bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of Maxwell displacement current across nematic liquid crystal cells triggered by conformational change of surface monolayer
Authors: Iwamoto Matsumasa , Kim Woo-Yoen , Wu Chen-Xu
Published: July 2002

The generation of Maxwell displacement current (MDC) across nematic liquid crystal (LC) cells triggered by the conformational change of surface monolayers is analyzed. It is theoretically shown that the change in the internal electric field formed in LC cells by the polarized surface monolayers and the change in the capacitance of LC cells due to the change in the orientational alignment of LCs are responsible for the generation of MDC. Based on this theoretical analysis, we discuss MDCs across p-pentyl-p-cyano-biphenyl LC cells induced by photoisomerization of surface azobenzene monolayers.
Broadening of CH3F in presence of Stark fields. I. Self-broadening and self-shifting of isolated components
Authors: Lemaire V. , Dore L. , Cazzoli G. , Buffa G. , Tarrini O. , Belli S.
Published: July 2002

Self-broadening and shifting of methyl fluoride CH312F have been investigated in the 50–408 GHz range for the rotational transitions J=0→1, J=1→2, J=2→3, J=5→6, and J=7→8 and for their ΔM=0 and ΔM=±1 Stark components, as they were resolved by an appropriate field. The different results obtained for M-degenerated and M-resolved cases, as well as the M dependence of the results has been interpreted theoretically within semiclassical perturbative approximations. Evidence of collisional coupling effects between Stark components was obtained.
The effect of two- and three-body interactions in ArnCO2 (n=1,2) on the asymmetric stretching CO2 coordinate: An ab initio study
Authors: Rak Janusz , Szczȩśniak M. M. , Chal/asiński Grzegorz , Cybulski Sl/awomir M.
Published: July 2002

The dependence of the two-body and three-body interactions in the ArnCO2 cluster upon the intramolecular asymmetric stretching coordinate of CO2 is studied by the ab initio method. In the T-shaped binary complex Ar–CO2, the influence of the components of the interaction energy on the shift of the asymmetric stretching frequency of CO2 (ν3) is estimated within a one-dimensional vibrational model and compared with the experimental data of Sperhac, Weida, and Nesbitt [J. Chem. Phys. 104, 2202 (1996)]. The interaction energy is dissected into Heitler–London, induction, and dispersion energies and their respective intrasystem correlation corrections. The redshift represents a delicate balance of these effects on the v=0 and v=1 levels. The highly correlated treatment is required to describe the dependence of two-body potential upon the stretching coordinate. The supermolecular coupled cluster calculations with the single, double, and noniterative triple excitations reproduce the shift observed by Sperhac et al. with excellent accuracy. In the Ar2CO2 trimer with the two Ar atoms in equatorial positions, the influence of the three-body interaction components on the v=0 and v=1 levels is analyzed. A model of the three-body potential, including three nonadditive components, exchange, induction, and dispersion is applied. It describes the departure from additivity of the two-body shifts observed by Sperhac et al. with excellent accuracy. The analytical models of the energy components are also discussed.
Comment on “New strategies to incorporate the solvent polarization in self-consistent reaction field and free-energy perturbation simulations” [J. Chem. Phys. 103, 10183 (1995)]
Authors: Ángyán János G.
Published: July 2002

Expressions for the solute polarization and distortion components of the free energy of solvation, proposed by Luque, Bofill, and Orozco, are analyzed in terms of the Rayleigh–Schrödinger perturbation theory applied to the nonlinear Schrödinger equation of the self-consistent reaction field model. At the second order, as far as nonlinearity effects are neglected, the distortion energy is equal to the negative of the polarization energy. This equality does not hold for the self-consistent second-order case.
Dielectric relaxation in the fragile viscous liquid state of toluene
Authors: Dö\S Axel , Hinze Gerald , Schiener Bernd , Hemberger Joachim , Böhmer Roland
Published: July 2002

Dielectric measurements were carried out on viscous toluene covering a frequency range from 0.1 Hz to 1 MHz. In order to suppress the pronounced crystallization tendency of this supercooled liquid it was contained in thin walled capillaries with outer diameters of 300 μm. From the temperature dependence of the characteristic dielectric relaxation times it was found that toluene is one of the most fragile low molecular weight glass-forming liquids, with a fragility index m=105. By comparison with time constants available from other experimental techniques it appears that near the glass transition the dielectric relaxation mode is not the slowest one.
A molecular beam photoionization mass spectrometric study of Cr(CO)6, Mo(CO)6, and W(CO)6
Authors: Chen Y.-J. , Liao C.-L. , Ng C. Y.
Published: July 2002

The photoionization efficiency (PIE) spectra for M(CO)n+ (n=0–6) from M(CO)6, M=Cr, Mo, and W, have been measured in the photon energy range of 650–1600 Å. Based on the ionization energies for M(CO)6 and appearance energies (AEs) for M(CO)n+ (n=0–5) determined here, we have obtained estimates for the sequential bond dissociation energies (D0) for CO–M(CO)n−1+ (n=1–6). The comparison between the D0 values for the Cr(CO)6+ system obtained here and in the recent collisional induced dissociation and theoretical studies suggests that D0 values for CO–M(CO)n−1+ (n=3–6) based on this PIE experiment are reliable. The PIE results reveal the general trend for individual D0 values that D0[CO–Cr(CO)n−1+]<D0[CO–Mo(CO)n−1+]<D0[CO–W(CO)n−1+] (n=3–6). The comparison of the first D0 values for M(CO)6+ obtained here and those for M(CO)6 reported previously provides strong support for the theoretical analysis that the importance of relativistic effects, which give rise to more efficient M to CO π-back-donation in M(CO)6, is in the order W(CO)6>Mo(CO)6>Cr(CO)6.
Electron transfer via bridges
Authors: Bixon M. , Jortner Joshua
Published: July 2002

In this paper we explore the energetic control of sequential and superexchange electron transfer (ET) mechanisms on the basis of quantum-mechanical simulations and calculations for long-range ET in DBA systems, where the donor (D) and the acceptor (A) are separated by a bridge (B). We studied ET dynamics in a Franck–Condon (FC) system characterized by three multi-dimensional displaced harmonic potential surfaces, where an initial single vibronic doorway state |α〉 (with energy Eα) in the DBA (≡D) electronic state is coupled to the mediating {|β〉} vibronic quasicontinuum of the D+B−A (≡B) electronic state, which in turn is coupled to the final {|γ〉} vibronic quasicontinuum of the D+BA− (≡A) electronic state. The level structure was described by the vibrational frequencies (for a four-mode harmonic system) and the energy gaps ΔGDB and ΔGDA between the origins of the corresponding electronic states (with nα=1–50, nβ=1000–2000, and nγ=1000–2000 states in the {|α〉}, {|β〉}, and {|γ〉} manifolds, respectively), while the couplings were characterized by the spectral densities and by the pair correlations (specified in terms of correlation parameters ηαα′ and ηββ′) between states belonging to the same manifold. The correlation parameters ηαα′ (α,α′=1–40) for the doorway-quasicontinuum coupling and ηββ′ (β,β′=150–190) for the interquasicontinuum coupling are considerably lower than unity (|ηαα′|⩽0.4 and |ηββ′|⩽0.3), obeying propensity rules with the highest values of |ηαα′| and |ηββ′| which correspond to a single vibrational quantum difference, while for multimode changes between α and α′ or between β and β′ very low values of |ηαα′| or |ηββ′| are exhibited. Radiationless transitions theory was applied for quantum-mechanical simulations based on the dynamcis of wave packets of molecular eigenstates for resonance (ΔGDBEα) coupling. Resonance |α〉–{|β〉}–{|γ〉} coupling results in two-step sequential ET kinetics for all doorway states |α〉, manifesting phase erosion due to weakly correlated intercontinuum coupling, without the need of intermediate state phonon induced thermalization. Off-resonance |α〉–{|β〉} coupling in conjunction with {|β〉}–{|γ〉} resonance interactions results in unistep superexchange ET kinetics. The simulated sequential ET rates and the superexchange rate are in good agreement with the calculated quantum-mechanical rates obtained using the electronic couplings and FC densities. The energy-gap (ΔGDB) dependence of the simulated and the calculated ET rates from a single doorway state reveal a “transition” from sequential to superexchange ET with increasing ΔGDB. For a finite-temperature system, characterized by a fixed ΔGDB (>0) small energy gap, the thermally averaged rate from a canonical ensemble of doorway states will result in the superposition of both superexchange and sequential mechanisms.
Spatiotemporal behavior of convective Turing patterns in porous media
Authors: Marlow Matthew , Sasaki Yuji , Vasquez Desiderio A.
Published: July 2002

We investigate the effects of convection on Turing patterns in porous media. We show that convectionless patterns can only exist confined to small domains. These patterns are unstable to convection if the density gradients are large enough. The numerical solution of the Schnackenberg model coupled to Darcy’s law shows that the convective pattern is either steady, oscillatory, or reverses direction, depending on the density gradient. In larger domains, we find that convection leads to an oscillatory state which becomes steady for large density gradients.
Resonant effects in photoionization of H2 and D2
Authors: Sánchez I. , Martı́n F.
Published: July 2002

We have evaluated photoionization cross sections of H2 and D2 with emphasis in resonant effects through intermediate doubly excited states. The theoretical method describes consistently the interference between resonant and nonresonant ionization processes, and includes both dissociative and nondissociative channels. We show that resonant peaks are rather apparent in vibrationally resolved spectra associated to the Σu+1 symmetry. In contrast, no resonant structure is observed in the total photoionization spectra, in agreement with recent experimental findings. Finally, we show that the ratios of dissociated to undissociated photoions strongly depend on resonant effects.
Chemical potential gradient driven permeation of small molecules through polymeric media
Authors: Sunderrajan Suresh , Hall Carol K. , Freeman Benny D.
Published: July 2002

Grand canonical molecular dynamics techniques are used to study small molecular penetrant permeation through polymeric media. Penetrants are modeled as hard spheres and square-well spheres. The polymer is modeled as a collection of hard chains and square-well chains. Glassy polymers are modeled using stationary chains while rubbery polymers are modeled using mobile chains. Facilitated transport polymers are also modeled by varying the square-well depth for specific sites along polymer chains. Penetrant partitioning, mutual diffusivity, solubility, and permeability (taken to be a product of diffusivity and solubility) are calculated as a function of reservoir chemical potential, barrier mobility, and, for the facilitating polymer case, the strength of the penetrant/facilitating site attraction. Penetrant diffusivity, solubility, and hence permeability are greater in a mobile barrier than in a stationary barrier. Diffusivity decreases and solubility increases upon the addition of facilitating sites to the barrier or upon increasing the strength of the penetrant/facilitating site attraction. Permeability decreases in these cases, contradicting our expectations concerning the phenomenon of facilitated transport. Comparisons are made between the permeation results presented here and experimental facilitated transport systems.
Coherent and diffusive features of polaron transport in exciton–phonon systems: A Mori approach
Authors: Dolderer H. , Wagner M.
Published: July 2002

Employing Mori-formalism we calculate the polaron transport of an exciton–phonon system with linear site-occupational coupling. The main feature of our approach is a consistent treatment of the memory function, which is of 2nd order in an effective “smallness parameter” appearing after a Lang–Firsov transformation has been made. The memory function is divided into a quasicoherent part, which can be interpreted as a band type motion, decreasing with increasing temperature, an incoherent part describing usual hopping dynamics and two crossterms. In contrast to previous similar approaches our results do not display any divergence problem. It is found that the two cross terms yield a small transport contribution of coherent nature, at least within the accuracy of the present approximation. Due to this term, although higher order effects may result in a damping, the transport will display coherent features for a long time.
Characterization of four phase transitions in Pb5Al3F19, including a new transition at 670 K, by impedance and NMR spectroscopy
Authors: El Omari Mohamed , Réau Jean-Maurice , Sénégas Jean , Ravez Jean , Abrahams S. C. , Nadiri Abdellah , Yacoubi Abdelmajid
Published: July 2002

The ac (alternating current) conductivity of Pb5Al3F19 between 293 and 743 K has been analyzed within a combined complex impedance, modulus, and permittivity formalism. The antiferroelectric phase IV to ferroelastic phase III, and the phase III to paraelastic phase II, transitions have been characterized; the latter is shown to be accompanied by higher F− ion mobility above T=368±5 K. A new dielectric anomaly observed isochronously in ε′(f,T) at 670 K corresponds to an expected, but hitherto unobserved, transition from phase II to the paraelectric prototype phase I of Pb5Al3F19. A 19F NMR investigation between 125 and 430 K of F− ion diffusion properties in Pb5Al3F19 shows that half the anions have become mobile at the ferroelectric phase V to phase IV transition temperature and that almost all the anions are mobile at the phase IV to phase III transition temperature. These motions are shown to be short range, and no correlation is found between phase formation and F− ion diffusion properties in Pb5Al3F19 in the temperature range below 320 K.
Density functional study of endohedral complexes M@C60 (M=Li, Na, K, Be, Mg, Ca, La, B, Al): Electronic properties, ionization potentials, and electron affinities
Authors: Brocl/awik Ewa , Eilmes Andrzej
Published: July 2002

Electronic structures of C60 and its endohedral complexes with Li, Na, K, Be, Mg, Ca, La, B, and Al have been calculated by means of density functional methodology. The electronic distributions have been analyzed from the point of view of the guest-host electron transfer. Alkali metals and Al have been found to donate their outermost electron to the fullerene cage. There is no electron transfer for Be, while for B, Mg, Ca, and La the transfer is only partial. For the entire series, ionization potentials and electron affinities have been calculated which are in excellent agreement with the few values known from experiments. The results are consistent with the donor-acceptor properties of the guest atom.
Momentum studies of electron correlation in the ground state of the H3+ molecule ion
Authors: Banyard K. E. , Sanders J.
Published: July 2002

The consequences of Coulomb correlation in the momentum space description of H3+ is examined in depth. Such an appraisal complements a previous analysis in position space for this simplest of polyatomic systems. As before, the ground state of the equilateral triangular conformation of H3+ is described by a wave function which recovers almost 99% of the correlation energy. The momentum representations of this wave function and a Hartree–Fock counterpart are obtained by using a Dirac–Fourier transform. The correlation effects are assessed by evaluating the changes which occur in the one- and two-particle probability densities over preselected momentum planes. The associated radial and angular partial planar Coulomb shifts are also calculated. In each instance, different regions of p-space can be sampled by giving a “test” or “reference” electron one of several specific fixed momentum values. Comparisons are made with an earlier and similar study of the effect of electron correlation for the ground state of H2. The correlation characteristics for H3+ are also compared, briefly, with those obtained in p-space for the ground states of HeH+ and some simple isoelectronic atomic ions.
Dynamic dipole polarizabilities for a lithium isoelectronic series in their ground and two first doublet excited states
Authors: Mérawa Mohammadou , Rérat Michel
Published: July 2002

A time-dependent gauge-invariant variational method (TDGI) has been used to calculate the length, mixed and velocity dipole polarizability of the lithium isoelectronic series LiI, BeII, BIII, and CIV in their ground (2 2S) and excited (2 2P and 3 2S) states. The results we have obtained are compared with previous experimental (LiI) and theoretical (LiI,BeII,BIII,CIV) data available in the literature. The variation of the polarizabilities versus imaginary frequencies have been investigated and figures displaying the behavior of the spherical components αLL′ of their polarizability are given.
Influence of thermal and optical properties on the visibility of liquid-crystal phase transitions in photoacoustic spectroscopy
Authors: Puccetti Germain , Leblanc Roger M.
Published: July 2002

The thermal, optical light absorption and light scattering properties of a liquid crystal can be distinguished and separately studied by a single technique, pulsed photoacoustic spectroscopy (PPAS), by using different excitation/detection configurations. The advantages of performing complementary thermal and optical measurements without modifying the sample environment are demonstrated in the case of a typical liquid crystal: 4-octyl-4′-cyanobiphenyl (8CB). Results show that light scattering can strongly influence the PPAS response signal through the light absorption profile, especially at phase transitions subjected to small changes in thermal properties, such as the Nematic/isotrope phase transition.
Tracer diffusion of Kr85 in liquid Ar, N2, and O2
Authors: Dunlop Peter J. , Bignell Caroline M.
Published: July 2002

Tracer diffusion coefficients are reported for Kr85 in liquid argon and in liquid nitrogen near their normal boiling points at a pressure of 1.25 atm. Results are also reported for Kr85 in liquid oxygen at the same temperatures.
Periodic modulation induced increase of reaction rates in autocatalytic systems
Authors: Alarcón T. , Pérez-Madrid A. , Rubı́ J. M.
Published: July 2002

We propose a new mechanism to increase the reactions rates in multistable autocatalytic systems. The mechanism is based upon the possibility for the enhancement of the response of the system due to the cooperative behavior between the noise and an external periodic modulation. In order to illustrate this feature we compute the reaction velocities for the particular case of the Sel’Kov model, showing that they increase significantly when the periodic modulation is introduced. This behavior originates from the existence of a minimum in the mean first passage time, one of the signatures of stochastic resonance.
Sensitivity of optical methods to the homogeneity of particulate layers
Authors: Mann E. K. , Heinrich L. , Semmler M. , Voegel J. C. , Schaaf P.
Published: July 2002

For stratified layers, it was recently demonstrated that information about the structure of thin dielectric layers could be determined from scanning angle reflectometry measurements independent of any model of the interface. A measure of the deviation of the film from uniform was defined and demonstrated to be measurable. This analysis is extended here to the case of particulate layers, where we show that the same uniformity parameter can give information about the deviation of the distribution of particles on the surface from a uniform one. Preliminary optical data on moderately sized polystyrene particles, 116 nm in diameter, suggest that the closest approach of two particles on the surface is considerably greater than their diameter. This is in agreement with direct measurements of the radial distribution functions of these particles adsorbed on surfaces under similar conditions.
Angle-resolved spin-orbit autoionization dynamics of Rydberg electron wave packets in Ar: A time-dependent MQDT approach
Authors: Ramswell J. A. , Fielding H. H.
Published: July 2002

The spin-orbit autoionization dynamics of a Rydberg electron wave packet in Ar have been investigated using time-dependent multichannel quantum defect theory (TD-MQDT). The time-dependent photoionization cross sections reveal interesting interference patterns in the recurrence spectra which can be accounted for in terms of the quantum defects of the autoionizing channels. An expression for the time- and energy-dependent asymmetry parameter β(E,t) has been derived to calculate time-resolved photoelectron angular distributions (PADs) for the first time. An oscillation in the time-resolved PAD is observed, with a period which is inversely proportional to the difference between the quantum defects of the autoionizing channels.
Accurate modified configuration interaction single-centered calculations for H2+
Authors: Kempe J. A. , Goldman S. P.
Published: July 2002

An accurate single centered method for molecular calculations, based on the modified configuration interaction method with infinite angular expansions, is introduced. A calculation of the ground state energy of the hydrogen molecular ion in the Born–Oppenheimer approximation is presented as an example yielding an accuracy for the energy of 7×10−8 a.u. which rivals the best multi-centered Gaussian or Slater calculations. Not only is the single-centered calculation simpler than the multi-centered ones but it allows one to introduce rigorously the vibrational-rotational motion of the nuclei. A discussion of the work in progress on the inclusion of nuclear motion using the newer radially uncoupled configuration interaction method is presented.
Multiple quantum filtering and spin exchange in solid state nuclear magnetic resonance
Authors: Ba Yong , Ripmeester John A.
Published: July 2002

Multiple quantum NMR of coupled spin-1/2 nuclei in a solid has been exploited as a filter to separate different magnetization components in a spatially heterogeneous system. After filtering, the spins labeled according to coherence orders were used to follow the track of spin exchange between different domains. In order to avoid time-consuming two-dimensional experiments, this exchange was detected in a one-dimensional experiment via selective detection of multiple quantum coherences. The technique was demonstrated for samples of solid adamantane in contact with its saturated solution in benzene-d6 and for a high-density polyoxymethylene.
Single crystal Raman spectra of 1,2,4,5-tetrabromobenzene: Calculational and experimental assignment of the internal modes
Authors: White K. M. , Eckhardt C. J.
Published: July 2002

A detailed study of the fundamental internal modes of the 1,2,4,5-tetrabromobenzene crystal has provided assignments of Raman-active modes consistent with polarized single-crystal measurements. A normal-mode calculation based on a valence force field derived from brominated and chlorinated benzenes is found to be in excellent agreement with the assignments. Additional bands arising from anharmonic interactions of the fundamentals are also assigned.
Surface induced segregation of hard-sphere chain mixtures
Authors: Vakarin E. V.
Published: July 2002

Structure and segregation properties of a hard-sphere chain fluid dissolved in a polymeric solvent near a hard wall are investigated using the Percus–Yevick (PY) approximation for the associative Henderson-Abraham-Barker (HAB) equation. Both the polymer and the solvent are modeled as freely jointed hard-sphere chains of different lengths mp and ms and with different bead diameters dp and ds. It is shown that there are two segregation factors: the bead diameter and the chain length difference, which cause structural features of the interface. It is determined that at low densities the segregation is mainly due to the chain-chain length difference, while the difference in the segment diameters is dominant at high densities. The structure is analyzed for various overall densities ζ3 and polymer concentrations xp. The partial density profiles as well as the segregation profiles are compared with Monte Carlo data to test the accuracy of the PY theory.
Effect of gravity on contact angle: A theoretical investigation
Authors: Ward C. A. , Sasges M. R.
Published: July 2002

Using the Gibbs description of an interphase, the necessary conditions for equilibrium of a closed, two-phase fluid system in the presence of gravity are the Laplace and Young equations and a condition on the chemical potentials. The last condition has been neglected in all previous examinations of contact angles in a gravitational field. After introducing explicit expressions for the chemical potentials, we find that the condition on the chemical potentials can be used to determine the pressure profile within the system. In a “two-interface” system in which a liquid phase is both above and below a vapor phase and the vapor phase forms a solid–vapor interphase in one region, the pressure profile in the liquid phases is the same as it would have been if the vapor phase were not there; thus in a gravitational field, the pressure is smaller in the liquid phase above the vapor phase than it is in the liquid phase below the vapor phase. This results in the contact angle at the upper three-phase line necessarily being smaller than that at the lower three-phase line. This difference in contact angles is conventionally referred to as contact angle hysteresis; however, we show that it is simply an equilibrium property of a capillary system in a gravitational field. The contact angle difference predicted to exist in the presence of gravity does not violate the Young equation, but the Young equation does impose a restriction on the equilibrium adsorption isotherms at the solid–vapor and solid–liquid interfaces.
Effect of gravity on contact angle: An experimental investigation
Authors: Sasges M. R. , Ward C. A.
Published: July 2002

A capillary system is considered that is exposed to a gravitational field and contains a single component fluid. A portion of the liquid is above and a portion below the vapor. A three-phase line is formed at both places. A previous thermodynamic analysis of such a system, based on the Gibbs model of the interphase and the resulting necessary conditions for equilibrium, has led to the prediction that the equilibrium contact angle at the upper three-phase line is necessarily smaller than that at the lower three-phase line, and to a method that can be used to predict the contact angle at the lower interface for a given value of the contact angle at the upper interface. Even in the absence of any line tension effects, this contact angle difference is predicted to exist for a solid surface that is both smooth and homogeneous. The experimental investigation reported herein supports this prediction and shows that if the system is displaced from this configuration that the system returns to the same configuration. This suggests that the configuration in which the contact angle is smaller at the upper three-phase line than at the lower three-phase line not only satisfies the necessary conditions for equilibrium, but that this configuration is the stable equilibrium configuration for the system.
The intermolecular vibrations of the NO dimer
Authors: East A. L. L. , McKellar A. R. W. , Watson J. K. G.
Published: July 2002

The far infrared spectrum of the NO dimer in the gas phase has been observed for the first time, using a low temperature (100 K) long-path (180 m) absorption cell. The spectroscopic results are used, together with mid-infrared observations and high level CCSD(T) ab initio calculations, to locate the four low frequency intermolecular vibrational modes of (NO)2: ν2 (symmetric bend)=239.361 cm−1, ν3 (intermolecular stretch)=134.503 cm−1, ν4 (out-of-plane torsion)≈117 cm−1, and ν6 (antisymmetric bend)=429.140 cm−1. These values agree poorly with previous determinations based on condensed-phase spectra, indicating that there are large shifts in going from the isolated gas-phase dimer to the liquid, solid, or matrix-isolated environments.
Intramolecular vibrational energy redistribution in the acetylenic C–H and hydroxyl stretches of propynol
Authors: Hudspeth Evan , McWhorter David A. , Pate Brooks H.
Published: July 2002

The high-resolution infrared spectra of the acetylenic C–H and O–H stretches of propynol have been measured using an electric-resonance optothermal molecular beam spectrometer (EROS). Both spectra display extensive fragmentation of the hydride-stretch oscillator strength characteristic of the intramolecular vibrational energy redistribution (IVR) process. The IVR lifetime is strongly mode-specific. The IVR lifetime of the acetylenic C–H stretch is approximately 400 ps, with a slight increase in the lifetime with increasing values of the Ka quantum number. The lifetime of the O–H stretch is 60 ps and is independent of the rotational quantum numbers. The experimental upper limit for the anharmonic state densities are 30 and 40 states/cm−1 for the acetylenic C–H and O–H stretches, respectively. These values are in good agreement with the values obtained by a direct state count (19 and 32 states/cm−1, respectively). The measured density of states increases with an approximate (2J+1)-dependence. These results indicate that all energetically accessible states are involved in the IVR dynamics. However, neither the acetylenic C–H nor the O–H stretch shows a decrease in lifetime as the total angular momentum (J) increases. This result shows that Coriolis coupling of these two hydride stretches to the near-resonant bath states is much weaker than the anharmonic coupling. For the O–H stretch, we are able to obtain the root-mean-squared (rms) matrix element for the Coriolis coupling prefactor, 0.0015(5) cm−1. The rms anharmonic coupling matrix element is 0.03 cm−1. For the low J values measured in the O–H spectrum, the Coriolis-induced IVR rate is much slower than the initial redistribution rate resulting from the stronger anharmonic interactions leading to an IVR process with two distinct time scales.
Reactions of N(2 2D) with methane and deuterated methanes
Authors: Umemoto Hironobu , Nakae Takanobu , Hashimoto Hideomi , Kongo Koichi , Kawasaki Masahiro
Published: July 2002

NH radicals and H atoms were identified as products in the reaction of N(2D) with CH4. The absolute yields of NH and H were determined to be 0.3±0.1 and 0.8±0.2, respectively. The rotational state distributions of NH (v″=0) for CH4 and partially deuterated methanes, CHnD4−n (n=1–3), are single modal and peak at N″=13±2, while the vibrational population ratios, NH (v″=1)/NH (v″=0), are 0.6. The rotational energy distribution of ND (v″=0) from CD4 is similar to that for NH (v″=0). The ND (v″=1)/ND (v″=0) ratio is 0.6. Doppler profiles of H and D atoms were measured to evaluate the energy released to the translational mode. It was between 72 and 85 kJ mol−1, suggesting the following H atom production mechanism: N(2D)+CH4→H+CH2NH. The H+CH3N channel is minor. In the reactions with partially deuterated methanes, the isotopic ratios, [H]/[D] and [NH]/[ND], are larger than the stoichiometric ratios in the reactant methanes. These results suggest that C–H bonds are more reactive to N(2D) than C–D bonds, while H atoms are ejected more preferentially than D atoms.
Spin polarization and the energetics of chlorine adsorption on the GaAs(110) surface
Authors: Chuan Kang H.
Published: July 2002

We have investigated the energetics of chlorine adsorption on the GaAs(110) surface using first principles plane-wave pseudopotential calculations. Both spin-polarized and spin-compensated calculations were performed for a set of eight different configurations of adsorbed chlorine atoms. We find that spin polarization plays an important role in the energetics of adsorption, particularly when we compare chlorine adatom binding energies for configurations with different numbers of “half-filled” dangling bonds. Our calculations show that scanning tunneling microscopy data of chlorine adatom configurations, with observations of islands at lower temperatures and chains at higher temperatures, can be understood in terms of the energetics. Previous calculations, which did not include spin-polarization effects, were not able to provide a definitive conclusion on these observations. From the energetics of the various configurations studied, we can extract a set of effective lateral interaction parameters between chlorine atoms adsorbed on GaAs(110). However, analysis of this set of parameters shows that the range of the effective lateral interactions probably extends to larger distances than the unit cells used in our calculations.
Simulation of structure and interaction forces for surfaces coated with grafted chains in a compressible solvent
Authors: Carson Meredith J. , Sanchez Isaac C. , Johnston Keith P. , de Pablo Juan J.
Published: July 2002

Lennard-Jones chains grafted to solid surfaces in a supercritical solvent are simulated with a continuum grand canonical Monte Carlo method. The force of interaction between two surfaces is calculated as a function of solvent density and temperature and analyzed as a function of the conformational properties of the grafted chains. At high, liquidlike bulk solvent densities, the chains are solvated and the interaction forces are repulsive. As the solvent density is lowered, the chains collapse, and the surfaces become attractive, indicating flocculation. The critical flocculation density coincides with the critical solution density for a bulk mixture of chains and solvent (corrected for local density enhancement). The bulk critical solution density, in turn, corresponds to the coil-to-globule transition of a single chain in bulk solution. The predicted correspondence between these properties agrees with results from lattice-fluid self-consistent field theory and colloid stability experiments. In good and poor solvents, the range of the interaction force between surfaces is much longer than the length of the grafted chains, due to expulsion of solvent from the interface as the surfaces are compressed. Very similar ranges were seen for forces measured with the surface forces apparatus in liquid solvent [G. Hadziioannou et al., J. Am. Chem. Soc. 108, 2869 (1986)].
A Monte Carlo study of metastable structures of the cyanoadamantane crystal
Authors: Kuchta Bogdan , Descamps Marc , Affouard Frédéric
Published: July 2002

We report results of the Monte Carlo simulations of stable and metastable phases of cyanoadamantane crystal, which is a model of an orientational glass forming system. The structures which have been found stable within the Monte Carlo runs include the plastic cubic phase, two different monoclinic phases, and an orthorhombic phase. The calculations have shown that all those structures remain metastable in the whole studied range of temperatures between 100 and 400 K. The orthorhombic arrangement is energetically the most favorable but the cubic phase is the most stable phase at high temperature because of the disorder and large entropy term in the free energy. The transient orthorhombic phase is playing an important role in the transformations from quenched cubic phase into stable low temperature structure. A possibility of a disorder in the orthorhombic phase is discussed and pointed out as a factor which may affect the mechanism and kinetics of the transformation between quenched cubic structure and the low temperature ordered phase. The present model is not able to stabilize thermodynamically the monoclinic structure at low temperature, and the simplified representation of the intermolecular interaction seems to be the cause.
Dynamics of HI photodissociation in the
A
band absorption via H-atom Doppler spectroscopy
Authors: Gendron Denis J. , Hepburn J. W.
Published: July 2002

The dynamics of HI photodissociation following absorption in the A band (200 to 300 nm) was investigated by Doppler spectroscopy at the Lyman α transition of the H-atom photofragments. Measurements of both the branching ratios for the formation of spin-orbit excited I(2P1/2) and ground state I(2P3/2) atoms and the angular distributions of the recoil velocity for these two photofragment channels were obtained at nine photolysis wavelengths between 212.5 and 266 nm. These results show that ground state products result from a transition of perpendicular symmetry (ΔΩ=1), while the excited state atoms are produced from a parallel transition (ΔΩ=0). These experimental results, in combination with total absorption cross section data obtained prior to the present study, have enabled a calculation of the potential curves for the dissociative excited states. The outcome of the calculation was found to be in good qualitative agreement with the model of HI electronic structure originally suggested by Mulliken.
Diabatic analysis of the electronic states of hydrogen chloride
Authors: Liyanage Rohana , Gordon Robert J. , Field Robert W.
Published: July 2002

An effective Hamiltonian method was used to deperturb the (X 2Πi)4pσ and (X 2Πi)4pπ Rydberg states and the V 1Σ+ valence state of HCl and DCl. A least-squares fit of the eigenvalues of the Hamiltonian to the experimental term energies was performed, and the zero-order diabatic characters of the basis states were determined. These deperturbed states were used to fit the observed Λ-doublings, to predict the energies of unknown states, to model spectroscopic intensity anomalies, and to explain the spin-orbit branching ratio for predissociation.
Potential functions for describing intermolecular interactions in cyanoacetylene clusters
Authors: Cabaleiro-Lago Enrique M. , Rı́os Miguel A.
Published: July 2002

Potential functions for describing interactions between cyanoacetylene molecules based on ab initio determined molecular properties and IMPT calculations are proposed. Electrostatic interactions are described by a multipole expansion on atoms and midbond points; dispersion is expressed by a London-type function of atomic polarizabilities and induction is considered via a series of polarizabilities distributed over the atoms. The repulsion contribution was determined by using a test-particle model involving a helium atom as probe particle. Two functions based on two basis sets of different size, viz. 6-311G** and 5S4P2D/3S2P, were used. Cyanoacetylene dimer exhibits two minima corresponding to a linear and an antiparallel configuration, respectively. The proposed functions accurately reproduce the characteristics of the dimer minima as derived from ab initio calculations at the Mo/ller–Plesset (MP2) level. In addition, they can describe cooperativeness in larger clusters; specifically, the dipole moment and interaction energy per molecule increase with increasing number of constituent units in the cluster. The behavior observed is similar to previously reported findings for HCN clusters.
A comparison of ZnO and ZnO−
Authors: Bauschlicher Charles W. , Partridge Harry
Published: July 2002

Ab initio electronic structure calculations are performed to support and to help interpret the experimental work reported in the preceding manuscript. The coupled cluster singles and doubles [CCSD(T)] approach, in conjunction with a large basis set, is used to compute spectroscopic constants for the X 1Σ+ and Π3 states of ZnO and the X 2Σ+ state of ZnO−. The spectroscopic constants, including the electron affinity (E.A.) are in good agreement with experiment. The ZnO E.A. is significantly larger than that of O, thus relative to the atomic ground-state asymptotes, ZnO− has a larger D0 than the Σ+1 state, despite the fact that the extra electron goes into an antibonding orbital. The changes in spectroscopic constants can be understood in terms of the X 1Σ+ formally dissociating to Zn S1+O 1D, while the Π3 and Σ+2 states dissociate to Zn 1S+O 3P and Zn 1S and O− 2P, respectively.
Formation and desorption dynamics of photoproducts in the ablation of van der Waals films of chlorobenzene at 248 nm
Authors: Georgiou Savas , Koubenakis Antonis , Labrakis John , Lassithiotaki Maria
Published: July 2002

The ablation of thick C6H5Cl films at 248 nm is studied with emphasis on the formation efficiency and desorption dynamics of the observed photoproducts. To this end, the desorbates are probed as a function of the laser fluence via time-of-flight quadrupole mass spectrometry. At low fluences, indicated to represent the subablation regime, we observe a strong induction effect for the parent molecule and desorption of only one new species, namely of HCl. Phenyl products are indicated to be formed, but they start desorbing only as parent peak induction becomes significant. In contrast, at higher fluences, desorption of the parent molecule from freshly deposited films is intense from the very first laser pulse, and a number of additional photoproducts are observed. Three main species, namely (C6H5)2, C6H4Cl2, and C12H9Cl, are observed, while Cl and C12H8Cl2 are detected mainly at higher fluences. All products are compatible with the known gas phase and solution chemistry of C6H5Cl, indicating that no new reaction channels open up above the ablation threshold. Over the full fluence range, the phenyl photoproducts differ significantly from HCl in their desorption observables. Specifically, the phenyl photoproducts are described by nearly the same velocity distribution as the parent molecule and their desorption efficiency closely correlates to that of C6H5Cl. In contrast, HCl exhibits a much more complicated translational behavior, but generally it is nearly equilibrated with the parent molecule. We argue that these differences derive mainly from the much higher volatility of HCl as compared with that of the phenyl photoproducts. Thus, for the phenyl derivatives, the “driving force” for their ejection is evidently their entrainment in the C6H5Cl jet, whereas for HCl, being quite volatile, its desorption appears to be largely independent of that of the parent molecule.
Photodynamics in superfluid helium: Femtosecond laser-induced ionization, charge recombination, and preparation of molecular Rydberg states
Authors: Benderskii A. V. , Zadoyan R. , Schwentner N. , Apkarian V. A.
Published: July 2002

Femtosecond pulses (790 nm) are used for nonresonant laser excitation of superfluid liquid helium to prepare ionic and neutral excited states at energies above 18 eV. Measurements of laser-induced fluorescence and photocurrent enable a detailed description of the primary photoprocesses. A controllable excitation regime unique to femtosecond pulses is realized at laser intensities below the dielectric breakdown threshold, I<5×1013 W/cm2. A steady state of the long-lived triplet excimers He2*(3a) (lowest Rydberg state) is established; the concentration decays between laser pulses through diffusion-controlled bimolecular annihilation to ∼1012 cm−3 at a laser repetition rate of 500 Hz. The triplet population is amplified with each pulse in a sequence that involves: (1) ionization of the Rydberg electron of He2* via complete Coulomb barrier suppression; (2) cascade electron impact ionization of the ground-state He atoms by the ponderomotively accelerated quasifree electrons in liquid He; (3) localization and thermalization of the “hot” electrons and He+ cations to form electron “bubble” and He3+ “snowball” states; (4) recombination of these elementary charge carriers to form He2*. The amplification factor for the triplets M=2m characterizes the excitation sequence: m is the number of generations in the cascade (m=5 at I=4.5×1013 W/cm2), and m is proportional to the laser intensity and temporal pulse width. The laser-induced ionization cascade prepares an inhomogeneous initial distribution of spatially separated ions on three length scales: clumps of positive charges with an interionic separation determined by the cascade length of 60 Å; a cloud of electrons surrounding the clump at the electron thermalization length ∼103 Å; and interclump separation dictated by the concentration of the He2* precursors, ∼104 Å.
Glass transition in hard disc mixtures
Authors: Speedy Robin J.
Published: July 2002

Binary hard disc mixtures show eutectic freezing when the diameters differ by more than 30% and near the eutectic composition glasses can be formed without complications due to freezing. The glasses are characterized by measuring their equation of state and entropy. The glass transition is described by including the fluid and its many glasses within a unified formalism that represents the fluid as the collection of all its glasses. This leads to an equation of state that describes the fluid and the glasses from the ideal-gas, low-density limit to an ideal-glass high-density limit, with an ideal glass transition, where the heat capacity decreases sharply. The ideal transition is pre-empted by a kinetic glass transition that occurs when the root-mean-squared displacement of discs, on the simulation time scale, is comparable to a disc diameter.
Comment on: “Accurate universal Gaussian basis set for all atoms of the periodic table” [J. Chem. Phys. 108, 5225 (1998)]
Authors: Wilson S. , Moncrieff D.
Published: July 2002

The universal basis set of de Castro and Jorge has been investigated in matrix Hartree–Fock calculations, without truncation for specific atomic species, in electron correlation studies, and in systematic schemes for basis set extension.
Comparison of aqueous and native oxide formation on Cu(111)
Authors: Chu Y. S. , Robinson I. K. , Gewirth A. A.
Published: July 2002

We present the results of an x-ray diffraction investigation of the formation of oxide on electropolished Cu(111) surfaces, both in situ at pH 4.5 and in air. In both cases the oxide is found to be crystalline cuprite and epitaxially aligned with the substrate, but with two possible epitaxial orientations. We followed the progress of oxidation by monitoring the shapes of the diffraction peaks for the two orientations as a function of time and potential. There is a narrow potential region where the oxide is a single monolayer thick. Beyond that, only one of the two oxide orientations becomes thicker, and does so in an inhomogeneous manner, thickening in narrow regions before it spreads.
Reaction of HF molecules with metal impurities on Si surfaces
Authors: Hoshino T. , Nishioka Y.
Published: July 2002

First-principles quantum chemical calculations have been performed to reveal the reaction mechanisms of HF molecules with metal impurities (Al or Cu) on Si surfaces. The lowest energy paths of this HF attack reaction have been obtained in two situations when a metal atom adheres to a Si surface with and without an O atom incorporated between the metal atom and the surface. The potential energy changes along these reaction paths were evaluated. It was revealed that a HF molecule has the ability to etch an Al monoxide on the Si surface. On the other hand, HF is not effective in the etching of a metal atom (Al or Cu) which has directly bonded to a Si surface.