymer from a nematic phase rich in a semiflexible polymer. We find that the interfacial tension is always lower when the director is parallel to the plane of the interface than when it is perpendicular. The ratio of the tension in the perpendicular to parallel case can be quite large, of order 5 or so, depending on the strength of the nematic interactions. We also find that the interfacial width is always lower in the parallel case even though the tension is lower; this is because the orientational density prefers parallel alignment of chains at the interface.
Molecular dynamics simulations of sodium chloride solutions in water–dimethyl sulphoxide mixtures: Potentials of mean force and solvation structures
Authors: Das Ashok K. , Tembe B. L.
Published: July 2002

Five solutions of sodium chloride in mixtures of water and dimethyl sulphoxide (DMSO) have been simulated using the conventional molecular dynamics technique. The potentials of mean force (PMFs) of the sodium chloride ion pair in the presence of the five water–DMSO mixtures with DMSO mole fractions (xDMSO) of 0.10, 0.21, 0.35, 0.48 and 0.91 have been computed. The derived PMFs have been confirmed by the long time dynamical ion-pair trajectories. The solvation structures of the ions in the presence of these mixtures have been analyzed using the ion-solvent radial distribution functions and the corresponding integration numbers. It has been found that the Na+ ion is always preferentially solvated by the water molecules in all the water–DMSO solvent mixtures. The Cl− ion is slightly preferred by the DMSO molecules in these mixed solvents.
Coordination and reaction mechanism of furan on Ru(001)
Authors: Yan F. Q. , Qiao M. H. , Wei X. M. , Liu Q. P. , Deng J. F. , Xu G. Q.
Published: July 2002

The surface chemistry of furan adsorbed on Ru(001) at 80 K has been explored using thermal desorption spectroscopy, x-ray and ultraviolet photoelectron spectroscopies and high resolution electron energy loss spectroscopy (HREELS). Furan adsorption follows a three-dimensional island growth mode. Three desorption peaks, corresponding to multilayer, second layer, and monolayer were identified at 117, 142 and 220 K, respectively. Monolayer species strongly bind to the surface, showing a much lower O(1s) binding energy (532.9 eV) as compared to the physisorbed furan (534.8 eV). HREELS reveals that within the monolayer, there are two different chemisorbed species with molecular rings oriented either tilted or parallel to the surface. By annealing the surface to 180 K, the parallel furan π complex undergoes α-C electrophilic addition and H elimination, forming the tilted α-furyl species and Ru–H. The α-furyl can either recombine with hydrogen to form furan desorbing between 180 and 250 K, or decompose to give adsorbed oxygen atoms and a metallocyclelike intermediate through a direct oxygen abstraction mechanism. Further thermal annealing leads to the formation of carbon monoxide and hydrogen desorbing from the surface.
Spin–spin model for two-level system/bath problems: A numerical study
Authors: Golosov Andrei A. , Tsonchev Stefan I. , Pechukas Philip , Friesner Richard A.
Published: July 2002

We study a new model for treating quantum dissipative systems, in which the bath is modeled as a collection of spins coupled to the system of interest. We develop a quasiclassical method to study this model, approximating the quantum Heisenberg equations by the classical ones, supplemented with stochastic initial conditions carefully chosen so that the results obtained from the classical equations are as close as possible to the quantum results. Using this method we compare the dynamics of such a spin–spin system with that of a spin–boson system, in which the bath is modeled as a collection of harmonic oscillators. We verify numerically that when the system-bath coupling is spread over many bath spins (the Brownian motion limit), the spin–spin model can be mapped on the spin–boson model (although with a temperature dependent spectral density). We also demonstrate that the two dissipative models are qualitatively very different in a non-Brownian motion regime.
Resonant degenerate four-wave mixing spectroscopy of transitions with degenerate energy levels: Saturation and polarization effects
Authors: Reichardt Thomas A. , Lucht Robert P.
Published: July 2002

The physics of the degenerate four-wave mixing process for resonant transitions between two degenerate energy levels is investigated by direct numerical integration of the time-dependent density matrix equations. The Zeeman structure of the upper and lower energy levels is included in a multistate formulation of the density matrix equations. The inclusion of the Zeeman structure enables the investigation of the degenerate four-wave mixing process for different polarization configurations of the forward pump, backward pump, and probe beams. Saturation curves and lineshapes are calculated for different polarization configurations and for numerous low-J transitions. At low laser intensity, the results of our calculations are in excellent agreement with perturbation theory in terms of the relative intensities of the degenerate four-wave mixing signal for linear polarization configurations. As the laser intensity increases and the resonance starts to saturate, we find in general that the relative degenerate four-wave mixing reflectivity increases for the crossed polarization configurations compared to the parallel polarization configuration because the saturation intensity is higher. However, for some resonance transitions, some of the crossed polarization configurations saturate at lower laser intensities than the parallel polarization configuration, even though the reflectivity for these crossed polarization configurations is much lower than for the parallel polarization configuration in the perturbative intensity limit. This result is explained in terms of the coupling of the various Zeeman states during the degenerate four-wave mixing interaction for specific polarization configurations. The effect of saturation on the resonance line shapes for the different polarization configurations is also investigated. Finally, a limited number of calculations are performed for resonances that are Doppler broadened as well as collision broadened. The effect of saturation on the reflectivity of the crossed polarization configurations compared to the parallel polarization configuration is even more significant for resonances with comparable Doppler and collisional broadening.
Vibronic interactions in the C36 tri- and tetra-anions
Authors: Yoshizawa Kazunari , Tachibana Masamitsu , Yamabe Tokio
Published: July 2002

We address the intramolecular vibronic interactions in the C36 tri- and tetra-anions to understand the Jahn–Teller effects and possible superconductivity in “electron-doped” C36 solids. We use the B3LYP hybrid Hartree–Fock/density-functional-theory method for our theoretical analyses. Neither the highest occupied molecular orbital (HOMO) nor the lowest unoccupied molecular orbital (LUMO) of the C36 molecule with D6h symmetry are degenerate, but the next LUMO is twofold degenerate. One can therefore expect Jahn–Teller distortions and interesting electronic properties in the C36 anions. Computed vibronic and electron–phonon coupling constants of the tetra-anion are about twice as large as those of the tri-anion. The second lowest Jahn–Teller active E2g mode of 561 cm−1 is predicted to have the largest coupling constants in both anions. We calculate superconducting transition temperature Tc from McMillan’s formula using the coupling constants as well as electronic densities of states at the Fermi level and Coulomb pseudopotentials as parameters.
Improvement on Lennard-Jones–Devonshire theory for predicting liquid–solid phase transition
Authors: Modarress Hamid , Ahmadnia Ebrahim , Mansoori G. Ali
Published: July 2002

In this paper the potential energy of the Lennard-Jones and Devonshire (LJD) model has been corrected to increase its PVT prediction abilities. The correction of potential energy is done in two ways (i) taking the coordination number as a function of temperature and volume to improve the second virial prediction ability and (ii) taking pair correlation effects into account to improve the phase transition prediction ability. The results of calculations indicate improvement both on PVT behavior in the above range of variation especially on the phase transition prediction ability in solid–liquid and vapor–liquid regions.
Simulation of crystal growth from Lennard-Jones solutions
Authors: Huitema H. E. A. , van Hengstum B. , van der Eerden J. P.
Published: July 2002

We simulate crystal growth from solution using the Monte Carlo method in the semigrand-isobaric–isothermal ensemble. All crystals are grown in the face-centered-cubic (100) direction, while varying the solubility and temperature. This enables us to change the growth mode from linear to nonlinear. In order to simulate at time scales necessary for growth from solution, we devised and used smart Monte Carlo moves. These moves enhance the solute–solvent interdiffusion processes, similar to convection in experimental situations, while leaving the kinetics in the adsorption layer between the crystal and the solution unaffected. These kinetics then become the rate determining step. The structure and dynamics of the interfacial region is investigated quantitatively, leading to the conclusion that especially during rough, three-dimensional growth, trapping of solvent particles in newly grown crystal layers is the rate determining process.
Interaction of gas-phase atomic chlorine with a silicon surface: Reactions on bare and hydrogen-chemisorbed surface sites
Authors: Ree J. , Shin H. K.
Published: July 2002

The reaction of gas-phase atomic chlorine with hydrogen atoms chemisorbed on a silicon surface is studied by use of the classical trajectory approach. In the model the gas atom interacts with the preadsorbed hydrogen atom and adjacent bare surface sites. The reaction zone atoms are configured to interact with a finite number of primary-system silicon atoms, which are coupled to the heat bath. The study shows that the chemisorption of Cl(g) is of major importance. Nearly all of the chemisorption events accompany the desorption of H(ad), i.e., a displacement reaction. Although it is much less important than the displacement reaction, the formation of HCl(g) is the second most significant reaction pathway. At a gas temperature of 1500 K and surface temperature 300 K, the probabilities of these two reactions are 0.829 and 0.082, respectively. The chemisorption of Cl(g) without dissociating H(ad) and collision-induced dissociation of H(ad) are found to be negligible. In the reaction pathway forming HCl, most of the reaction energy is carried by HCl(g). The ensemble-averaged vibrational, rotational, and translational energies are 37.4%, 35.6%, 18.3% of the liberated energy, respectively. Less than 9% of the energy dissipates into the solid phase. Although the majority of HCl produced in the gas phase belongs to a fast component of the time-of-flight distribution for a direct-mode reaction, there is a significant amount of HCl belonging to a slow component, which is characteristic of complex-mode collisions.
Photochemistry of disilane adsorbed on a H terminated Si(100) surface
Authors: Wright Steven , Hasselbrink Eckart
Published: July 2002

The photochemistry of Si2H6 adsorbed on H terminated Si(100) has been investigated using the time-of-flight (TOF) technique and high resolution electron energy loss spectroscopy (HREELS). Intact Si2H6 desorbs via a photochemical mechanism during illumination with either 5.0 or 6.4 eV photons. Two cross sections differing by two orders of magnitude are required to describe photodesorption of the first weakly chemisorbed monolayer. It is likely that molecules adsorbed in this state adopt one of two orientations. We suggest that, as in the Antoniewcz model, the initial excitation involves temporary electron trapping and that the molecular orientation dependence of this process is reflected by the two cross sections. Photofragmentation is initiated by 6.4 eV photons but not by 5.0 eV photons and results in the desorption of mostly SiH4 and H2. TOF distributions corresponding to these two products are bimodal. The fast component corresponds to those fragments that are ejected with sufficient kinetic energy to overcome the desorption barrier directly whereas the slow component represents those fragments that become trapped and then thermally desorb. While the absence of fragmentation at a photon energy of 5.0 eV is consistent with the fact that the threshold for direct excitation is found to be at 5.2 eV, there is strong evidence that an indirect mechanism, again involving the formation of a temporary anion, is responsible for fragmentation. Specifically, HREELS shows that there are two resonances centered at 1.5 and 2.7 eV above the vacuum level. It appears that desorption can be initiated by electron trapping in either of these two resonances whereas fragmentation only occurs when a hot electron is trapped in the higher energy resonance. In common with photodesorption of intact Si2H6, photofragmentation is best described by two cross sections, although in this case they differ by a remarkable three orders of magnitude. There are at least two major Si2H6 fragmentation channels. It is proposed that one of these yields SiH4, Si and H2 when a short-lived electronic excitation causes the nuclei to accelerate along a reaction coordinate leading to these products whereas the other yields SiH4 and SiH2 as a result of randomized vibrational excitation.
Glancing incidence scattering of hyperthermal He+, Xe+, and C60+ from graphite: Angular and velocity distributions of neutrals
Authors: Hillenkamp Matthias , Pfister Jörg , Kappes Manfred M. , Webb Roger P.
Published: July 2002

In a comparative study of size and mass effects in the glancing incidence scattering of hyperthermal ions from a soft surface, He+, Xe+, and C60+ were scattered from highly oriented pyrolitic graphite (HOPG) over an energy range between 200 and 5000 eV. Predominantly neutral scattering products were observed for all projectiles. The corresponding angular and velocity distributions were recorded in the scattering plane. He+ gives rise to broad angular distributions conforming to the predictions of a simple binary collision model. With full widths at half maximum of less than 6 deg at typical collision energies, the angular distributions found for Xe+ and C60+ are very narrow. While Xe+ is always scattered near-specularly independent of collision energy, neutrals resulting from C60+ exit subspecularly—increasingly so the higher the collision energy. The experimental data are discussed in terms of dynamical simulations which indicate that Xe+ collides with a rigid ensemble of six carbon atoms while the fullerene scatters from a larger “moving target.”